Re: [PATCH] gpio: mmio: do not calculate bgpio_bits via "ngpios"

[Jonas Gorski <jonas.gorski@xxxxxxxxx>]

Hi Linus,

On Wed, 26 Jun 2024 at 14:06, Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
>
> Hi Shiji,
>
> thanks for your patch!
>
> On Tue, Jun 25, 2024 at 3:22 AM Shiji Yang <yangshiji66@xxxxxxxxxxx> wrote:
>
> > bgpio_bits must be aligned with the data bus width. For example, on a
> > 32 bit big endian system and we only have 16 GPIOs. If we only assume
> > bgpio_bits=16 we can never control the GPIO because the base address
> > is the lowest address.
> >
> > low address                          high address
> > -------------------------------------------------
> > |   byte3   |   byte2   |   byte1   |   byte0   |
> > -------------------------------------------------
> > |    NaN    |    NaN    |  gpio8-15 |  gpio0-7  |
> > -------------------------------------------------
> >
> > Fixes: 55b2395e4e92 ("gpio: mmio: handle "ngpios" properly in bgpio_init()")
> > Fixes: https://github.com/openwrt/openwrt/issues/15739
> > Reported-by: Mark Mentovai <mark@xxxxxxxxxxxx>
> > Signed-off-by: Shiji Yang <yangshiji66@xxxxxxxxxxx>
> > Suggested-By: Mark Mentovai <mark@xxxxxxxxxxxx>
> > Reviewed-by: Jonas Gorski <jonas.gorski@xxxxxxxxx>
> > Tested-by: Lóránd Horváth <lorand.horvath82@xxxxxxxxx>
>
> Commit  55b2395e4e92 also contains this:
>
> @@ -614,10 +616,15 @@ int bgpio_init(struct gpio_chip *gc, struct device *dev,
>         gc->parent = dev;
>         gc->label = dev_name(dev);
>         gc->base = -1;
> -       gc->ngpio = gc->bgpio_bits;
>         gc->request = bgpio_request;
>
> After this patch gc->ngpio will be unset for any GPIO chip that
> provides a ngpios property, so restore the above line too.

The patch only removes a line changing gc->bgpio_bits, not gc->ngpio.
gc->ngpio is untouched.

gc->ngpio will still be set by gpiochip_get_ngpios() if there is a
ngpio property. See the context of the patch:

> @@ -619,8 +619,6 @@ int bgpio_init(struct gpio_chip *gc, struct device *dev,
>         ret = gpiochip_get_ngpios(gc, dev); <--
>         if (ret)
>                 gc->ngpio = gc->bgpio_bits; <- and if it fails, fallback to bgpio_bits

>
> But maybe a better fix is:
>
> + #include <linux/types.h>
> (...)
> +  else
> +               gc->bgpio_bits = round_up(gc->ngpio, sizeof(phys_addr_t) * 8);
>
> ?

bgpio only supports a single register worth of gpios, so the limit is
1 * sizeof(phys_addr_t) * 8. So this would force force bgpio_bits to
sizeof(phys_addr_t) * 8. And this will break any bgpio users where the
gpio registers are less than phys_addr_t wide. Like 32 bit registers
on a 64 bit system, or 16 bit registers on 32 bit.

Therefore I think the most sane thing is to keep gc->bgpio_bits at sz * 8.

The only potentially reasonable thing that could be added here is a
check that gc->ngpio is at most bgpio_bits. But that would be an
additional check, not a fix per se.

Best Regards,
Jonas