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

[Brian Fundakowski Feldman]

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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