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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	==========================================
2	Xillybus driver for generic FPGA interface
3	==========================================
4	
5	:Author: Eli Billauer, Xillybus Ltd. (http://xillybus.com)
6	:Email:  eli.billauer@gmail.com or as advertised on Xillybus' site.
7	
8	.. Contents:
9	
10	 - Introduction
11	  -- Background
12	  -- Xillybus Overview
13	
14	 - Usage
15	  -- User interface
16	  -- Synchronization
17	  -- Seekable pipes
18	
19	 - Internals
20	  -- Source code organization
21	  -- Pipe attributes
22	  -- Host never reads from the FPGA
23	  -- Channels, pipes, and the message channel
24	  -- Data streaming
25	  -- Data granularity
26	  -- Probing
27	  -- Buffer allocation
28	  -- The "nonempty" message (supporting poll)
29	
30	
31	Introduction
32	============
33	
34	Background
35	----------
36	
37	An FPGA (Field Programmable Gate Array) is a piece of logic hardware, which
38	can be programmed to become virtually anything that is usually found as a
39	dedicated chipset: For instance, a display adapter, network interface card,
40	or even a processor with its peripherals. FPGAs are the LEGO of hardware:
41	Based upon certain building blocks, you make your own toys the way you like
42	them. It's usually pointless to reimplement something that is already
43	available on the market as a chipset, so FPGAs are mostly used when some
44	special functionality is needed, and the production volume is relatively low
45	(hence not justifying the development of an ASIC).
46	
47	The challenge with FPGAs is that everything is implemented at a very low
48	level, even lower than assembly language. In order to allow FPGA designers to
49	focus on their specific project, and not reinvent the wheel over and over
50	again, pre-designed building blocks, IP cores, are often used. These are the
51	FPGA parallels of library functions. IP cores may implement certain
52	mathematical functions, a functional unit (e.g. a USB interface), an entire
53	processor (e.g. ARM) or anything that might come handy. Think of them as a
54	building block, with electrical wires dangling on the sides for connection to
55	other blocks.
56	
57	One of the daunting tasks in FPGA design is communicating with a fullblown
58	operating system (actually, with the processor running it): Implementing the
59	low-level bus protocol and the somewhat higher-level interface with the host
60	(registers, interrupts, DMA etc.) is a project in itself. When the FPGA's
61	function is a well-known one (e.g. a video adapter card, or a NIC), it can
62	make sense to design the FPGA's interface logic specifically for the project.
63	A special driver is then written to present the FPGA as a well-known interface
64	to the kernel and/or user space. In that case, there is no reason to treat the
65	FPGA differently than any device on the bus.
66	
67	It's however common that the desired data communication doesn't fit any well-
68	known peripheral function. Also, the effort of designing an elegant
69	abstraction for the data exchange is often considered too big. In those cases,
70	a quicker and possibly less elegant solution is sought: The driver is
71	effectively written as a user space program, leaving the kernel space part
72	with just elementary data transport. This still requires designing some
73	interface logic for the FPGA, and write a simple ad-hoc driver for the kernel.
74	
75	Xillybus Overview
76	-----------------
77	
78	Xillybus is an IP core and a Linux driver. Together, they form a kit for
79	elementary data transport between an FPGA and the host, providing pipe-like
80	data streams with a straightforward user interface. It's intended as a low-
81	effort solution for mixed FPGA-host projects, for which it makes sense to
82	have the project-specific part of the driver running in a user-space program.
83	
84	Since the communication requirements may vary significantly from one FPGA
85	project to another (the number of data pipes needed in each direction and
86	their attributes), there isn't one specific chunk of logic being the Xillybus
87	IP core. Rather, the IP core is configured and built based upon a
88	specification given by its end user.
89	
90	Xillybus presents independent data streams, which resemble pipes or TCP/IP
91	communication to the user. At the host side, a character device file is used
92	just like any pipe file. On the FPGA side, hardware FIFOs are used to stream
93	the data. This is contrary to a common method of communicating through fixed-
94	sized buffers (even though such buffers are used by Xillybus under the hood).
95	There may be more than a hundred of these streams on a single IP core, but
96	also no more than one, depending on the configuration.
