docs/envvars.html - platform/external/mesa3d - Gitiles

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 <div class="header">
   <h1>The Mesa 3D Graphics Library</h1>
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 <iframe src="contents.html"></iframe>
 <div class="content">

 <h1>Environment Variables</h1>

 <p>
 Normally, no environment variables need to be set.  Most of the environment
 variables used by Mesa/Gallium are for debugging purposes, but they can
 sometimes be useful for debugging end-user issues.
 </p>


 <h2>LibGL environment variables</h2>

 <ul>
 <li>LIBGL_DEBUG - If defined debug information will be printed to stderr.
    If set to 'verbose' additional information will be printed.
 <li>LIBGL_DRIVERS_PATH - colon-separated list of paths to search for DRI drivers
 <li>LIBGL_ALWAYS_INDIRECT - forces an indirect rendering context/connection.
 <li>LIBGL_ALWAYS_SOFTWARE - if set, always use software rendering
 <li>LIBGL_NO_DRAWARRAYS - if set do not use DrawArrays GLX protocol (for debugging)
 <li>LIBGL_SHOW_FPS - print framerate to stdout based on the number of glXSwapBuffers
     calls per second.
 <li>LIBGL_DRI3_DISABLE - disable DRI3 if set (the value does not matter)
 </ul>


 <h2>Core Mesa environment variables</h2>

 <ul>
 <li>MESA_NO_ASM - if set, disables all assembly language optimizations
 <li>MESA_NO_MMX - if set, disables Intel MMX optimizations
 <li>MESA_NO_3DNOW - if set, disables AMD 3DNow! optimizations
 <li>MESA_NO_SSE - if set, disables Intel SSE optimizations
 <li>MESA_DEBUG - if set, error messages are printed to stderr.  For example,
    if the application generates a GL_INVALID_ENUM error, a corresponding error
    message indicating where the error occurred, and possibly why, will be
    printed to stderr.<br>

    For release builds, MESA_DEBUG defaults to off (no debug output).

    MESA_DEBUG accepts the following comma-separated list of named
    flags, which adds extra behaviour to just set MESA_DEBUG=1:
    <ul>
      <li>silent - turn off debug messages. Only useful for debug builds.</li>
      <li>flush - flush after each drawing command</li>
      <li>incomplete_tex - extra debug messages when a texture is incomplete</li>
      <li>incomplete_fbo - extra debug messages when a fbo is incomplete</li>
    </ul>
 <li>MESA_LOG_FILE - specifies a file name for logging all errors, warnings,
 etc., rather than stderr
 <li>MESA_TEX_PROG - if set, implement conventional texture env modes with
 fragment programs (intended for developers only)
 <li>MESA_TNL_PROG - if set, implement conventional vertex transformation
 operations with vertex programs (intended for developers only).
 Setting this variable automatically sets the MESA_TEX_PROG variable as well.
 <li>MESA_EXTENSION_OVERRIDE - can be used to enable/disable extensions.
 A value such as "GL_EXT_foo -GL_EXT_bar" will enable the GL_EXT_foo extension
 and disable the GL_EXT_bar extension.
 <li>MESA_EXTENSION_MAX_YEAR - The GL_EXTENSIONS string returned by Mesa is sorted
 by extension year.
 If this variable is set to year X, only extensions defined on or before year
 X will be reported.
 This is to work-around a bug in some games where the extension string is
 copied into a fixed-size buffer without truncating.
 If the extension string is too long, the buffer overrun can cause the game
 to crash.
 This is a work-around for that.
 <li>MESA_GL_VERSION_OVERRIDE - changes the value returned by
 glGetString(GL_VERSION) and possibly the GL API type.
 <ul>
 <li> The format should be MAJOR.MINOR[FC]
 <li> FC is an optional suffix that indicates a forward compatible context.
 This is only valid for versions &gt;= 3.0.
 <li> GL versions &lt; 3.0 are set to a compatibility (non-Core) profile
 <li> GL versions = 3.0, see below
 <li> GL versions &gt; 3.0 are set to a Core profile
 <li> Examples: 2.1, 3.0, 3.0FC, 3.1, 3.1FC
 <ul>
 <li> 2.1 - select a compatibility (non-Core) profile with GL version 2.1
 <li> 3.0 - select a compatibility (non-Core) profile with GL version 3.0
 <li> 3.0FC - select a Core+Forward Compatible profile with GL version 3.0
 <li> 3.1 - select a Core profile with GL version 3.1
 <li> 3.1FC - select a Core+Forward Compatible profile with GL version 3.1
 </ul>
 <li> Mesa may not really implement all the features of the given version.
 (for developers only)
 </ul>
 <li>MESA_GLES_VERSION_OVERRIDE - changes the value returned by
 glGetString(GL_VERSION) for OpenGL ES.
 <ul>
 <li> The format should be MAJOR.MINOR
 <li> Examples: 2.0, 3.0, 3.1
 <li> Mesa may not really implement all the features of the given version.
 (for developers only)
 </ul>
 <li>MESA_GLSL_VERSION_OVERRIDE - changes the value returned by
 glGetString(GL_SHADING_LANGUAGE_VERSION). Valid values are integers, such as
 "130".  Mesa will not really implement all the features of the given language version
 if it's higher than what's normally reported. (for developers only)
 <li>MESA_GLSL - <a href="shading.html#envvars">shading language compiler options</a>
 <li>MESA_NO_MINMAX_CACHE - when set, the minmax index cache is globally disabled.
 </ul>


