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Ian Munsiea9282d02014-10-08 19:55:05 +11001Coherent Accelerator Interface (CXL)
2====================================
3
4Introduction
5============
6
7 The coherent accelerator interface is designed to allow the
8 coherent connection of accelerators (FPGAs and other devices) to a
9 POWER system. These devices need to adhere to the Coherent
10 Accelerator Interface Architecture (CAIA).
11
12 IBM refers to this as the Coherent Accelerator Processor Interface
13 or CAPI. In the kernel it's referred to by the name CXL to avoid
14 confusion with the ISDN CAPI subsystem.
15
16 Coherent in this context means that the accelerator and CPUs can
17 both access system memory directly and with the same effective
18 addresses.
19
20
21Hardware overview
22=================
23
24 POWER8 FPGA
25 +----------+ +---------+
26 | | | |
27 | CPU | | AFU |
28 | | | |
29 | | | |
30 | | | |
31 +----------+ +---------+
32 | PHB | | |
33 | +------+ | PSL |
34 | | CAPP |<------>| |
35 +---+------+ PCIE +---------+
36
37 The POWER8 chip has a Coherently Attached Processor Proxy (CAPP)
38 unit which is part of the PCIe Host Bridge (PHB). This is managed
39 by Linux by calls into OPAL. Linux doesn't directly program the
40 CAPP.
41
42 The FPGA (or coherently attached device) consists of two parts.
43 The POWER Service Layer (PSL) and the Accelerator Function Unit
44 (AFU). The AFU is used to implement specific functionality behind
45 the PSL. The PSL, among other things, provides memory address
46 translation services to allow each AFU direct access to userspace
47 memory.
48
49 The AFU is the core part of the accelerator (eg. the compression,
50 crypto etc function). The kernel has no knowledge of the function
51 of the AFU. Only userspace interacts directly with the AFU.
52
53 The PSL provides the translation and interrupt services that the
54 AFU needs. This is what the kernel interacts with. For example, if
55 the AFU needs to read a particular effective address, it sends
56 that address to the PSL, the PSL then translates it, fetches the
57 data from memory and returns it to the AFU. If the PSL has a
58 translation miss, it interrupts the kernel and the kernel services
59 the fault. The context to which this fault is serviced is based on
60 who owns that acceleration function.
61
62
63AFU Modes
64=========
65
66 There are two programming modes supported by the AFU. Dedicated
67 and AFU directed. AFU may support one or both modes.
68
69 When using dedicated mode only one MMU context is supported. In
70 this mode, only one userspace process can use the accelerator at
71 time.
72
73 When using AFU directed mode, up to 16K simultaneous contexts can
74 be supported. This means up to 16K simultaneous userspace
75 applications may use the accelerator (although specific AFUs may
76 support fewer). In this mode, the AFU sends a 16 bit context ID
77 with each of its requests. This tells the PSL which context is
78 associated with each operation. If the PSL can't translate an
79 operation, the ID can also be accessed by the kernel so it can
80 determine the userspace context associated with an operation.
81
82
83MMIO space
84==========
85
86 A portion of the accelerator MMIO space can be directly mapped
87 from the AFU to userspace. Either the whole space can be mapped or
88 just a per context portion. The hardware is self describing, hence
89 the kernel can determine the offset and size of the per context
90 portion.
91
92
93Interrupts
94==========
95
96 AFUs may generate interrupts that are destined for userspace. These
97 are received by the kernel as hardware interrupts and passed onto
98 userspace by a read syscall documented below.
99
100 Data storage faults and error interrupts are handled by the kernel
101 driver.
102
103
104Work Element Descriptor (WED)
105=============================
106
107 The WED is a 64-bit parameter passed to the AFU when a context is
108 started. Its format is up to the AFU hence the kernel has no
109 knowledge of what it represents. Typically it will be the
110 effective address of a work queue or status block where the AFU
111 and userspace can share control and status information.
112
113
114
115
116User API
117========
118
119 For AFUs operating in AFU directed mode, two character device
120 files will be created. /dev/cxl/afu0.0m will correspond to a
121 master context and /dev/cxl/afu0.0s will correspond to a slave
122 context. Master contexts have access to the full MMIO space an
123 AFU provides. Slave contexts have access to only the per process
124 MMIO space an AFU provides.
125
126 For AFUs operating in dedicated process mode, the driver will
127 only create a single character device per AFU called
128 /dev/cxl/afu0.0d. This will have access to the entire MMIO space
129 that the AFU provides (like master contexts in AFU directed).
