spigen(4) SPI Generic IO interface -- need comments

[Brian Fundakowski Feldman]

I've added a couple more features:
 * clock adjustment via ioctl, independent per spigenN device
 * mmap(2) support for very low latency

On Tue, Aug 18, 2015 at 6:47 PM, Brian Fundakowski Feldman <
brianfundakowskifeldman at gmail.com> wrote:

> On Mon, Aug 17, 2015 at 12:04 PM Tom Jones <jones at sdf.org> wrote:
>
>> On Mon, Aug 17, 2015 at 10:00:26AM -0400, Brian Fundakowski Feldman wrote:
>> > I'm woefully out-of-practice with my kernel hackery (but still pretty
>> > proficient in jiggery-pokery) so I would like to get comments on a
>> little
>> > driver I just made for interfacing arbitrarily in userland with SPI
>> > components.  The only thing I'm exposing is a /dev/spigenN node with a
>> > single transfer ioctl and I put together a test circuit and program
>> with an
>> > MCP3008 10-bit ADC IC to validate that it basically works, other than
>> the
>> > limitation that the transfers must be octet-multiply-sized, but I
>> haven't
>> > looked at the SoC's (I'm using a Raspberry Pi 2) data sheet to tell
>> whether
>> > that's just a limit on the spibus(4) interface or the Broadcom SPI
>> driver
>> > or the Broadcom SoC itself.
>> >
>> > I hit one snag in development where I simply called the ioctl wrong and
>> > found copyin(9) to page fault HARD if given a bogus user address to copy
>> > from, and panic the kernel.  I can post up the test program if anyone
>> wants
>> > but it's very trivial: I just align the start bit and the command data
>> into
>> > the least significant bits of the first octet, shift it up two
>> positions so
>> > the NULs get clocked out as part of the command field, and provide two
>> > octets for the data field to retrieve back the 10-bit digital value.
>>
>> Oh, cool.
>>
>> I did the same earlier this year, have you seen[1]?.
>>
>> The FreeBSD i2c api is the same/very similar the linux one[2][3]. Have you
>> considered adding some of the ioctls[3] or the data structures to make it
>> easier to port code?
>>
>> [1]:
>> https://lists.freebsd.org/pipermail/freebsd-embedded/2015-April/002466.html
>> [2]: https://www.kernel.org/doc/Documentation/i2c/dev-interface
>> [3]:
>> https://www.freebsd.org/cgi/man.cgi?query=iic&apropos=0&sektion=0&manpath=FreeBSD+10.2-RELEASE&arch=default&format=html
>> [4]: https://www.kernel.org/doc/Documentation/spi/spidev
>
>
> I've iterated a bit on this to try to make some more sensible API,
> behaving reasonably about being able to set the SPI clock speed.  I'm going
> to implement an mmap handler so I can have my low-latency operation mode,
> as well.  I don't like the Linux APIs one bit because it's just not safe to
> allow all those configuration changes on a per-transfer basis...
>
> Moving this to -embedded because it's more apt than -hackers.
>
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