 <h2>Mesa Xlib driver environment variables</h2>

 <p>
 The following are only applicable to the Mesa Xlib software driver.
 See the <a href="xlibdriver.html">Xlib software driver page</a> for details.
 </p>
 <ul>
 <li>MESA_RGB_VISUAL - specifies the X visual and depth for RGB mode
 <li>MESA_CI_VISUAL - specifies the X visual and depth for CI mode
 <li>MESA_BACK_BUFFER - specifies how to implement the back color buffer,
     either "pixmap" or "ximage"
 <li>MESA_GAMMA - gamma correction coefficients for red, green, blue channels
 <li>MESA_XSYNC - enable synchronous X behavior (for debugging only)
 <li>MESA_GLX_FORCE_CI - if set, force GLX to treat 8bpp visuals as CI visuals
 <li>MESA_GLX_FORCE_ALPHA - if set, forces RGB windows to have an alpha channel.
 <li>MESA_GLX_DEPTH_BITS - specifies default number of bits for depth buffer.
 <li>MESA_GLX_ALPHA_BITS - specifies default number of bits for alpha channel.
 </ul>


 <h2>i945/i965 driver environment variables (non-Gallium)</h2>

 <ul>
 <li>INTEL_NO_HW - if set to 1, prevents batches from being submitted to the hardware.
    This is useful for debugging hangs, etc.</li>
 <li>INTEL_DEBUG - a comma-separated list of named flags, which do various things:
 <ul>
    <li>tex - emit messages about textures.</li>
    <li>state - emit messages about state flag tracking</li>
    <li>blit - emit messages about blit operations</li>
    <li>miptree - emit messages about miptrees</li>
    <li>perf - emit messages about performance issues</li>
    <li>perfmon - emit messages about AMD_performance_monitor</li>
    <li>bat - emit batch information</li>
    <li>pix - emit messages about pixel operations</li>
    <li>buf - emit messages about buffer objects</li>
    <li>fbo - emit messages about framebuffers</li>
    <li>fs - dump shader assembly for fragment shaders</li>
    <li>gs - dump shader assembly for geometry shaders</li>
    <li>sync - after sending each batch, emit a message and wait for that batch to finish rendering</li>
    <li>prim - emit messages about drawing primitives</li>
    <li>vert - emit messages about vertex assembly</li>
    <li>dri - emit messages about the DRI interface</li>
    <li>sf - emit messages about the strips &amp; fans unit (for old gens, includes the SF program)</li>
    <li>stats - enable statistics counters. you probably actually want perfmon or intel_gpu_top instead.</li>
    <li>urb - emit messages about URB setup</li>
    <li>vs - dump shader assembly for vertex shaders</li>
    <li>clip - emit messages about the clip unit (for old gens, includes the CLIP program)</li>
    <li>aub - dump batches into an AUB trace for use with simulation tools</li>
    <li>shader_time - record how much GPU time is spent in each shader</li>
    <li>no16 - suppress generation of 16-wide fragment shaders. useful for debugging broken shaders</li>
    <li>blorp - emit messages about the blorp operations (blits &amp; clears)</li>
    <li>nodualobj - suppress generation of dual-object geometry shader code</li>
    <li>optimizer - dump shader assembly to files at each optimization pass and iteration that make progress</li>
    <li>ann - annotate IR in assembly dumps</li>
    <li>no8 - don't generate SIMD8 fragment shader</li>
    <li>vec4 - force vec4 mode in vertex shader</li>
    <li>spill_fs - force spilling of all registers in the scalar backend (useful to debug spilling code)</li>
    <li>spill_vec4 - force spilling of all registers in the vec4 backend (useful to debug spilling code)</li>
    <li>cs - dump shader assembly for compute shaders</li>
    <li>hex - print instruction hex dump with the disassembly</li>
    <li>nocompact - disable instruction compaction</li>
    <li>tcs - dump shader assembly for tessellation control shaders</li>
    <li>tes - dump shader assembly for tessellation evaluation shaders</li>
    <li>l3 - emit messages about the new L3 state during transitions</li>
    <li>do32 - generate compute shader SIMD32 programs even if workgroup size doesn't exceed the SIMD16 limit</li>
    <li>norbc - disable single sampled render buffer compression</li>
 </ul>
 </ul>


