[MinnowBoard] SPI questions

Kevin Shelton kmshelton at gmail.com
Tue Jan 27 06:38:52 UTC 2015

Thanks John and Darren.  I will play with spid_devtest and have
reached out to linux-spi at vger.kernel.org.

A minnowmax-specific question: In the Baytrail block diagram at
does the Marvell pxa27x correspond to the "Low Power IO Controller"?

On Mon, Jan 26, 2015 at 6:33 PM, Darren Hart <dvhart at linux.intel.com> wrote:
> On 1/26/15, 5:52 PM, "John Hawley" <john.hawley at intel.com> wrote:
>>On 01/26/2015 05:47 PM, Kevin Shelton wrote:
>>> On Fri, Jan 23, 2015 at 2:41 PM, John Hawley <john.hawley at intel.com
>>> <mailto:john.hawley at intel.com>> wrote:
>>>     > I saw the thread 'Adding an SPI device to the Minnowboard' from
>>>     > 2013 and 'SPI support on minnowboard v1' from Aug 2014.
>>>     >
>>>     > Darren Hart notes:
>>>     > Ultimately we want to do things like this without board files by
>>>     > the _DSD mechanisms introduced by the ACPI 5.1 specification last
>>>     >
>>>     > I just wanted to confirm the ACPI mechanism is not the
>>>recommended way
>>>     > yet, and that using low-speed-spidev.c as a template is still the
>>>way to go.
>>>     The answer to that will depend on what kernel you are intending to
>>>     target.  Kernel's with ACPI 5.1 _DSD support, I think you'd want to
>>>     on that.  Older kernels without that, likely spidev or a more
>>>     driver.
>>> Currently, I am targeting 3.17.  3.17 does not have ACPI 5.1 _DSD
>>> support, correct?
>>Off the top of my head that came in in 3.18, so yes that's correct.
> 3.19 iirc.
> Also, using _DSD required a firmware change, or at least a DSDT update.
>>>     > Additional q:
>>>     > How do you tell the SPI controller that you have an active-high
>>>     > of the usual active-low device?  Is it correct to do a bitwise or
>>>     > SPI_CS_HIGH (0x4) with your SPI_MODE in your spi_board_info
>>>struct, like:
>>>     > .mode = SPI_MODE_0 | SPI_CS_HIGH
>>>     That should work, but take my statement with a grain of salt as I
>>>     haven't tried it with a device.
>>> It seems to have no effect that I can discern.  Pin 5 idles at 3.3V
>>> whether I have
>>>  .mode = SPI_MODE_3 | SPI_CS_HIGH
>>> -or-
>>> .mode = SPI_MODE_3
>>> I threw in a
>>> pr_info("SPI mode=%i\n", cod_spi_board_info.mode);
>>> to sanity check that I am setting the mode to what I think I am (3 or
>>> Any debugging ideas?
>>> What is the best way to learn more about the SPI master?  It's built
>>> into the CPU, correct?
>>> This smells in the ballpark of
>>> relatedness:
>>> I don't grok that patch, but I confirmed my version of pxa2xx.c in my
>>> 3.17 tree appears to contain that change.
>>The SPI interface is indeed built into the CPU.  It's the pxa2xx core,
>>which it looks like you've found.  I'd have to punt to someone else, as
>>I'll admit, I don't know the SPI code well enough to say what's going
>>on.  I've CC'ed Darren Hart, he might know who to check with next.
> Those are some very specific SPI usage questions that I don't know the
> answer to off the top of my head. To find out, I would:
> 1) Search for other drivers in tree and externally that use active-high
> First hit looks interesting:
> https://www.kernel.org/doc/Documentation/spi/spidev_test.c
> 2) Lookup the right mailing lists for SPI Linux kernel development and ask
> the same question there
> --
> Darren Hart
> Intel Open Source Technology Center
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