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1
2 ==========================================
3 Xillybus driver for generic FPGA interface
4 ==========================================
5
6Author: Eli Billauer, Xillybus Ltd. (http://xillybus.com)
7Email: eli.billauer@gmail.com or as advertised on Xillybus' site.
8
9Contents:
10
11 - Introduction
12 -- Background
13 -- Xillybus Overview
14
15 - Usage
16 -- User interface
17 -- Synchronization
18 -- Seekable pipes
19
20- Internals
21 -- Source code organization
22 -- Pipe attributes
23 -- Host never reads from the FPGA
24 -- Channels, pipes, and the message channel
25 -- Data streaming
26 -- Data granularity
27 -- Probing
28 -- Buffer allocation
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29 -- The "nonempty" message (supporting poll)
30
31
32INTRODUCTION
33============
34
35Background
36----------
37
38An FPGA (Field Programmable Gate Array) is a piece of logic hardware, which
39can be programmed to become virtually anything that is usually found as a
40dedicated chipset: For instance, a display adapter, network interface card,
41or even a processor with its peripherals. FPGAs are the LEGO of hardware:
42Based upon certain building blocks, you make your own toys the way you like
43them. It's usually pointless to reimplement something that is already
44available on the market as a chipset, so FPGAs are mostly used when some
45special functionality is needed, and the production volume is relatively low
46(hence not justifying the development of an ASIC).
47
48The challenge with FPGAs is that everything is implemented at a very low
49level, even lower than assembly language. In order to allow FPGA designers to
50focus on their specific project, and not reinvent the wheel over and over
51again, pre-designed building blocks, IP cores, are often used. These are the
52FPGA parallels of library functions. IP cores may implement certain
53mathematical functions, a functional unit (e.g. a USB interface), an entire
54processor (e.g. ARM) or anything that might come handy. Think of them as a
55building block, with electrical wires dangling on the sides for connection to
56other blocks.
57
58One of the daunting tasks in FPGA design is communicating with a fullblown
59operating system (actually, with the processor running it): Implementing the
60low-level bus protocol and the somewhat higher-level interface with the host
61(registers, interrupts, DMA etc.) is a project in itself. When the FPGA's
62function is a well-known one (e.g. a video adapter card, or a NIC), it can
63make sense to design the FPGA's interface logic specifically for the project.
64A special driver is then written to present the FPGA as a well-known interface
65to the kernel and/or user space. In that case, there is no reason to treat the
66FPGA differently than any device on the bus.
67
68It's however common that the desired data communication doesn't fit any well-
69known peripheral function. Also, the effort of designing an elegant
70abstraction for the data exchange is often considered too big. In those cases,
71a quicker and possibly less elegant solution is sought: The driver is
72effectively written as a user space program, leaving the kernel space part
73with just elementary data transport. This still requires designing some
74interface logic for the FPGA, and write a simple ad-hoc driver for the kernel.
75
76Xillybus Overview
77-----------------
78
79Xillybus is an IP core and a Linux driver. Together, they form a kit for
80elementary data transport between an FPGA and the host, providing pipe-like
81data streams with a straightforward user interface. It's intended as a low-
82effort solution for mixed FPGA-host projects, for which it makes sense to
83have the project-specific part of the driver running in a user-space program.
84
85Since the communication requirements may vary significantly from one FPGA
86project to another (the number of data pipes needed in each direction and
87their attributes), there isn't one specific chunk of logic being the Xillybus
88IP core. Rather, the IP core is configured and built based upon a
89specification given by its end user.
90
91Xillybus presents independent data streams, which resemble pipes or TCP/IP
92communication to the user. At the host side, a character device file is used
93just like any pipe file. On the FPGA side, hardware FIFOs are used to stream
94the data. This is contrary to a common method of communicating through fixed-
95sized buffers (even though such buffers are used by Xillybus under the hood).
96There may be more than a hundred of these streams on a single IP core, but
97also no more than one, depending on the configuration.