 <h2>Radeon driver environment variables (radeon, r200, and r300g)</h2>

 <ul>
 <li>RADEON_NO_TCL - if set, disable hardware-accelerated Transform/Clip/Lighting.
 </ul>


 <h2>EGL environment variables</h2>

 <p>
 Mesa EGL supports different sets of environment variables.  See the
 <a href="egl.html">Mesa EGL</a> page for the details.
 </p>


 <h2>Gallium environment variables</h2>

 <ul>
 <li>GALLIUM_HUD - draws various information on the screen, like framerate,
     cpu load, driver statistics, performance counters, etc.
     Set GALLIUM_HUD=help and run e.g. glxgears for more info.
 <li>GALLIUM_HUD_PERIOD - sets the hud update rate in seconds (float). Use zero
     to update every frame. The default period is 1/2 second.
 <li>GALLIUM_HUD_VISIBLE - control default visibility, defaults to true.
 <li>GALLIUM_HUD_TOGGLE_SIGNAL - toggle visibility via user specified signal.
     Especially useful to toggle hud at specific points of application and
     disable for unencumbered viewing the rest of the time. For example, set
     GALLIUM_HUD_VISIBLE to false and GALLIUM_HUD_TOGGLE_SIGNAL to 10 (SIGUSR1).
     Use kill -10 <pid> to toggle the hud as desired.
 <li>GALLIUM_DRIVER - useful in combination with LIBGL_ALWAYS_SOFTWARE=1 for
     choosing one of the software renderers "softpipe", "llvmpipe" or "swr".
 <li>GALLIUM_LOG_FILE - specifies a file for logging all errors, warnings, etc.
     rather than stderr.
 <li>GALLIUM_PRINT_OPTIONS - if non-zero, print all the Gallium environment
     variables which are used, and their current values.
 <li>GALLIUM_DUMP_CPU - if non-zero, print information about the CPU on start-up
 <li>TGSI_PRINT_SANITY - if set, do extra sanity checking on TGSI shaders and
     print any errors to stderr.
 <LI>DRAW_FSE - ???
 <LI>DRAW_NO_FSE - ???
 <li>DRAW_USE_LLVM - if set to zero, the draw module will not use LLVM to execute
     shaders, vertex fetch, etc.
 <li>ST_DEBUG - controls debug output from the Mesa/Gallium state tracker.
 Setting to "tgsi", for example, will print all the TGSI shaders.
 See src/mesa/state_tracker/st_debug.c for other options.
 </ul>

 <h3>Softpipe driver environment variables</h3>
 <ul>
 <li>SOFTPIPE_DUMP_FS - if set, the softpipe driver will print fragment shaders
     to stderr
 <li>SOFTPIPE_DUMP_GS - if set, the softpipe driver will print geometry shaders
     to stderr
 <li>SOFTPIPE_NO_RAST - if set, rasterization is no-op'd.  For profiling purposes.
 <li>SOFTPIPE_USE_LLVM - if set, the softpipe driver will try to use LLVM JIT for
     vertex shading processing.
 </ul>


