Re: [2/2] spi: imx: fix use of native chip-selects with devicetree

[Trent Piepho <tpiepho-cgc2CodaaHDQT0dZR+AlfA@public.gmane.org>]

On Fri, 2017-03-17 at 15:03 +1000, Greg Ungerer wrote:
> You can successfully specify native chip-selects if using a platform
> setup by specifying the cs-gpio as negative offset by 32. And that
> works correctly. You cannot use the same method in devicetree.

I came across some spi-imx bugs in an older kernel and in checking the
latest kernel, found this patch.  It fixed the issue with dt not
working for older spi imx devices (the imx51+ config function never
used the platform data 32 offset thing), but fails to address non-dt
platform data based users of the spi imx driver.

For instance, include/linux/platform_data/spi-imx.h still refers to the
offset by 32 for native cs:

 * @chipselect: Array of chipselects for this master. Numbers >= 0 mean gpio
 *              pins, numbers < 0 mean internal CSPI chipselects according
 *              to MXC_SPI_CS(). Normally you want to use gpio based chip

#define MXC_SPI_CS(no)  ((no) - 32)

After this patch, for no-DT users, any negative (or rather not a valid
gpio) value in chipselect will cause the native chipselect
corresponding to the spi slave's chipselect.  It's no longer possible
to specify which native cs to use in the spi-imx platform data.  So in
something like:

arch/arm/mach-imx/mach-mx31moboard.c-static int moboard_spi1_cs[] = {
arch/arm/mach-imx/mach-mx31moboard.c:   MXC_SPI_CS(0),
arch/arm/mach-imx/mach-mx31moboard.c:   MXC_SPI_CS(2),
arch/arm/mach-imx/mach-mx31moboard.c-};

The spi device configured to use chip select 1 will no longer use
native chip select line 2.

Looking at all in-tree users of spi imx platform data, it appears that
this board is the only one in which the cs selected does not match the
array index.  Though there are still several which still use
MSC_SPI_CS() even though it doesn't really work anymore.