[MinnowBoard] SPI questions

Darren Hart dvhart at linux.intel.com
Tue Jan 27 07:35:30 UTC 2015

The SPI bus controlled by the pxa2xx driver is on the LPSS, which, yes,
corresponds to the Low Power IO Controller in that block diagram.

On 1/26/15, 10:38 PM, "Kevin Shelton" <kmshelton at gmail.com> wrote:

>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>
>> 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
>>>>     >
>>>>     > 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
>>>>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
>>>>     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
>>>>     > 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
>> the same question there
>> --
>> Darren Hart
>> Intel Open Source Technology Center
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Darren Hart
Intel Open Source Technology Center

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