98
99In order to ease the deployment of the Xillybus IP core, it contains a simple
100data structure which completely defines the core's configuration. The Linux
101driver fetches this data structure during its initialization process, and sets
102up the DMA buffers and character devices accordingly. As a result, a single
103driver is used to work out of the box with any Xillybus IP core.
104
105The data structure just mentioned should not be confused with PCI's
106configuration space or the Flattened Device Tree.
107
108USAGE
109=====
110
111User interface
112--------------
113
114On the host, all interface with Xillybus is done through /dev/xillybus_*
115device files, which are generated automatically as the drivers loads. The
116names of these files depend on the IP core that is loaded in the FPGA (see
117Probing below). To communicate with the FPGA, open the device file that
118corresponds to the hardware FIFO you want to send data or receive data from,
119and use plain write() or read() calls, just like with a regular pipe. In
120particular, it makes perfect sense to go:
121
122$ cat mydata > /dev/xillybus_thisfifo
123
124$ cat /dev/xillybus_thatfifo > hisdata
125
126possibly pressing CTRL-C as some stage, even though the xillybus_* pipes have
127the capability to send an EOF (but may not use it).
128
129The driver and hardware are designed to behave sensibly as pipes, including:
130
131* Supporting non-blocking I/O (by setting O_NONBLOCK on open() ).
132
133* Supporting poll() and select().
134
135* Being bandwidth efficient under load (using DMA) but also handle small
136 pieces of data sent across (like TCP/IP) by autoflushing.
137
138A device file can be read only, write only or bidirectional. Bidirectional
139device files are treated like two independent pipes (except for sharing a
140"channel" structure in the implementation code).
141
142Synchronization
143---------------
144
145Xillybus pipes are configured (on the IP core) to be either synchronous or
146asynchronous. For a synchronous pipe, write() returns successfully only after
147some data has been submitted and acknowledged by the FPGA. This slows down
148bulk data transfers, and is nearly impossible for use with streams that
149require data at a constant rate: There is no data transmitted to the FPGA
150between write() calls, in particular when the process loses the CPU.
151
152When a pipe is configured asynchronous, write() returns if there was enough
153room in the buffers to store any of the data in the buffers.
154
155For FPGA to host pipes, asynchronous pipes allow data transfer from the FPGA
156as soon as the respective device file is opened, regardless of if the data
157has been requested by a read() call. On synchronous pipes, only the amount
158of data requested by a read() call is transmitted.
159
160In summary, for synchronous pipes, data between the host and FPGA is
161transmitted only to satisfy the read() or write() call currently handled
162by the driver, and those calls wait for the transmission to complete before
163returning.
164
165Note that the synchronization attribute has nothing to do with the possibility
166that read() or write() completes less bytes than requested. There is a
167separate configuration flag ("allowpartial") that determines whether such a
168partial completion is allowed.
169
170Seekable pipes
171--------------
172
173A synchronous pipe can be configured to have the stream's position exposed
174to the user logic at the FPGA. Such a pipe is also seekable on the host API.
175With this feature, a memory or register interface can be attached on the
176FPGA side to the seekable stream. Reading or writing to a certain address in
177the attached memory is done by seeking to the desired address, and calling
178read() or write() as required.
179
180
181INTERNALS
182=========
183
184Source code organization
185------------------------
186
187The Xillybus driver consists of a core module, xillybus_core.c, and modules
188that depend on the specific bus interface (xillybus_of.c and xillybus_pcie.c).
189
190The bus specific modules are those probed when a suitable device is found by
191the kernel. Since the DMA mapping and synchronization functions, which are bus
192dependent by their nature, are used by the core module, a
193xilly_endpoint_hardware structure is passed to the core module on
194initialization. This structure is populated with pointers to wrapper functions
195which execute the DMA-related operations on the bus.
196
197Pipe attributes
198---------------
199
200Each pipe has a number of attributes which are set when the FPGA component
201(IP core) is built. They are fetched from the IDT (the data structure which
202defines the core's configuration, see Probing below) by xilly_setupchannels()
203in xillybus_core.c as follows:
204
205* is_writebuf: The pipe's direction. A non-zero value means it's an FPGA to
206 host pipe (the FPGA "writes").
