1==========================================
2Xillybus 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
31Introduction
32============
33
34Background
35----------
36
37An FPGA (Field Programmable Gate Array) is a piece of logic hardware, which
38can be programmed to become virtually anything that is usually found as a
39dedicated chipset: For instance, a display adapter, network interface card,
40or even a processor with its peripherals. FPGAs are the LEGO of hardware:
41Based upon certain building blocks, you make your own toys the way you like
42them. It's usually pointless to reimplement something that is already
43available on the market as a chipset, so FPGAs are mostly used when some
44special functionality is needed, and the production volume is relatively low
45(hence not justifying the development of an ASIC).
46
47The challenge with FPGAs is that everything is implemented at a very low
48level, even lower than assembly language. In order to allow FPGA designers to
49focus on their specific project, and not reinvent the wheel over and over
50again, pre-designed building blocks, IP cores, are often used. These are the
51FPGA parallels of library functions. IP cores may implement certain
52mathematical functions, a functional unit (e.g. a USB interface), an entire
53processor (e.g. ARM) or anything that might come handy. Think of them as a
54building block, with electrical wires dangling on the sides for connection to
55other blocks.
56
57One of the daunting tasks in FPGA design is communicating with a fullblown
58operating system (actually, with the processor running it): Implementing the
59low-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
61function is a well-known one (e.g. a video adapter card, or a NIC), it can
62make sense to design the FPGA's interface logic specifically for the project.
63A special driver is then written to present the FPGA as a well-known interface
64to the kernel and/or user space. In that case, there is no reason to treat the
65FPGA differently than any device on the bus.
66
67It's however common that the desired data communication doesn't fit any well-
68known peripheral function. Also, the effort of designing an elegant
69abstraction for the data exchange is often considered too big. In those cases,
70a quicker and possibly less elegant solution is sought: The driver is
71effectively written as a user space program, leaving the kernel space part
72with just elementary data transport. This still requires designing some
73interface logic for the FPGA, and write a simple ad-hoc driver for the kernel.
74
75Xillybus Overview
76-----------------
77
78Xillybus is an IP core and a Linux driver. Together, they form a kit for
79elementary data transport between an FPGA and the host, providing pipe-like
80data streams with a straightforward user interface. It's intended as a low-
81effort solution for mixed FPGA-host projects, for which it makes sense to
82have the project-specific part of the driver running in a user-space program.
83
84Since the communication requirements may vary significantly from one FPGA
85project to another (the number of data pipes needed in each direction and
86their attributes), there isn't one specific chunk of logic being the Xillybus
87IP core. Rather, the IP core is configured and built based upon a
88specification given by its end user.
89
90Xillybus presents independent data streams, which resemble pipes or TCP/IP
91communication to the user. At the host side, a character device file is used
92just like any pipe file. On the FPGA side, hardware FIFOs are used to stream
93the data. This is contrary to a common method of communicating through fixed-
94sized buffers (even though such buffers are used by Xillybus under the hood).
95There may be more than a hundred of these streams on a single IP core, but
96also no more than one, depending on the configuration.
97
98In order to ease the deployment of the Xillybus IP core, it contains a simple
99data structure which completely defines the core's configuration. The Linux
100driver fetches this data structure during its initialization process, and sets
101up the DMA buffers and character devices accordingly. As a result, a single
102driver is used to work out of the box with any Xillybus IP core.
103
104The data structure just mentioned should not be confused with PCI's
105configuration space or the Flattened Device Tree.
106
107Usage
108=====
109
110User interface
111--------------
112
113On the host, all interface with Xillybus is done through /dev/xillybus_*
114device files, which are generated automatically as the drivers loads. The
115names of these files depend on the IP core that is loaded in the FPGA (see
116Probing below). To communicate with the FPGA, open the device file that
117corresponds to the hardware FIFO you want to send data or receive data from,
118and use plain write() or read() calls, just like with a regular pipe. In
119particular, it makes perfect sense to go::
120
121	$ cat mydata > /dev/xillybus_thisfifo
122
123	$ cat /dev/xillybus_thatfifo > hisdata
124
125possibly pressing CTRL-C as some stage, even though the xillybus_* pipes have
126the capability to send an EOF (but may not use it).
127
128The 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
137A device file can be read only, write only or bidirectional. Bidirectional
138device files are treated like two independent pipes (except for sharing a
139"channel" structure in the implementation code).
