Re: [PATCH] spi: bcm2835: Convert to use CS GPIO descriptors

[Martin Sperl <kernel@xxxxxxxxxxxxxxxx>]

> On 20.04.2019, at 13:08, Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
> 
> This converts the BCM2835 SPI master driver to use GPIO
> descriptors for chip select handling.
> 
> The BCM2835 driver was relying on the core to drive the
> CS high/low so very small changes were needed for this
> part. If it managed to request the CS from the device tree
> node, all is pretty straight forward.
> 
> However for native GPIOs this driver has a quite unorthodox
> loopback to request some GPIOs from the SoC GPIO chip by
> looking it up from the device tree using gpiochip_find()
> and then offseting hard into its numberspace. This has
> been augmented a bit by using gpiochip_request_own_desc()
> but this code really needs to be verified. If "native CS"
> is actually an SoC GPIO, why is it even done this way?
> Should this GPIO not just be defined in the device tree
> like any other CS GPIO? I'm confused.

It has been done to keep dt-backward compatibility.

When the driver was written dt had configured native-cs.

A native-cs implementation comes with lots of
limitations and some of which - especially dma mode
has some quirks with regards to native-cs (glitches,
deasserts after each finished dma transfer,...)

The only way to make this work properly was to use
Gpio.

So to properly support old device-trees with native-cs 
this gpio translation is needed, as we should not break
things for old dt...

Maybe this rational helps...

Martin