207
208* channelnum: The pipe's identification number in communication between the
209 host and FPGA.
210
211* format: The underlying data width. See Data Granularity below.
212
213* allowpartial: A non-zero value means that a read() or write() (whichever
214 applies) may return with less than the requested number of bytes. The common
215 choice is a non-zero value, to match standard UNIX behavior.
216
217* synchronous: A non-zero value means that the pipe is synchronous. See
218 Syncronization above.
219
220* bufsize: Each DMA buffer's size. Always a power of two.
221
222* bufnum: The number of buffers allocated for this pipe. Always a power of two.
223
224* exclusive_open: A non-zero value forces exclusive opening of the associated
225 device file. If the device file is bidirectional, and already opened only in
226 one direction, the opposite direction may be opened once.
227
228* seekable: A non-zero value indicates that the pipe is seekable. See
229 Seekable pipes above.
230
231* supports_nonempty: A non-zero value (which is typical) indicates that the
232 hardware will send the messages that are necessary to support select() and
233 poll() for this pipe.
234
235Host never reads from the FPGA
236------------------------------
237
238Even though PCI Express is hotpluggable in general, a typical motherboard
239doesn't expect a card to go away all of the sudden. But since the PCIe card
240is based upon reprogrammable logic, a sudden disappearance from the bus is
241quite likely as a result of an accidental reprogramming of the FPGA while the
242host is up. In practice, nothing happens immediately in such a situation. But
243if the host attempts to read from an address that is mapped to the PCI Express
244device, that leads to an immediate freeze of the system on some motherboards,
245even though the PCIe standard requires a graceful recovery.
246
247In order to avoid these freezes, the Xillybus driver refrains completely from
248reading from the device's register space. All communication from the FPGA to
249the host is done through DMA. In particular, the Interrupt Service Routine
250doesn't follow the common practice of checking a status register when it's
251invoked. Rather, the FPGA prepares a small buffer which contains short
252messages, which inform the host what the interrupt was about.
253
254This mechanism is used on non-PCIe buses as well for the sake of uniformity.
255
256
257Channels, pipes, and the message channel
258----------------------------------------
259
260Each of the (possibly bidirectional) pipes presented to the user is allocated
261a data channel between the FPGA and the host. The distinction between channels
262and pipes is necessary only because of channel 0, which is used for interrupt-
263related messages from the FPGA, and has no pipe attached to it.
264
265Data streaming
266--------------
267
268Even though a non-segmented data stream is presented to the user at both
269sides, the implementation relies on a set of DMA buffers which is allocated
270for each channel. For the sake of illustration, let's take the FPGA to host
271direction: As data streams into the respective channel's interface in the
272FPGA, the Xillybus IP core writes it to one of the DMA buffers. When the
273buffer is full, the FPGA informs the host about that (appending a
274XILLYMSG_OPCODE_RELEASEBUF message channel 0 and sending an interrupt if
275necessary). The host responds by making the data available for reading through
276the character device. When all data has been read, the host writes on the
277the FPGA's buffer control register, allowing the buffer's overwriting. Flow
278control mechanisms exist on both sides to prevent underflows and overflows.
279
280This is not good enough for creating a TCP/IP-like stream: If the data flow
281stops momentarily before a DMA buffer is filled, the intuitive expectation is
282that the partial data in buffer will arrive anyhow, despite the buffer not
283being completed. This is implemented by adding a field in the
284XILLYMSG_OPCODE_RELEASEBUF message, through which the FPGA informs not just
285which buffer is submitted, but how much data it contains.
286
287But the FPGA will submit a partially filled buffer only if directed to do so
288by the host. This situation occurs when the read() method has been blocking
289for XILLY_RX_TIMEOUT jiffies (currently 10 ms), after which the host commands
290the FPGA to submit a DMA buffer as soon as it can. This timeout mechanism
291balances between bus bandwidth efficiency (preventing a lot of partially
292filled buffers being sent) and a latency held fairly low for tails of data.