 <h3>LLVMpipe driver environment variables</h3>
 <ul>
 <li>LP_NO_RAST - if set LLVMpipe will no-op rasterization
 <li>LP_DEBUG - a comma-separated list of debug options is accepted.  See the
     source code for details.
 <li>LP_PERF - a comma-separated list of options to selectively no-op various
     parts of the driver.  See the source code for details.
 <li>LP_NUM_THREADS - an integer indicating how many threads to use for rendering.
     Zero turns off threading completely.  The default value is the number of CPU
     cores present.
 </ul>

 <h3>VMware SVGA driver environment variables</h3>
 <ul>
 <li>SVGA_FORCE_SWTNL - force use of software vertex transformation
 <li>SVGA_NO_SWTNL - don't allow software vertex transformation fallbacks
 (will often result in incorrect rendering).
 <li>SVGA_DEBUG - for dumping shaders, constant buffers, etc.  See the code
 for details.
 <li>See the driver code for other, lesser-used variables.
 </ul>


 <h3>VA-API state tracker environment variables</h3>
 <ul>
 <li>VAAPI_MPEG4_ENABLED - enable MPEG4 for VA-API, disabled by default.
 </ul>


 <h3>VC4 driver environment variables</h3>
 <ul>
 <li>VC4_DEBUG - a comma-separated list of named flags, which do various things:
 <ul>
    <li>cl - dump command list during creation</li>
    <li>qpu - dump generated QPU instructions</li>
    <li>qir - dump QPU IR during program compile</li>
    <li>nir - dump NIR during program compile</li>
    <li>tgsi - dump TGSI during program compile</li>
    <li>shaderdb - dump program compile information for shader-db analysis</li>
    <li>perf - print during performance-related events</li>
    <li>norast - skip actual hardware execution of commands</li>
    <li>always_flush - flush after each draw call</li>
    <li>always_sync - wait for finish after each flush</li>
    <li>dump - write a GPU command stream trace file (VC4 simulator only)</li>
 </ul>
 </ul>


 <p>
 Other Gallium drivers have their own environment variables.  These may change
 frequently so the source code should be consulted for details.
 </p>

 </div>
 </body>
 </html>
	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
	<html lang="en">
	<head>
	<meta http-equiv="content-type" content="text/html; charset=utf-8">
	<title>Environment Variables</title>
	<link rel="stylesheet" type="text/css" href="mesa.css">
	</head>
	<body>

	<div class="header">
	<h1>The Mesa 3D Graphics Library</h1>
	</div>

	<iframe src="contents.html"></iframe>
	<div class="content">

	<h1>Environment Variables</h1>

	<p>
	Normally, no environment variables need to be set. Most of the environment
	variables used by Mesa/Gallium are for debugging purposes, but they can
	sometimes be useful for debugging end-user issues.
	</p>


	<h2>LibGL environment variables</h2>

	<ul>
	<li>LIBGL_DEBUG - If defined debug information will be printed to stderr.
	If set to 'verbose' additional information will be printed.
	<li>LIBGL_DRIVERS_PATH - colon-separated list of paths to search for DRI drivers
	<li>LIBGL_ALWAYS_INDIRECT - forces an indirect rendering context/connection.
	<li>LIBGL_ALWAYS_SOFTWARE - if set, always use software rendering
	<li>LIBGL_NO_DRAWARRAYS - if set do not use DrawArrays GLX protocol (for debugging)
	<li>LIBGL_SHOW_FPS - print framerate to stdout based on the number of glXSwapBuffers
	calls per second.
	<li>LIBGL_DRI3_DISABLE - disable DRI3 if set (the value does not matter)
	</ul>



	<h2>Core Mesa environment variables</h2>

	<ul>
	<li>MESA_NO_ASM - if set, disables all assembly language optimizations
	<li>MESA_NO_MMX - if set, disables Intel MMX optimizations
	<li>MESA_NO_3DNOW - if set, disables AMD 3DNow! optimizations
	<li>MESA_NO_SSE - if set, disables Intel SSE optimizations
	<li>MESA_DEBUG - if set, error messages are printed to stderr. For example,
	if the application generates a GL_INVALID_ENUM error, a corresponding error
	message indicating where the error occurred, and possibly why, will be
	printed to stderr.<br>

	For release builds, MESA_DEBUG defaults to off (no debug output).

