linux-kernel - Re: [PATCH v3] gpio: mmio: handle "ngpios" properly in bgpio

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Date:   Mon, 6 Mar 2023 22:16:13 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Asmaa Mnebhi <asmaa@...dia.com>
Cc:     "linus.walleij@...aro.org" <linus.walleij@...aro.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] gpio: mmio: handle "ngpios" properly in bgpio_init()

On Mon, Mar 6, 2023 at 8:47 PM Asmaa Mnebhi <asmaa@...dia.com> wrote:

...

> > > +       ret = gpiochip_get_ngpios(gc, dev);
> > > +       if (ret)
> > > +               gc->ngpio = gc->bgpio_bits;
> >
> > But this doesn't update bgpio_bits in the success case. Can you explain why
> > it's not a problem (should be at least in the code as a comment).
>
> In the success rate, the bgpio_bits would also be equal to "sz * 8" anyways.
> The argument " unsigned long sz" passed in bgpio_init is specifically for this purpose. That tells the gpio library the gpio register access size.
> if (!is_power_of_2(sz))
>                  return -EINVAL;
>  gc->bgpio_bits = sz * 8;
>
> If in the success case, we make it dependent on the ngpio value, we would need to round it up anyways to the closest (power of 2 && multiple of 8) which is the same as "sz * 8"
> I will add a comment in the code in my next patch.

I believe we should use only a single source for what we need. If we
rely on ngpios, the bgpio_bits should be recalculated based on it. The
expression doing this is very simple. Something like round_up(ngpios,
8);

...

> > > +               ret = device_property_read_u32(dev, "ngpios", &ngpios);
> > > +               if (ret) {
> > > +                       chip_err(gc, "Failed to get ngpios property\n");
> > > +                       return -EINVAL;
> > > +               }
> >
> > This is not an equivalent to what was in the GPIO library. Why is it so?
>
> Sure. I will keep it the same in my next patch.

No, you should take care about error codes properly. Now you are
shadowing anything to -EINVAL. With this you must keep the comment in
the code untouched (moved, but untouched).

> The reason I didn’t is because I noticed that the final result of the logic is the same i.e. " goto err_free_dev_name"
> "if(ret == -ENODATA)" is handled separately is to add an informative message: chip_err(gc, "tried to insert a GPIO chip with zero lines\n"); and return ret = -EINVAL.

Yes, but you missed one out of three cases.

> > > +               gc->ngpio = ngpios;
> > > +       }

...

> > >                 pr_err("%s: GPIOs %d..%d (%s) failed to register, %d\n", __func__,
> > > -                      base, base + (int)ngpios - 1,
> > > +                      base, base + (int)gc->ngpio - 1,
> > >                        gc->label ? : "generic", ret);
> >
> > AFAIU this will give a different result to what was previous in one of the error
> > cases.
>
> this one provides the "local" gpio pin id i.e. 0->31 for example.
> chip_warn(gc, "line cnt %u is greater than fast path cnt %u\n", gc->ngpio, FASTPATH_NGPIO);
>
> while this one provides the "global" gpio pin id. when bgpio_init sets the base : gc->base = -1; and gpiochip_add_data_with_key applies this logic:
> pr_err("%s: GPIOs %d..%d (%s) failed to register, %d\n", __func__, base, base + (int)gc->ngpio - 1,
> base = gc->base;
>          if (base < 0) {
>                  base = gpiochip_find_base(gc->ngpio);
> Then the base would be = GPIO_DYNAMIC_BASE
>
> Apologies if I misunderstood your question?

I'm talking about your change. It behaves differently in case of
different errors and contents of gc->ngpio and the ngpios local
variable.

...

Please, check again carefully all possible branches and cases (there
are few of them, not just a couple).

-- 
With Best Regards,
Andy Shevchenko