[MinnowBoard] SPI questions

Kevin Shelton kmshelton at gmail.com
Tue Jan 27 01:47:28 UTC 2015

On Fri, Jan 23, 2015 at 2:41 PM, John Hawley <john.hawley at intel.com> wrote:

> > I saw the thread 'Adding an SPI device to the Minnowboard' from late
> > 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 using
> > the _DSD mechanisms introduced by the ACPI 5.1 specification last week
> >
> > 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 push
> on that.  Older kernels without that, likely spidev or a more targeted
> driver.

Currently, I am targeting 3.17.  3.17 does not have ACPI 5.1 _DSD support,

> > Additional q:
> > How do you tell the SPI controller that you have an active-high instead
> > of the usual active-low device?  Is it correct to do a bitwise or with
> > 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
.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 7).

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.

> - John 'Warthog9' Hawley
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