140
141Synchronization
142---------------
143
144Xillybus pipes are configured (on the IP core) to be either synchronous or
145asynchronous. For a synchronous pipe, write() returns successfully only after
146some data has been submitted and acknowledged by the FPGA. This slows down
147bulk data transfers, and is nearly impossible for use with streams that
148require data at a constant rate: There is no data transmitted to the FPGA
149between write() calls, in particular when the process loses the CPU.
150
151When a pipe is configured asynchronous, write() returns if there was enough
152room in the buffers to store any of the data in the buffers.
153
154For FPGA to host pipes, asynchronous pipes allow data transfer from the FPGA
155as soon as the respective device file is opened, regardless of if the data
156has been requested by a read() call. On synchronous pipes, only the amount
157of data requested by a read() call is transmitted.
158
159In summary, for synchronous pipes, data between the host and FPGA is
160transmitted only to satisfy the read() or write() call currently handled
161by the driver, and those calls wait for the transmission to complete before
162returning.
163
164Note that the synchronization attribute has nothing to do with the possibility
165that read() or write() completes less bytes than requested. There is a
166separate configuration flag ("allowpartial") that determines whether such a
167partial completion is allowed.
168
169Seekable pipes
170--------------
171
172A synchronous pipe can be configured to have the stream's position exposed
173to the user logic at the FPGA. Such a pipe is also seekable on the host API.
174With this feature, a memory or register interface can be attached on the
175FPGA side to the seekable stream. Reading or writing to a certain address in
176the attached memory is done by seeking to the desired address, and calling
177read() or write() as required.
178
179
180Internals
181=========
182
183Source code organization
184------------------------
185
186The Xillybus driver consists of a core module, xillybus_core.c, and modules
187that depend on the specific bus interface (xillybus_of.c and xillybus_pcie.c).
188
189The bus specific modules are those probed when a suitable device is found by
190the kernel. Since the DMA mapping and synchronization functions, which are bus
191dependent by their nature, are used by the core module, a
192xilly_endpoint_hardware structure is passed to the core module on
193initialization. This structure is populated with pointers to wrapper functions
194which execute the DMA-related operations on the bus.
195
196Pipe attributes
197---------------
198
199Each 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
201defines the core's configuration, see Probing below) by xilly_setupchannels()
202in 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
234Host never reads from the FPGA
235------------------------------
236
237Even though PCI Express is hotpluggable in general, a typical motherboard
238doesn't expect a card to go away all of the sudden. But since the PCIe card
239is based upon reprogrammable logic, a sudden disappearance from the bus is
240quite likely as a result of an accidental reprogramming of the FPGA while the
241host is up. In practice, nothing happens immediately in such a situation. But
242if the host attempts to read from an address that is mapped to the PCI Express
243device, that leads to an immediate freeze of the system on some motherboards,
244even though the PCIe standard requires a graceful recovery.
245
246In order to avoid these freezes, the Xillybus driver refrains completely from
247reading from the device's register space. All communication from the FPGA to
248the host is done through DMA. In particular, the Interrupt Service Routine
249doesn't follow the common practice of checking a status register when it's
250invoked. Rather, the FPGA prepares a small buffer which contains short
251messages, which inform the host what the interrupt was about.
252
253This mechanism is used on non-PCIe buses as well for the sake of uniformity.
254
255
256Channels, pipes, and the message channel
257----------------------------------------
258
259Each of the (possibly bidirectional) pipes presented to the user is allocated
260a data channel between the FPGA and the host. The distinction between channels
261and pipes is necessary only because of channel 0, which is used for interrupt-
262related messages from the FPGA, and has no pipe attached to it.
263
264Data streaming
265--------------
266
267Even though a non-segmented data stream is presented to the user at both
268sides, the implementation relies on a set of DMA buffers which is allocated
269for each channel. For the sake of illustration, let's take the FPGA to host
270direction: As data streams into the respective channel's interface in the
271FPGA, the Xillybus IP core writes it to one of the DMA buffers. When the
272buffer is full, the FPGA informs the host about that (appending a
273XILLYMSG_OPCODE_RELEASEBUF message channel 0 and sending an interrupt if
274necessary). The host responds by making the data available for reading through
275the character device. When all data has been read, the host writes on the
276the FPGA's buffer control register, allowing the buffer's overwriting. Flow
277control mechanisms exist on both sides to prevent underflows and overflows.
278
279This is not good enough for creating a TCP/IP-like stream: If the data flow
280stops momentarily before a DMA buffer is filled, the intuitive expectation is
281that the partial data in buffer will arrive anyhow, despite the buffer not
282being completed. This is implemented by adding a field in the
283XILLYMSG_OPCODE_RELEASEBUF message, through which the FPGA informs not just
284which buffer is submitted, but how much data it contains.