130
131 The types described below are defined in include/uapi/misc/cxl.h
132
133 The following file operations are supported on both slave and
134 master devices.
135
Masanari Iidadc12f202015-07-06 23:41:57 +0900136 A userspace library libcxl is available here:
Michael Neulingaee85fb2015-05-27 16:07:01 +1000137 https://github.com/ibm-capi/libcxl
138 This provides a C interface to this kernel API.
Ian Munsiea9282d02014-10-08 19:55:05 +1100139
140open
141----
142
143 Opens the device and allocates a file descriptor to be used with
144 the rest of the API.
145
146 A dedicated mode AFU only has one context and only allows the
147 device to be opened once.
148
149 An AFU directed mode AFU can have many contexts, the device can be
150 opened once for each context that is available.
151
152 When all available contexts are allocated the open call will fail
153 and return -ENOSPC.
154
155 Note: IRQs need to be allocated for each context, which may limit
156 the number of contexts that can be created, and therefore
157 how many times the device can be opened. The POWER8 CAPP
158 supports 2040 IRQs and 3 are used by the kernel, so 2037 are
159 left. If 1 IRQ is needed per context, then only 2037
160 contexts can be allocated. If 4 IRQs are needed per context,
161 then only 2037/4 = 509 contexts can be allocated.
162
163
164ioctl
165-----
166
167 CXL_IOCTL_START_WORK:
168 Starts the AFU context and associates it with the current
169 process. Once this ioctl is successfully executed, all memory
170 mapped into this process is accessible to this AFU context
171 using the same effective addresses. No additional calls are
172 required to map/unmap memory. The AFU memory context will be
173 updated as userspace allocates and frees memory. This ioctl
174 returns once the AFU context is started.
175
176 Takes a pointer to a struct cxl_ioctl_start_work:
177
178 struct cxl_ioctl_start_work {
179 __u64 flags;
180 __u64 work_element_descriptor;
181 __u64 amr;
182 __s16 num_interrupts;
183 __s16 reserved1;
184 __s32 reserved2;
185 __u64 reserved3;
186 __u64 reserved4;
187 __u64 reserved5;
188 __u64 reserved6;
189 };
190
191 flags:
192 Indicates which optional fields in the structure are
193 valid.
194
195 work_element_descriptor:
196 The Work Element Descriptor (WED) is a 64-bit argument
197 defined by the AFU. Typically this is an effective
198 address pointing to an AFU specific structure
199 describing what work to perform.
200
201 amr:
202 Authority Mask Register (AMR), same as the powerpc
203 AMR. This field is only used by the kernel when the
204 corresponding CXL_START_WORK_AMR value is specified in
205 flags. If not specified the kernel will use a default
206 value of 0.
207
208 num_interrupts:
209 Number of userspace interrupts to request. This field
210 is only used by the kernel when the corresponding
211 CXL_START_WORK_NUM_IRQS value is specified in flags.
212 If not specified the minimum number required by the
213 AFU will be allocated. The min and max number can be
214 obtained from sysfs.
215
216 reserved fields:
217 For ABI padding and future extensions
218
219 CXL_IOCTL_GET_PROCESS_ELEMENT:
220 Get the current context id, also known as the process element.
221 The value is returned from the kernel as a __u32.
222
223
224mmap
225----
226
227 An AFU may have an MMIO space to facilitate communication with the
228 AFU. If it does, the MMIO space can be accessed via mmap. The size
229 and contents of this area are specific to the particular AFU. The
230 size can be discovered via sysfs.
231
232 In AFU directed mode, master contexts are allowed to map all of
233 the MMIO space and slave contexts are allowed to only map the per
234 process MMIO space associated with the context. In dedicated
235 process mode the entire MMIO space can always be mapped.
236
237 This mmap call must be done after the START_WORK ioctl.
238
239 Care should be taken when accessing MMIO space. Only 32 and 64-bit
240 accesses are supported by POWER8. Also, the AFU will be designed
241 with a specific endianness, so all MMIO accesses should consider
242 endianness (recommend endian(3) variants like: le64toh(),
243 be64toh() etc). These endian issues equally apply to shared memory
244 queues the WED may describe.
