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1 | Coherent Accelerator Interface (CXL) |
2 | ==================================== | |
3 | ||
4 | Introduction | |
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 | ||
21 | Hardware 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 | ||
63 | AFU 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 | ||
83 | MMIO 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 | ||
93 | Interrupts | |
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 | ||
104 | Work 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 | ||
116 | User 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 | ||
136 | ||
137 | open | |
138 | ---- | |
139 | ||
140 | Opens the device and allocates a file descriptor to be used with | |
141 | the rest of the API. | |
142 | ||
143 | A dedicated mode AFU only has one context and only allows the | |
144 | device to be opened once. | |
145 | ||
146 | An AFU directed mode AFU can have many contexts, the device can be | |
147 | opened once for each context that is available. | |
148 | ||
149 | When all available contexts are allocated the open call will fail | |
150 | and return -ENOSPC. | |
151 | ||
152 | Note: IRQs need to be allocated for each context, which may limit | |
153 | the number of contexts that can be created, and therefore | |
154 | how many times the device can be opened. The POWER8 CAPP | |
155 | supports 2040 IRQs and 3 are used by the kernel, so 2037 are | |
156 | left. If 1 IRQ is needed per context, then only 2037 | |
157 | contexts can be allocated. If 4 IRQs are needed per context, | |
158 | then only 2037/4 = 509 contexts can be allocated. | |
159 | ||
160 | ||
161 | ioctl | |
162 | ----- | |
163 | ||
164 | CXL_IOCTL_START_WORK: | |
165 | Starts the AFU context and associates it with the current | |
166 | process. Once this ioctl is successfully executed, all memory | |
167 | mapped into this process is accessible to this AFU context | |
168 | using the same effective addresses. No additional calls are | |
169 | required to map/unmap memory. The AFU memory context will be | |
170 | updated as userspace allocates and frees memory. This ioctl | |
171 | returns once the AFU context is started. | |
172 | ||
173 | Takes a pointer to a struct cxl_ioctl_start_work: | |
174 | ||
175 | struct cxl_ioctl_start_work { | |
176 | __u64 flags; | |
177 | __u64 work_element_descriptor; | |
178 | __u64 amr; | |
179 | __s16 num_interrupts; | |
180 | __s16 reserved1; | |
181 | __s32 reserved2; | |
182 | __u64 reserved3; | |
183 | __u64 reserved4; | |
184 | __u64 reserved5; | |
185 | __u64 reserved6; | |
186 | }; | |
187 | ||
188 | flags: | |
189 | Indicates which optional fields in the structure are | |
190 | valid. | |
191 | ||
192 | work_element_descriptor: | |
193 | The Work Element Descriptor (WED) is a 64-bit argument | |
194 | defined by the AFU. Typically this is an effective | |
195 | address pointing to an AFU specific structure | |
196 | describing what work to perform. | |
197 | ||
198 | amr: | |
199 | Authority Mask Register (AMR), same as the powerpc | |
200 | AMR. This field is only used by the kernel when the | |
201 | corresponding CXL_START_WORK_AMR value is specified in | |
202 | flags. If not specified the kernel will use a default | |
203 | value of 0. | |
204 | ||
205 | num_interrupts: | |
206 | Number of userspace interrupts to request. This field | |
207 | is only used by the kernel when the corresponding | |
208 | CXL_START_WORK_NUM_IRQS value is specified in flags. | |
209 | If not specified the minimum number required by the | |
210 | AFU will be allocated. The min and max number can be | |
211 | obtained from sysfs. | |
212 | ||
213 | reserved fields: | |
214 | For ABI padding and future extensions | |
215 | ||
216 | CXL_IOCTL_GET_PROCESS_ELEMENT: | |
217 | Get the current context id, also known as the process element. | |
218 | The value is returned from the kernel as a __u32. | |
219 | ||
220 | ||
221 | mmap | |
222 | ---- | |
223 | ||
224 | An AFU may have an MMIO space to facilitate communication with the | |
225 | AFU. If it does, the MMIO space can be accessed via mmap. The size | |
226 | and contents of this area are specific to the particular AFU. The | |
227 | size can be discovered via sysfs. | |
228 | ||
229 | In AFU directed mode, master contexts are allowed to map all of | |
230 | the MMIO space and slave contexts are allowed to only map the per | |
231 | process MMIO space associated with the context. In dedicated | |
232 | process mode the entire MMIO space can always be mapped. | |
233 | ||
234 | This mmap call must be done after the START_WORK ioctl. | |
235 | ||
236 | Care should be taken when accessing MMIO space. Only 32 and 64-bit | |
237 | accesses are supported by POWER8. Also, the AFU will be designed | |