293
294A similar setting is used in the host to FPGA direction. The handling of
295partial DMA buffers is somewhat different, though. The user can tell the
296driver to submit all data it has in the buffers to the FPGA, by issuing a
297write() with the byte count set to zero. This is similar to a flush request,
298but it doesn't block. There is also an autoflushing mechanism, which triggers
299an equivalent flush roughly XILLY_RX_TIMEOUT jiffies after the last write().
300This allows the user to be oblivious about the underlying buffering mechanism
301and yet enjoy a stream-like interface.
302
303Note that the issue of partial buffer flushing is irrelevant for pipes having
304the "synchronous" attribute nonzero, since synchronous pipes don't allow data
305to lay around in the DMA buffers between read() and write() anyhow.
306
307Data granularity
308----------------
309
310The data arrives or is sent at the FPGA as 8, 16 or 32 bit wide words, as
311configured by the "format" attribute. Whenever possible, the driver attempts
312to hide this when the pipe is accessed differently from its natural alignment.
313For example, reading single bytes from a pipe with 32 bit granularity works
314with no issues. Writing single bytes to pipes with 16 or 32 bit granularity
315will also work, but the driver can't send partially completed words to the
316FPGA, so the transmission of up to one word may be held until it's fully
317occupied with user data.
318
319This somewhat complicates the handling of host to FPGA streams, because
320when a buffer is flushed, it may contain up to 3 bytes don't form a word in
321the FPGA, and hence can't be sent. To prevent loss of data, these leftover
322bytes need to be moved to the next buffer. The parts in xillybus_core.c
323that mention "leftovers" in some way are related to this complication.
324
325Probing
326-------
327
328As mentioned earlier, the number of pipes that are created when the driver
329loads and their attributes depend on the Xillybus IP core in the FPGA. During
330the driver's initialization, a blob containing configuration info, the
331Interface Description Table (IDT), is sent from the FPGA to the host. The
332bootstrap process is done in three phases:
333
3341. Acquire the length of the IDT, so a buffer can be allocated for it. This
335 is done by sending a quiesce command to the device, since the acknowledge
336 for this command contains the IDT's buffer length.
337
3382. Acquire the IDT itself.
339
3403. Create the interfaces according to the IDT.
341
342Buffer allocation
343-----------------
344
345In order to simplify the logic that prevents illegal boundary crossings of
346PCIe packets, the following rule applies: If a buffer is smaller than 4kB,
347it must not cross a 4kB boundary. Otherwise, it must be 4kB aligned. The
348xilly_setupchannels() functions allocates these buffers by requesting whole
349pages from the kernel, and diving them into DMA buffers as necessary. Since
350all buffers' sizes are powers of two, it's possible to pack any set of such
351buffers, with a maximal waste of one page of memory.
352
353All buffers are allocated when the driver is loaded. This is necessary,
354since large continuous physical memory segments are sometimes requested,
355which are more likely to be available when the system is freshly booted.
356
357The allocation of buffer memory takes place in the same order they appear in
358the IDT. The driver relies on a rule that the pipes are sorted with decreasing
359buffer size in the IDT. If a requested buffer is larger or equal to a page,
360the necessary number of pages is requested from the kernel, and these are
361used for this buffer. If the requested buffer is smaller than a page, one
362single page is requested from the kernel, and that page is partially used.
363Or, if there already is a partially used page at hand, the buffer is packed
364into that page. It can be shown that all pages requested from the kernel
365(except possibly for the last) are 100% utilized this way.
366
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367The "nonempty" message (supporting poll)
368---------------------------------------
369
370In order to support the "poll" method (and hence select() ), there is a small
371catch regarding the FPGA to host direction: The FPGA may have filled a DMA
372buffer with some data, but not submitted that buffer. If the host waited for
373the buffer's submission by the FPGA, there would be a possibility that the
374FPGA side has sent data, but a select() call would still block, because the
375host has not received any notification about this. This is solved with
376XILLYMSG_OPCODE_NONEMPTY messages sent by the FPGA when a channel goes from
377completely empty to containing some data.
378
379These messages are used only to support poll() and select(). The IP core can
380be configured not to send them for a slight reduction of bandwidth.