	MESA_DEBUG accepts the following comma-separated list of named
	flags, which adds extra behaviour to just set MESA_DEBUG=1:
	<ul>
	<li>silent - turn off debug messages. Only useful for debug builds.</li>
	<li>flush - flush after each drawing command</li>
	<li>incomplete_tex - extra debug messages when a texture is incomplete</li>
	<li>incomplete_fbo - extra debug messages when a fbo is incomplete</li>
	</ul>
	<li>MESA_LOG_FILE - specifies a file name for logging all errors, warnings,
	etc., rather than stderr
	<li>MESA_TEX_PROG - if set, implement conventional texture env modes with
	fragment programs (intended for developers only)
	<li>MESA_TNL_PROG - if set, implement conventional vertex transformation
	operations with vertex programs (intended for developers only).
	Setting this variable automatically sets the MESA_TEX_PROG variable as well.
	<li>MESA_EXTENSION_OVERRIDE - can be used to enable/disable extensions.
	A value such as "GL_EXT_foo -GL_EXT_bar" will enable the GL_EXT_foo extension
	and disable the GL_EXT_bar extension.
	<li>MESA_EXTENSION_MAX_YEAR - The GL_EXTENSIONS string returned by Mesa is sorted
	by extension year.
	If this variable is set to year X, only extensions defined on or before year
	X will be reported.
	This is to work-around a bug in some games where the extension string is
	copied into a fixed-size buffer without truncating.
	If the extension string is too long, the buffer overrun can cause the game
	to crash.
	This is a work-around for that.
	<li>MESA_GL_VERSION_OVERRIDE - changes the value returned by
	glGetString(GL_VERSION) and possibly the GL API type.
	<ul>
	<li> The format should be MAJOR.MINOR[FC]
	<li> FC is an optional suffix that indicates a forward compatible context.
	This is only valid for versions >= 3.0.
	<li> GL versions < 3.0 are set to a compatibility (non-Core) profile
	<li> GL versions = 3.0, see below
	<li> GL versions > 3.0 are set to a Core profile
	<li> Examples: 2.1, 3.0, 3.0FC, 3.1, 3.1FC
	<ul>
	<li> 2.1 - select a compatibility (non-Core) profile with GL version 2.1
	<li> 3.0 - select a compatibility (non-Core) profile with GL version 3.0
	<li> 3.0FC - select a Core+Forward Compatible profile with GL version 3.0
	<li> 3.1 - select a Core profile with GL version 3.1
	<li> 3.1FC - select a Core+Forward Compatible profile with GL version 3.1
	</ul>
	<li> Mesa may not really implement all the features of the given version.
	(for developers only)
	</ul>
	<li>MESA_GLES_VERSION_OVERRIDE - changes the value returned by
	glGetString(GL_VERSION) for OpenGL ES.
	<ul>
	<li> The format should be MAJOR.MINOR
	<li> Examples: 2.0, 3.0, 3.1
	<li> Mesa may not really implement all the features of the given version.
	(for developers only)
	</ul>
	<li>MESA_GLSL_VERSION_OVERRIDE - changes the value returned by
	glGetString(GL_SHADING_LANGUAGE_VERSION). Valid values are integers, such as
	"130". Mesa will not really implement all the features of the given language version
	if it's higher than what's normally reported. (for developers only)
	<li>MESA_GLSL - <a href="shading.html#envvars">shading language compiler options</a>
	<li>MESA_NO_MINMAX_CACHE - when set, the minmax index cache is globally disabled.
	</ul>


	<h2>Mesa Xlib driver environment variables</h2>

	<p>
	The following are only applicable to the Mesa Xlib software driver.
	See the <a href="xlibdriver.html">Xlib software driver page</a> for details.
	</p>
	<ul>
	<li>MESA_RGB_VISUAL - specifies the X visual and depth for RGB mode
	<li>MESA_CI_VISUAL - specifies the X visual and depth for CI mode
	<li>MESA_BACK_BUFFER - specifies how to implement the back color buffer,
	either "pixmap" or "ximage"
	<li>MESA_GAMMA - gamma correction coefficients for red, green, blue channels
	<li>MESA_XSYNC - enable synchronous X behavior (for debugging only)
	<li>MESA_GLX_FORCE_CI - if set, force GLX to treat 8bpp visuals as CI visuals
	<li>MESA_GLX_FORCE_ALPHA - if set, forces RGB windows to have an alpha channel.
	<li>MESA_GLX_DEPTH_BITS - specifies default number of bits for depth buffer.
	<li>MESA_GLX_ALPHA_BITS - specifies default number of bits for alpha channel.
	</ul>