97	
98	In order to ease the deployment of the Xillybus IP core, it contains a simple
99	data structure which completely defines the core's configuration. The Linux
100	driver fetches this data structure during its initialization process, and sets
101	up the DMA buffers and character devices accordingly. As a result, a single
102	driver is used to work out of the box with any Xillybus IP core.
103	
104	The data structure just mentioned should not be confused with PCI's
105	configuration space or the Flattened Device Tree.
106	
107	Usage
108	=====
109	
110	User interface
111	--------------
112	
113	On the host, all interface with Xillybus is done through /dev/xillybus_*
114	device files, which are generated automatically as the drivers loads. The
115	names of these files depend on the IP core that is loaded in the FPGA (see
116	Probing below). To communicate with the FPGA, open the device file that
117	corresponds to the hardware FIFO you want to send data or receive data from,
118	and use plain write() or read() calls, just like with a regular pipe. In
119	particular, it makes perfect sense to go::
120	
121		$ cat mydata > /dev/xillybus_thisfifo
122	
123		$ cat /dev/xillybus_thatfifo > hisdata
124	
125	possibly pressing CTRL-C as some stage, even though the xillybus_* pipes have
126	the capability to send an EOF (but may not use it).
127	
128	The driver and hardware are designed to behave sensibly as pipes, including:
129	
130	* Supporting non-blocking I/O (by setting O_NONBLOCK on open() ).
131	
132	* Supporting poll() and select().
133	
134	* Being bandwidth efficient under load (using DMA) but also handle small
135	  pieces of data sent across (like TCP/IP) by autoflushing.
136	
137	A device file can be read only, write only or bidirectional. Bidirectional
138	device files are treated like two independent pipes (except for sharing a
139	"channel" structure in the implementation code).
140	
141	Synchronization
142	---------------
143	
144	Xillybus pipes are configured (on the IP core) to be either synchronous or
145	asynchronous. For a synchronous pipe, write() returns successfully only after
146	some data has been submitted and acknowledged by the FPGA. This slows down
147	bulk data transfers, and is nearly impossible for use with streams that
148	require data at a constant rate: There is no data transmitted to the FPGA
149	between write() calls, in particular when the process loses the CPU.
150	
151	When a pipe is configured asynchronous, write() returns if there was enough
152	room in the buffers to store any of the data in the buffers.
153	
154	For FPGA to host pipes, asynchronous pipes allow data transfer from the FPGA
155	as soon as the respective device file is opened, regardless of if the data
156	has been requested by a read() call. On synchronous pipes, only the amount
157	of data requested by a read() call is transmitted.
158	
159	In summary, for synchronous pipes, data between the host and FPGA is
160	transmitted only to satisfy the read() or write() call currently handled
161	by the driver, and those calls wait for the transmission to complete before
162	returning.
163	
164	Note that the synchronization attribute has nothing to do with the possibility
165	that read() or write() completes less bytes than requested. There is a
166	separate configuration flag ("allowpartial") that determines whether such a
167	partial completion is allowed.
168	
169	Seekable pipes
170	--------------
171	
172	A synchronous pipe can be configured to have the stream's position exposed
173	to the user logic at the FPGA. Such a pipe is also seekable on the host API.
174	With this feature, a memory or register interface can be attached on the
175	FPGA side to the seekable stream. Reading or writing to a certain address in
176	the attached memory is done by seeking to the desired address, and calling
177	read() or write() as required.
178	
179	
180	Internals
181	=========
182	
183	Source code organization
184	------------------------
185	
186	The Xillybus driver consists of a core module, xillybus_core.c, and modules
187	that depend on the specific bus interface (xillybus_of.c and xillybus_pcie.c).
188	
189	The bus specific modules are those probed when a suitable device is found by
190	the kernel. Since the DMA mapping and synchronization functions, which are bus
191	dependent by their nature, are used by the core module, a
192	xilly_endpoint_hardware structure is passed to the core module on
193	initialization. This structure is populated with pointers to wrapper functions
194	which execute the DMA-related operations on the bus.
195	
196	Pipe attributes
197	---------------
198	
199	Each pipe has a number of attributes which are set when the FPGA component
200	(IP core) is built. They are fetched from the IDT (the data structure which
201	defines the core's configuration, see Probing below) by xilly_setupchannels()
202	in xillybus_core.c as follows:
203	
204	* is_writebuf: The pipe's direction. A non-zero value means it's an FPGA to
205	  host pipe (the FPGA "writes").
