spi: imx: native chip selects and devicetree

Selecting native chip selects for iMX SPI devices in a devicetree configuration does not work. That is the case for imx25 based SoC parts, and I think for imx31 SoC as well. There is no problem with configuring SPI ports to use a GPIO as chip select.

Selecting native chip selects via the old platform setup code will work, it is only devicetree configurations that are broken.

With platform configuration you specify a native chip select by setting the cs_gpio for the SPI device to be "32 - chipselect", This will be a negative number - and thus not a valid GPIO number. And that "chipselect" is the actual hardware native chip select number.

You cannot specify the cs_gpio in dvicetree as a negative number, so this whole scheme does not work. The common method in devicetree is to set the cs_gpio entry for your device to be "<0>". If you do this to configure your SPI device to use a native chip select it is valid, but the SPI device cannot be accessed (no valid read data returned from it).

The problem lies in the way the spi-imx.c driver sets up the controlling registers of the iMX SPI block. It doesn't have the correct logic for using nativce chip selects in the devicetree case.

The best fix is to use the "chip_select" associated with the SPI device as passed in from the devicetree (as the "reg" tag) or directly in the platform data structure. The hitch with that is that field is not set correctly by some platform device code.

The following patches fix the platform setup code to correctly set the "chip_select" for the SPI device and to set the SPI registers correctly based on that.

These changes affect the iMX machine code and drivers/spi subsystem so I have sent this to both lists for feedback.

Signed-off-by Greg Ungerer <[email protected]>