Re: [PATCH v4 2/5] nvmem: add support for the write-protect pin

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Bartosz,

On Mon, Feb 17, 2020 at 3:34 PM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> czw., 30 sty 2020 o 09:06 Geert Uytterhoeven <geert@linux-m68k.org> napisał(a):
> > On Tue, Jan 7, 2020 at 10:30 AM Khouloud Touil <ktouil@baylibre.com> wrote:
> > > The write-protect pin handling looks like a standard property that
> > > could benefit other users if available in the core nvmem framework.
> > >
> > > Instead of modifying all the memory drivers to check this pin, make
> > > the NVMEM subsystem check if the write-protect GPIO being passed
> > > through the nvmem_config or defined in the device tree and pull it
> > > low whenever writing to the memory.
> > >
> > > There was a suggestion for introducing the gpiodesc from pdata, but
> > > as pdata is already removed it could be replaced by adding it to
> > > nvmem_config.
> > >
> > > Reference: https://lists.96boards.org/pipermail/dev/2018-August/001056.html
> > >
> > > Signed-off-by: Khouloud Touil <ktouil@baylibre.com>
> > > Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
> > > Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
> >
> > Thanks for your patch!
> >
> > > --- a/drivers/nvmem/core.c
> > > +++ b/drivers/nvmem/core.c
> > > @@ -15,6 +15,7 @@
> > >  #include <linux/module.h>
> > >  #include <linux/nvmem-consumer.h>
> > >  #include <linux/nvmem-provider.h>
> > > +#include <linux/gpio/consumer.h>
> > >  #include <linux/of.h>
> > >  #include <linux/slab.h>
> > >  #include "nvmem.h"
> > > @@ -54,8 +55,14 @@ static int nvmem_reg_read(struct nvmem_device *nvmem, unsigned int offset,
> > >  static int nvmem_reg_write(struct nvmem_device *nvmem, unsigned int offset,
> > >                            void *val, size_t bytes)
> > >  {
> > > -       if (nvmem->reg_write)
> > > -               return nvmem->reg_write(nvmem->priv, offset, val, bytes);
> > > +       int ret;
> > > +
> > > +       if (nvmem->reg_write) {
> > > +               gpiod_set_value_cansleep(nvmem->wp_gpio, 0);
> > > +               ret = nvmem->reg_write(nvmem->priv, offset, val, bytes);
> > > +               gpiod_set_value_cansleep(nvmem->wp_gpio, 1);
> > > +               return ret;
> > > +       }
> > >
> > >         return -EINVAL;
> > >  }
> > > @@ -338,6 +345,14 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
> > >                 kfree(nvmem);
> > >                 return ERR_PTR(rval);
> > >         }
> > > +       if (config->wp_gpio)
> > > +               nvmem->wp_gpio = config->wp_gpio;
> > > +       else
> > > +               nvmem->wp_gpio = gpiod_get_optional(config->dev, "wp",
> > > +                                                   GPIOD_OUT_HIGH);
> >
> > Shouldn't this GPIO be released in nvmem_release(), by calling gpiod_put()?
> >
>
> Hi Geert,
>
> Khouloud already sent out a patch but I think it still doesn't fix all
> the problems.
>
> While we should call gpiod_put() for the descs we request - we must
> not do it for the desc we get over the config structure. Unless... we

That's true.

> make descs reference counted with kref and add gpiod_ref() helper.
> That way we could increase the reference counter in the upper branch
> of the if and not do it in the lower. Calling gpiod_put() would
> internally call kref_put(). Does it make sense? I think that a
> function that's called gpiod_put() but doesn't really use reference
> counting is misleading anyway.

Yep.

> > Once that's implemented, I assume it will be auto-released on registration
> > failure by the call to put_device()?
>
> No, I think this is another leak - why would put_device() lead to
> freeing any resources? Am I missing something?

Sorry, I don't remember why I wrote that part...

Anyway, requested GPIOs should be released on failure, and on
unregistration.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

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