[MinnowBoard] SPI questions

Kevin Shelton kmshelton at gmail.com
Tue Jan 27 23:11:25 UTC 2015

I realized that in my BIOS settings, I was in ACPI mode.  Do I need to
be in PCI mode to use a board file?  What does that ACPI-PCI switch in
the BIOS do?  thanks for all the help

On Mon, Jan 26, 2015 at 11:35 PM, Darren Hart <dvhart at linux.intel.com> wrote:
> 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
>>> _______________________________________________
>>> elinux-MinnowBoard mailing list
>>> elinux-MinnowBoard at lists.elinux.org
>>> http://lists.elinux.org/mailman/listinfo/elinux-minnowboard
> --
> Darren Hart
> Intel Open Source Technology Center

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