Re: [PATCH 1/4 v3] spi: Support high CS when using descriptors

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

Hi Linus,

On Wed, Apr 3, 2019 at 6:24 PM Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
> On Wed, Apr 3, 2019 at 3:35 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > On Wed, Jan 16, 2019 at 7:27 PM Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
> > > All controllers using GPIO descriptors can by definition
> > > support high CS connections, so just enforce this when
> > > registering an SPI controller.
> >
> > But that is guaranteed to be true only for chip selects handled by a GPIO,
> > right?
> > Native chip selects may still not support SPI_CS_HIGH, depending
> > on the controller.
> > Before, the bad_bits check in spi_setup() would detect this, and return
> > an error. After, this will fail silently.
>
> Yes but only for systems that use descriptors all the way. So dealing
> with this is part of the process of converting to using descriptors.
> (Thus the other patches in this series.)

Well, if the it worked before (no error), it should work after the conversion.
The error is handy for new (future) users.

> Do we have some hardware that supports only active low native
> CS but also want to use GPIOs? Because then maybe I should
> take a stab at that in particular. If I keep converting drivers to this
> then I will hit my head into it sooner or later I suppose.

There may be hardware like that.  The integrator should take care of
it[*].

[*] Until we manage to describe functional relations in DT instead of explicit
    GPIO/native <whatever>/IRQn relations. After that
      - drivers can switch from native to GPIO chip select automatically, when
        some native feature is missing,
      - the system can choose to use spi-gpio or i2c-gpio if no driver is
        available for the hardware SPI or I2C controller on the same pins,
      - the system can pinmux between a GPIO with interrupt functionality
        or a real interrupt controller pin, depending on availability.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds