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Bernie Thompsondf9be302010-09-05 16:34:58 -07001
2What is udlfb?
3===============
4
5This is a driver for DisplayLink USB 2.0 era graphics chips.
6
7DisplayLink chips provide simple hline/blit operations with some compression,
8pairing that with a hardware framebuffer (16MB) on the other end of the
9USB wire. That hardware framebuffer is able to drive the VGA, DVI, or HDMI
10monitor with no CPU involvement until a pixel has to change.
11
Eric Engestromad6a2d02016-04-25 01:24:21 +010012The CPU or other local resource does all the rendering; optionally compares the
Bernie Thompsondf9be302010-09-05 16:34:58 -070013result with a local shadow of the remote hardware framebuffer to identify
14the minimal set of pixels that have changed; and compresses and sends those
15pixels line-by-line via USB bulk transfers.
16
17Because of the efficiency of bulk transfers and a protocol on top that
18does not require any acks - the effect is very low latency that
19can support surprisingly high resolutions with good performance for
20non-gaming and non-video applications.
21
22Mode setting, EDID read, etc are other bulk or control transfers. Mode
23setting is very flexible - able to set nearly arbitrary modes from any timing.
24
25Advantages of USB graphics in general:
26
27 * Ability to add a nearly arbitrary number of displays to any USB 2.0
28 capable system. On Linux, number of displays is limited by fbdev interface
29 (FB_MAX is currently 32). Of course, all USB devices on the same
30 host controller share the same 480Mbs USB 2.0 interface.
31
32Advantages of supporting DisplayLink chips with kernel framebuffer interface:
33
34 * The actual hardware functionality of DisplayLink chips matches nearly
35 one-to-one with the fbdev interface, making the driver quite small and
36 tight relative to the functionality it provides.
37 * X servers and other applications can use the standard fbdev interface
38 from user mode to talk to the device, without needing to know anything
39 about USB or DisplayLink's protocol at all. A "displaylink" X driver
40 and a slightly modified "fbdev" X driver are among those that already do.
41
42Disadvantages:
43
44 * Fbdev's mmap interface assumes a real hardware framebuffer is mapped.
45 In the case of USB graphics, it is just an allocated (virtual) buffer.
46 Writes need to be detected and encoded into USB bulk transfers by the CPU.
47 Accurate damage/changed area notifications work around this problem.
48 In the future, hopefully fbdev will be enhanced with an small standard
49 interface to allow mmap clients to report damage, for the benefit
50 of virtual or remote framebuffers.
51 * Fbdev does not arbitrate client ownership of the framebuffer well.
52 * Fbcon assumes the first framebuffer it finds should be consumed for console.
53 * It's not clear what the future of fbdev is, given the rise of KMS/DRM.
54
55How to use it?
56==============
57
58Udlfb, when loaded as a module, will match against all USB 2.0 generation
59DisplayLink chips (Alex and Ollie family). It will then attempt to read the EDID
60of the monitor, and set the best common mode between the DisplayLink device
61and the monitor's capabilities.
62
63If the DisplayLink device is successful, it will paint a "green screen" which
64means that from a hardware and fbdev software perspective, everything is good.
65
66At that point, a /dev/fb? interface will be present for user-mode applications
67to open and begin writing to the framebuffer of the DisplayLink device using
68standard fbdev calls. Note that if mmap() is used, by default the user mode
Eric Engestromad6a2d02016-04-25 01:24:21 +010069application must send down damage notifications to trigger repaints of the
Bernie Thompsondf9be302010-09-05 16:34:58 -070070changed regions. Alternatively, udlfb can be recompiled with experimental
71defio support enabled, to support a page-fault based detection mechanism
Eric Engestromad6a2d02016-04-25 01:24:21 +010072that can work without explicit notification.
Bernie Thompsondf9be302010-09-05 16:34:58 -070073
74The most common client of udlfb is xf86-video-displaylink or a modified
75xf86-video-fbdev X server. These servers have no real DisplayLink specific
76code. They write to the standard framebuffer interface and rely on udlfb
77to do its thing. The one extra feature they have is the ability to report
78rectangles from the X DAMAGE protocol extension down to udlfb via udlfb's
79damage interface (which will hopefully be standardized for all virtual
80framebuffers that need damage info). These damage notifications allow
81udlfb to efficiently process the changed pixels.
82
83Module Options
84==============
85
86Special configuration for udlfb is usually unnecessary. There are a few
87options, however.
88
89From the command line, pass options to modprobe
Bernie Thompson9f811b72011-08-21 13:35:38 -070090modprobe udlfb fb_defio=0 console=1 shadow=1
Bernie Thompsondf9be302010-09-05 16:34:58 -070091
Bernie Thompson9f811b72011-08-21 13:35:38 -070092Or modify options on the fly at /sys/module/udlfb/parameters directory via
93sudo nano fb_defio
94change the parameter in place, and save the file.
Bernie Thompsondf9be302010-09-05 16:34:58 -070095
Bernie Thompson9f811b72011-08-21 13:35:38 -070096Unplug/replug USB device to apply with new settings
97
98Or for permanent option, create file like /etc/modprobe.d/udlfb.conf with text
99options udlfb fb_defio=0 console=1 shadow=1
100
101Accepted boolean options:
Bernie Thompsondf9be302010-09-05 16:34:58 -0700102
103fb_defio Make use of the fb_defio (CONFIG_FB_DEFERRED_IO) kernel
104 module to track changed areas of the framebuffer by page faults.
Bernie Thompson9f811b72011-08-21 13:35:38 -0700105 Standard fbdev applications that use mmap but that do not
106 report damage, should be able to work with this enabled.
107 Disable when running with X server that supports reporting
108 changed regions via ioctl, as this method is simpler,
109 more stable, and higher performance.
Bernie Thompson4aa7faf2011-08-21 13:35:39 -0700110 default: fb_defio=1
Bernie Thompsondf9be302010-09-05 16:34:58 -0700111
Bernie Thompson4aa7faf2011-08-21 13:35:39 -0700112console Allow fbcon to attach to udlfb provided framebuffers.
113 Can be disabled if fbcon and other clients
114 (e.g. X with --shared-vt) are in conflict.
115 default: console=1
Bernie Thompsondf9be302010-09-05 16:34:58 -0700116
Stuart Hopkinsd3189542011-08-21 13:34:17 -0700117shadow Allocate a 2nd framebuffer to shadow what's currently across
118 the USB bus in device memory. If any pixels are unchanged,
119 do not transmit. Spends host memory to save USB transfers.
120 Enabled by default. Only disable on very low memory systems.
Bernie Thompson4aa7faf2011-08-21 13:35:39 -0700121 default: shadow=1
Stuart Hopkinsd3189542011-08-21 13:34:17 -0700122
Bernie Thompsondf9be302010-09-05 16:34:58 -0700123Sysfs Attributes
124================
125
126Udlfb creates several files in /sys/class/graphics/fb?
127Where ? is the sequential framebuffer id of the particular DisplayLink device
128
129edid If a valid EDID blob is written to this file (typically
130 by a udev rule), then udlfb will use this EDID as a
131 backup in case reading the actual EDID of the monitor
132 attached to the DisplayLink device fails. This is
133 especially useful for fixed panels, etc. that cannot
134 communicate their capabilities via EDID. Reading
135 this file returns the current EDID of the attached
136 monitor (or last backup value written). This is
137 useful to get the EDID of the attached monitor,
138 which can be passed to utilities like parse-edid.
139
140metrics_bytes_rendered 32-bit count of pixel bytes rendered
141
142metrics_bytes_identical 32-bit count of how many of those bytes were found to be
143 unchanged, based on a shadow framebuffer check
144
145metrics_bytes_sent 32-bit count of how many bytes were transferred over
146 USB to communicate the resulting changed pixels to the
147 hardware. Includes compression and protocol overhead
148
149metrics_cpu_kcycles_used 32-bit count of CPU cycles used in processing the
150 above pixels (in thousands of cycles).
151
152metrics_reset Write-only. Any write to this file resets all metrics
153 above to zero. Note that the 32-bit counters above
154 roll over very quickly. To get reliable results, design
155 performance tests to start and finish in a very short
156 period of time (one minute or less is safe).
157
158--
159Bernie Thompson <bernie@plugable.com>