	<h2>i945/i965 driver environment variables (non-Gallium)</h2>

	<ul>
	<li>INTEL_NO_HW - if set to 1, prevents batches from being submitted to the hardware.
	This is useful for debugging hangs, etc.</li>
	<li>INTEL_DEBUG - a comma-separated list of named flags, which do various things:
	<ul>
	<li>tex - emit messages about textures.</li>
	<li>state - emit messages about state flag tracking</li>
	<li>blit - emit messages about blit operations</li>
	<li>miptree - emit messages about miptrees</li>
	<li>perf - emit messages about performance issues</li>
	<li>perfmon - emit messages about AMD_performance_monitor</li>
	<li>bat - emit batch information</li>
	<li>pix - emit messages about pixel operations</li>
	<li>buf - emit messages about buffer objects</li>
	<li>fbo - emit messages about framebuffers</li>
	<li>fs - dump shader assembly for fragment shaders</li>
	<li>gs - dump shader assembly for geometry shaders</li>
	<li>sync - after sending each batch, emit a message and wait for that batch to finish rendering</li>
	<li>prim - emit messages about drawing primitives</li>
	<li>vert - emit messages about vertex assembly</li>
	<li>dri - emit messages about the DRI interface</li>
	<li>sf - emit messages about the strips & fans unit (for old gens, includes the SF program)</li>
	<li>stats - enable statistics counters. you probably actually want perfmon or intel_gpu_top instead.</li>
	<li>urb - emit messages about URB setup</li>
	<li>vs - dump shader assembly for vertex shaders</li>
	<li>clip - emit messages about the clip unit (for old gens, includes the CLIP program)</li>
	<li>aub - dump batches into an AUB trace for use with simulation tools</li>
	<li>shader_time - record how much GPU time is spent in each shader</li>
	<li>no16 - suppress generation of 16-wide fragment shaders. useful for debugging broken shaders</li>
	<li>blorp - emit messages about the blorp operations (blits & clears)</li>
	<li>nodualobj - suppress generation of dual-object geometry shader code</li>
	<li>optimizer - dump shader assembly to files at each optimization pass and iteration that make progress</li>
	<li>ann - annotate IR in assembly dumps</li>
	<li>no8 - don't generate SIMD8 fragment shader</li>
	<li>vec4 - force vec4 mode in vertex shader</li>
	<li>spill_fs - force spilling of all registers in the scalar backend (useful to debug spilling code)</li>
	<li>spill_vec4 - force spilling of all registers in the vec4 backend (useful to debug spilling code)</li>
	<li>cs - dump shader assembly for compute shaders</li>
	<li>hex - print instruction hex dump with the disassembly</li>
	<li>nocompact - disable instruction compaction</li>
	<li>tcs - dump shader assembly for tessellation control shaders</li>
	<li>tes - dump shader assembly for tessellation evaluation shaders</li>
	<li>l3 - emit messages about the new L3 state during transitions</li>
	<li>do32 - generate compute shader SIMD32 programs even if workgroup size doesn't exceed the SIMD16 limit</li>
	<li>norbc - disable single sampled render buffer compression</li>
	</ul>
	</ul>


	<h2>Radeon driver environment variables (radeon, r200, and r300g)</h2>

	<ul>
	<li>RADEON_NO_TCL - if set, disable hardware-accelerated Transform/Clip/Lighting.
	</ul>


	<h2>EGL environment variables</h2>

	<p>
	Mesa EGL supports different sets of environment variables. See the
	<a href="egl.html">Mesa EGL</a> page for the details.
	</p>