238 | with a specific endianness, so all MMIO accesses should consider | |
239 | endianness (recommend endian(3) variants like: le64toh(), | |
240 | be64toh() etc). These endian issues equally apply to shared memory | |
241 | queues the WED may describe. | |
242 | ||
243 | ||
244 | read | |
245 | ---- | |
246 | ||
247 | Reads events from the AFU. Blocks if no events are pending | |
248 | (unless O_NONBLOCK is supplied). Returns -EIO in the case of an | |
249 | unrecoverable error or if the card is removed. | |
250 | ||
251 | read() will always return an integral number of events. | |
252 | ||
253 | The buffer passed to read() must be at least 4K bytes. | |
254 | ||
255 | The result of the read will be a buffer of one or more events, | |
256 | each event is of type struct cxl_event, of varying size. | |
257 | ||
258 | struct cxl_event { | |
259 | struct cxl_event_header header; | |
260 | union { | |
261 | struct cxl_event_afu_interrupt irq; | |
262 | struct cxl_event_data_storage fault; | |
263 | struct cxl_event_afu_error afu_error; | |
264 | }; | |
265 | }; | |
266 | ||
267 | The struct cxl_event_header is defined as: | |
268 | ||
269 | struct cxl_event_header { | |
270 | __u16 type; | |
271 | __u16 size; | |
272 | __u16 process_element; | |
273 | __u16 reserved1; | |
274 | }; | |
275 | ||
276 | type: | |
277 | This defines the type of event. The type determines how | |
278 | the rest of the event is structured. These types are | |
279 | described below and defined by enum cxl_event_type. | |
280 | ||
281 | size: | |
282 | This is the size of the event in bytes including the | |
283 | struct cxl_event_header. The start of the next event can | |
284 | be found at this offset from the start of the current | |
285 | event. | |
286 | ||
287 | process_element: | |
288 | Context ID of the event. | |
289 | ||
290 | reserved field: | |
291 | For future extensions and padding. | |
292 | ||
293 | If the event type is CXL_EVENT_AFU_INTERRUPT then the event | |
294 | structure is defined as: | |
295 | ||
296 | struct cxl_event_afu_interrupt { | |
297 | __u16 flags; | |
298 | __u16 irq; /* Raised AFU interrupt number */ | |
299 | __u32 reserved1; | |
300 | }; | |
301 | ||
302 | flags: | |
303 | These flags indicate which optional fields are present | |
304 | in this struct. Currently all fields are mandatory. | |
305 | ||
306 | irq: | |
307 | The IRQ number sent by the AFU. | |
308 | ||
309 | reserved field: | |
310 | For future extensions and padding. | |
311 | ||
312 | If the event type is CXL_EVENT_DATA_STORAGE then the event | |
313 | structure is defined as: | |
314 | ||
315 | struct cxl_event_data_storage { | |
316 | __u16 flags; | |
317 | __u16 reserved1; | |
318 | __u32 reserved2; | |
319 | __u64 addr; | |
320 | __u64 dsisr; | |
321 | __u64 reserved3; | |
322 | }; | |
323 | ||
324 | flags: | |
325 | These flags indicate which optional fields are present in | |
326 | this struct. Currently all fields are mandatory. | |
327 | ||
328 | address: | |
329 | The address that the AFU unsuccessfully attempted to | |
330 | access. Valid accesses will be handled transparently by the | |
331 | kernel but invalid accesses will generate this event. | |
332 | ||
333 | dsisr: | |
334 | This field gives information on the type of fault. It is a | |
335 | copy of the DSISR from the PSL hardware when the address | |
336 | fault occurred. The form of the DSISR is as defined in the | |
337 | CAIA. | |
338 | ||
339 | reserved fields: | |
340 | For future extensions | |
341 | ||
342 | If the event type is CXL_EVENT_AFU_ERROR then the event structure | |
343 | is defined as: | |
344 | ||
345 | struct cxl_event_afu_error { | |
346 | __u16 flags; | |
347 | __u16 reserved1; | |
348 | __u32 reserved2; | |
349 | __u64 error; | |
350 | }; | |
351 | ||
352 | flags: | |
353 | These flags indicate which optional fields are present in | |
354 | this struct. Currently all fields are Mandatory. | |
355 | ||
356 | error: | |
357 | Error status from the AFU. Defined by the AFU. | |
358 | ||
359 | reserved fields: | |
360 | For future extensions and padding | |
361 | ||
362 | Sysfs Class | |
363 | =========== | |
364 | ||
365 | A cxl sysfs class is added under /sys/class/cxl to facilitate | |
366 | enumeration and tuning of the accelerators. Its layout is | |
367 | described in Documentation/ABI/testing/sysfs-class-cxl | |
368 | ||
369 | Udev rules | |
370 | ========== | |
371 | ||
372 | The following udev rules could be used to create a symlink to the | |
373 | most logical chardev to use in any programming mode (afuX.Yd for | |
374 | dedicated, afuX.Ys for afu directed), since the API is virtually | |
375 | identical for each: | |
376 | ||
377 | SUBSYSTEM=="cxl", ATTRS{mode}=="dedicated_process", SYMLINK="cxl/%b" | |
378 | SUBSYSTEM=="cxl", ATTRS{mode}=="afu_directed", \ | |
379 | KERNEL=="afu[0-9]*.[0-9]*s", SYMLINK="cxl/%b" |