206	
207	* channelnum: The pipe's identification number in communication between the
208	  host and FPGA.
209	
210	* format: The underlying data width. See Data Granularity below.
211	
212	* allowpartial: A non-zero value means that a read() or write() (whichever
213	  applies) may return with less than the requested number of bytes. The common
214	  choice is a non-zero value, to match standard UNIX behavior.
215	
216	* synchronous: A non-zero value means that the pipe is synchronous. See
217	  Synchronization above.
218	
219	* bufsize: Each DMA buffer's size. Always a power of two.
220	
221	* bufnum: The number of buffers allocated for this pipe. Always a power of two.
222	
223	* exclusive_open: A non-zero value forces exclusive opening of the associated
224	  device file. If the device file is bidirectional, and already opened only in
225	  one direction, the opposite direction may be opened once.
226	
227	* seekable: A non-zero value indicates that the pipe is seekable. See
228	  Seekable pipes above.
229	
230	* supports_nonempty: A non-zero value (which is typical) indicates that the
231	  hardware will send the messages that are necessary to support select() and
232	  poll() for this pipe.
233	
234	Host never reads from the FPGA
235	------------------------------
236	
237	Even though PCI Express is hotpluggable in general, a typical motherboard
238	doesn't expect a card to go away all of the sudden. But since the PCIe card
239	is based upon reprogrammable logic, a sudden disappearance from the bus is
240	quite likely as a result of an accidental reprogramming of the FPGA while the
241	host is up. In practice, nothing happens immediately in such a situation. But
242	if the host attempts to read from an address that is mapped to the PCI Express
243	device, that leads to an immediate freeze of the system on some motherboards,
244	even though the PCIe standard requires a graceful recovery.
245	
246	In order to avoid these freezes, the Xillybus driver refrains completely from
247	reading from the device's register space. All communication from the FPGA to
248	the host is done through DMA. In particular, the Interrupt Service Routine
249	doesn't follow the common practice of checking a status register when it's
250	invoked. Rather, the FPGA prepares a small buffer which contains short
251	messages, which inform the host what the interrupt was about.
252	
253	This mechanism is used on non-PCIe buses as well for the sake of uniformity.
254	
255	
256	Channels, pipes, and the message channel
257	----------------------------------------
258	
259	Each of the (possibly bidirectional) pipes presented to the user is allocated
260	a data channel between the FPGA and the host. The distinction between channels
261	and pipes is necessary only because of channel 0, which is used for interrupt-
262	related messages from the FPGA, and has no pipe attached to it.
263	
264	Data streaming
265	--------------
266	
267	Even though a non-segmented data stream is presented to the user at both
268	sides, the implementation relies on a set of DMA buffers which is allocated
269	for each channel. For the sake of illustration, let's take the FPGA to host
270	direction: As data streams into the respective channel's interface in the
271	FPGA, the Xillybus IP core writes it to one of the DMA buffers. When the
272	buffer is full, the FPGA informs the host about that (appending a
273	XILLYMSG_OPCODE_RELEASEBUF message channel 0 and sending an interrupt if
274	necessary). The host responds by making the data available for reading through
275	the character device. When all data has been read, the host writes on the
276	the FPGA's buffer control register, allowing the buffer's overwriting. Flow
277	control mechanisms exist on both sides to prevent underflows and overflows.
278	
279	This is not good enough for creating a TCP/IP-like stream: If the data flow
280	stops momentarily before a DMA buffer is filled, the intuitive expectation is
281	that the partial data in buffer will arrive anyhow, despite the buffer not
282	being completed. This is implemented by adding a field in the
283	XILLYMSG_OPCODE_RELEASEBUF message, through which the FPGA informs not just
284	which buffer is submitted, but how much data it contains.
285	
286	But the FPGA will submit a partially filled buffer only if directed to do so
287	by the host. This situation occurs when the read() method has been blocking
288	for XILLY_RX_TIMEOUT jiffies (currently 10 ms), after which the host commands
289	the FPGA to submit a DMA buffer as soon as it can. This timeout mechanism
290	balances between bus bandwidth efficiency (preventing a lot of partially
291	filled buffers being sent) and a latency held fairly low for tails of data.