285
286But the FPGA will submit a partially filled buffer only if directed to do so
287by the host. This situation occurs when the read() method has been blocking
288for XILLY_RX_TIMEOUT jiffies (currently 10 ms), after which the host commands
289the FPGA to submit a DMA buffer as soon as it can. This timeout mechanism
290balances between bus bandwidth efficiency (preventing a lot of partially
291filled buffers being sent) and a latency held fairly low for tails of data.
292
293A similar setting is used in the host to FPGA direction. The handling of
294partial DMA buffers is somewhat different, though. The user can tell the
295driver to submit all data it has in the buffers to the FPGA, by issuing a
296write() with the byte count set to zero. This is similar to a flush request,
297but it doesn't block. There is also an autoflushing mechanism, which triggers
298an equivalent flush roughly XILLY_RX_TIMEOUT jiffies after the last write().
299This allows the user to be oblivious about the underlying buffering mechanism
300and yet enjoy a stream-like interface.
301
302Note that the issue of partial buffer flushing is irrelevant for pipes having
303the "synchronous" attribute nonzero, since synchronous pipes don't allow data
304to lay around in the DMA buffers between read() and write() anyhow.
305
306Data granularity
307----------------
308
309The data arrives or is sent at the FPGA as 8, 16 or 32 bit wide words, as
310configured by the "format" attribute. Whenever possible, the driver attempts
311to hide this when the pipe is accessed differently from its natural alignment.
312For example, reading single bytes from a pipe with 32 bit granularity works
313with no issues. Writing single bytes to pipes with 16 or 32 bit granularity
314will also work, but the driver can't send partially completed words to the
315FPGA, so the transmission of up to one word may be held until it's fully
316occupied with user data.
317
318This somewhat complicates the handling of host to FPGA streams, because
319when a buffer is flushed, it may contain up to 3 bytes don't form a word in
320the FPGA, and hence can't be sent. To prevent loss of data, these leftover
321bytes need to be moved to the next buffer. The parts in xillybus_core.c
322that mention "leftovers" in some way are related to this complication.
323
324Probing
325-------
326
327As mentioned earlier, the number of pipes that are created when the driver
328loads and their attributes depend on the Xillybus IP core in the FPGA. During
329the driver's initialization, a blob containing configuration info, the
330Interface Description Table (IDT), is sent from the FPGA to the host. The
331bootstrap process is done in three phases:
332
3331. 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
3372. Acquire the IDT itself.
338
3393. Create the interfaces according to the IDT.
340
341Buffer allocation
342-----------------
343
344In order to simplify the logic that prevents illegal boundary crossings of
345PCIe packets, the following rule applies: If a buffer is smaller than 4kB,
346it must not cross a 4kB boundary. Otherwise, it must be 4kB aligned. The
347xilly_setupchannels() functions allocates these buffers by requesting whole
348pages from the kernel, and diving them into DMA buffers as necessary. Since
349all buffers' sizes are powers of two, it's possible to pack any set of such
350buffers, with a maximal waste of one page of memory.
351
352All buffers are allocated when the driver is loaded. This is necessary,
353since large continuous physical memory segments are sometimes requested,
354which are more likely to be available when the system is freshly booted.
355
356The allocation of buffer memory takes place in the same order they appear in
357the IDT. The driver relies on a rule that the pipes are sorted with decreasing
358buffer size in the IDT. If a requested buffer is larger or equal to a page,
359the necessary number of pages is requested from the kernel, and these are
360used for this buffer. If the requested buffer is smaller than a page, one
361single page is requested from the kernel, and that page is partially used.
362Or, if there already is a partially used page at hand, the buffer is packed
363into 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
366The "nonempty" message (supporting poll)
367----------------------------------------
368
369In order to support the "poll" method (and hence select() ), there is a small
370catch regarding the FPGA to host direction: The FPGA may have filled a DMA
371buffer with some data, but not submitted that buffer. If the host waited for
372the buffer's submission by the FPGA, there would be a possibility that the
373FPGA side has sent data, but a select() call would still block, because the
374host has not received any notification about this. This is solved with
375XILLYMSG_OPCODE_NONEMPTY messages sent by the FPGA when a channel goes from
376completely empty to containing some data.
377
378These messages are used only to support poll() and select(). The IP core can
379be configured not to send them for a slight reduction of bandwidth.
380