	<h2>Gallium environment variables</h2>

	<ul>
	<li>GALLIUM_HUD - draws various information on the screen, like framerate,
	cpu load, driver statistics, performance counters, etc.
	Set GALLIUM_HUD=help and run e.g. glxgears for more info.
	<li>GALLIUM_HUD_PERIOD - sets the hud update rate in seconds (float). Use zero
	to update every frame. The default period is 1/2 second.
	<li>GALLIUM_HUD_VISIBLE - control default visibility, defaults to true.
	<li>GALLIUM_HUD_TOGGLE_SIGNAL - toggle visibility via user specified signal.
	Especially useful to toggle hud at specific points of application and
	disable for unencumbered viewing the rest of the time. For example, set
	GALLIUM_HUD_VISIBLE to false and GALLIUM_HUD_TOGGLE_SIGNAL to 10 (SIGUSR1).
	Use kill -10 <pid> to toggle the hud as desired.
	<li>GALLIUM_DRIVER - useful in combination with LIBGL_ALWAYS_SOFTWARE=1 for
	choosing one of the software renderers "softpipe", "llvmpipe" or "swr".
	<li>GALLIUM_LOG_FILE - specifies a file for logging all errors, warnings, etc.
	rather than stderr.
	<li>GALLIUM_PRINT_OPTIONS - if non-zero, print all the Gallium environment
	variables which are used, and their current values.
	<li>GALLIUM_DUMP_CPU - if non-zero, print information about the CPU on start-up
	<li>TGSI_PRINT_SANITY - if set, do extra sanity checking on TGSI shaders and
	print any errors to stderr.
	<LI>DRAW_FSE - ???
	<LI>DRAW_NO_FSE - ???
	<li>DRAW_USE_LLVM - if set to zero, the draw module will not use LLVM to execute
	shaders, vertex fetch, etc.
	<li>ST_DEBUG - controls debug output from the Mesa/Gallium state tracker.
	Setting to "tgsi", for example, will print all the TGSI shaders.
	See src/mesa/state_tracker/st_debug.c for other options.
	</ul>

	<h3>Softpipe driver environment variables</h3>
	<ul>
	<li>SOFTPIPE_DUMP_FS - if set, the softpipe driver will print fragment shaders
	to stderr
	<li>SOFTPIPE_DUMP_GS - if set, the softpipe driver will print geometry shaders
	to stderr
	<li>SOFTPIPE_NO_RAST - if set, rasterization is no-op'd. For profiling purposes.
	<li>SOFTPIPE_USE_LLVM - if set, the softpipe driver will try to use LLVM JIT for
	vertex shading processing.
	</ul>


	<h3>LLVMpipe driver environment variables</h3>
	<ul>
	<li>LP_NO_RAST - if set LLVMpipe will no-op rasterization
	<li>LP_DEBUG - a comma-separated list of debug options is accepted. See the
	source code for details.
	<li>LP_PERF - a comma-separated list of options to selectively no-op various
	parts of the driver. See the source code for details.
	<li>LP_NUM_THREADS - an integer indicating how many threads to use for rendering.
	Zero turns off threading completely. The default value is the number of CPU
	cores present.
	</ul>

	<h3>VMware SVGA driver environment variables</h3>
	<ul>
	<li>SVGA_FORCE_SWTNL - force use of software vertex transformation
	<li>SVGA_NO_SWTNL - don't allow software vertex transformation fallbacks
	(will often result in incorrect rendering).
	<li>SVGA_DEBUG - for dumping shaders, constant buffers, etc. See the code
	for details.
	<li>See the driver code for other, lesser-used variables.
	</ul>


	<h3>VA-API state tracker environment variables</h3>
	<ul>
	<li>VAAPI_MPEG4_ENABLED - enable MPEG4 for VA-API, disabled by default.
	</ul>


	<h3>VC4 driver environment variables</h3>
	<ul>
	<li>VC4_DEBUG - a comma-separated list of named flags, which do various things:
	<ul>
	<li>cl - dump command list during creation</li>
	<li>qpu - dump generated QPU instructions</li>
	<li>qir - dump QPU IR during program compile</li>
	<li>nir - dump NIR during program compile</li>
	<li>tgsi - dump TGSI during program compile</li>
	<li>shaderdb - dump program compile information for shader-db analysis</li>
	<li>perf - print during performance-related events</li>
	<li>norast - skip actual hardware execution of commands</li>
	<li>always_flush - flush after each draw call</li>
	<li>always_sync - wait for finish after each flush</li>
	<li>dump - write a GPU command stream trace file (VC4 simulator only)</li>
	</ul>
	</ul>


	<p>
	Other Gallium drivers have their own environment variables. These may change
	frequently so the source code should be consulted for details.
	</p>

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