245
246
247read
248----
249
250 Reads events from the AFU. Blocks if no events are pending
251 (unless O_NONBLOCK is supplied). Returns -EIO in the case of an
252 unrecoverable error or if the card is removed.
253
254 read() will always return an integral number of events.
255
256 The buffer passed to read() must be at least 4K bytes.
257
258 The result of the read will be a buffer of one or more events,
259 each event is of type struct cxl_event, of varying size.
260
261 struct cxl_event {
262 struct cxl_event_header header;
263 union {
264 struct cxl_event_afu_interrupt irq;
265 struct cxl_event_data_storage fault;
266 struct cxl_event_afu_error afu_error;
267 };
268 };
269
270 The struct cxl_event_header is defined as:
271
272 struct cxl_event_header {
273 __u16 type;
274 __u16 size;
275 __u16 process_element;
276 __u16 reserved1;
277 };
278
279 type:
280 This defines the type of event. The type determines how
281 the rest of the event is structured. These types are
282 described below and defined by enum cxl_event_type.
283
284 size:
285 This is the size of the event in bytes including the
286 struct cxl_event_header. The start of the next event can
287 be found at this offset from the start of the current
288 event.
289
290 process_element:
291 Context ID of the event.
292
293 reserved field:
294 For future extensions and padding.
295
296 If the event type is CXL_EVENT_AFU_INTERRUPT then the event
297 structure is defined as:
298
299 struct cxl_event_afu_interrupt {
300 __u16 flags;
301 __u16 irq; /* Raised AFU interrupt number */
302 __u32 reserved1;
303 };
304
305 flags:
306 These flags indicate which optional fields are present
307 in this struct. Currently all fields are mandatory.
308
309 irq:
310 The IRQ number sent by the AFU.
311
312 reserved field:
313 For future extensions and padding.
314
315 If the event type is CXL_EVENT_DATA_STORAGE then the event
316 structure is defined as:
317
318 struct cxl_event_data_storage {
319 __u16 flags;
320 __u16 reserved1;
321 __u32 reserved2;
322 __u64 addr;
323 __u64 dsisr;
324 __u64 reserved3;
325 };
326
327 flags:
328 These flags indicate which optional fields are present in
329 this struct. Currently all fields are mandatory.
330
331 address:
332 The address that the AFU unsuccessfully attempted to
333 access. Valid accesses will be handled transparently by the
334 kernel but invalid accesses will generate this event.
335
336 dsisr:
337 This field gives information on the type of fault. It is a
338 copy of the DSISR from the PSL hardware when the address
339 fault occurred. The form of the DSISR is as defined in the
340 CAIA.
341
342 reserved fields:
343 For future extensions
344
345 If the event type is CXL_EVENT_AFU_ERROR then the event structure
346 is defined as:
347
348 struct cxl_event_afu_error {
349 __u16 flags;
350 __u16 reserved1;
351 __u32 reserved2;
352 __u64 error;
353 };
354
355 flags:
356 These flags indicate which optional fields are present in
357 this struct. Currently all fields are Mandatory.
358
359 error:
360 Error status from the AFU. Defined by the AFU.
361
362 reserved fields:
363 For future extensions and padding
364
365Sysfs Class
366===========
367
368 A cxl sysfs class is added under /sys/class/cxl to facilitate
369 enumeration and tuning of the accelerators. Its layout is
370 described in Documentation/ABI/testing/sysfs-class-cxl
371
Michael Neulingaee85fb2015-05-27 16:07:01 +1000372
Ian Munsiea9282d02014-10-08 19:55:05 +1100373Udev rules
374==========
375
376 The following udev rules could be used to create a symlink to the
377 most logical chardev to use in any programming mode (afuX.Yd for
378 dedicated, afuX.Ys for afu directed), since the API is virtually
379 identical for each:
380
381 SUBSYSTEM=="cxl", ATTRS{mode}=="dedicated_process", SYMLINK="cxl/%b"
382 SUBSYSTEM=="cxl", ATTRS{mode}=="afu_directed", \
383 KERNEL=="afu[0-9]*.[0-9]*s", SYMLINK="cxl/%b"