292	
293	A similar setting is used in the host to FPGA direction. The handling of
294	partial DMA buffers is somewhat different, though. The user can tell the
295	driver to submit all data it has in the buffers to the FPGA, by issuing a
296	write() with the byte count set to zero. This is similar to a flush request,
297	but it doesn't block. There is also an autoflushing mechanism, which triggers
298	an equivalent flush roughly XILLY_RX_TIMEOUT jiffies after the last write().
299	This allows the user to be oblivious about the underlying buffering mechanism
300	and yet enjoy a stream-like interface.
301	
302	Note that the issue of partial buffer flushing is irrelevant for pipes having
303	the "synchronous" attribute nonzero, since synchronous pipes don't allow data
304	to lay around in the DMA buffers between read() and write() anyhow.
305	
306	Data granularity
307	----------------
308	
309	The data arrives or is sent at the FPGA as 8, 16 or 32 bit wide words, as
310	configured by the "format" attribute. Whenever possible, the driver attempts
311	to hide this when the pipe is accessed differently from its natural alignment.
312	For example, reading single bytes from a pipe with 32 bit granularity works
313	with no issues. Writing single bytes to pipes with 16 or 32 bit granularity
314	will also work, but the driver can't send partially completed words to the
315	FPGA, so the transmission of up to one word may be held until it's fully
316	occupied with user data.
317	
318	This somewhat complicates the handling of host to FPGA streams, because
319	when a buffer is flushed, it may contain up to 3 bytes don't form a word in
320	the FPGA, and hence can't be sent. To prevent loss of data, these leftover
321	bytes need to be moved to the next buffer. The parts in xillybus_core.c
322	that mention "leftovers" in some way are related to this complication.
323	
324	Probing
325	-------
326	
327	As mentioned earlier, the number of pipes that are created when the driver
328	loads and their attributes depend on the Xillybus IP core in the FPGA. During
329	the driver's initialization, a blob containing configuration info, the
330	Interface Description Table (IDT), is sent from the FPGA to the host. The
331	bootstrap process is done in three phases:
332	
333	1. Acquire the length of the IDT, so a buffer can be allocated for it. This
334	   is done by sending a quiesce command to the device, since the acknowledge
335	   for this command contains the IDT's buffer length.
336	
337	2. Acquire the IDT itself.
338	
339	3. Create the interfaces according to the IDT.
340	
341	Buffer allocation
342	-----------------
343	
344	In order to simplify the logic that prevents illegal boundary crossings of
345	PCIe packets, the following rule applies: If a buffer is smaller than 4kB,
346	it must not cross a 4kB boundary. Otherwise, it must be 4kB aligned. The
347	xilly_setupchannels() functions allocates these buffers by requesting whole
348	pages from the kernel, and diving them into DMA buffers as necessary. Since
349	all buffers' sizes are powers of two, it's possible to pack any set of such
350	buffers, with a maximal waste of one page of memory.
351	
352	All buffers are allocated when the driver is loaded. This is necessary,
353	since large continuous physical memory segments are sometimes requested,
354	which are more likely to be available when the system is freshly booted.
355	
356	The allocation of buffer memory takes place in the same order they appear in
357	the IDT. The driver relies on a rule that the pipes are sorted with decreasing
358	buffer size in the IDT. If a requested buffer is larger or equal to a page,
359	the necessary number of pages is requested from the kernel, and these are
360	used for this buffer. If the requested buffer is smaller than a page, one
361	single page is requested from the kernel, and that page is partially used.
362	Or, if there already is a partially used page at hand, the buffer is packed
363	into that page. It can be shown that all pages requested from the kernel
364	(except possibly for the last) are 100% utilized this way.
365	
366	The "nonempty" message (supporting poll)
367	----------------------------------------
368	
369	In order to support the "poll" method (and hence select() ), there is a small
370	catch regarding the FPGA to host direction: The FPGA may have filled a DMA
371	buffer with some data, but not submitted that buffer. If the host waited for
372	the buffer's submission by the FPGA, there would be a possibility that the
373	FPGA side has sent data, but a select() call would still block, because the
374	host has not received any notification about this. This is solved with
375	XILLYMSG_OPCODE_NONEMPTY messages sent by the FPGA when a channel goes from
376	completely empty to containing some data.
377	
378	These messages are used only to support poll() and select(). The IP core can
379	be configured not to send them for a slight reduction of bandwidth.
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