linux-kernel - Re: [PATCH 1/3] gpio: regmap: Support registers with more than one bit per GPIO

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Date:   Mon, 04 Jul 2022 17:03:44 +0100
From:   Aidan MacDonald <aidanmacdonald.0x0@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Michael Walle <michael@...le.cc>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] gpio: regmap: Support registers with more than one
 bit per GPIO


Andy Shevchenko <andy.shevchenko@...il.com> writes:

> On Sun, Jul 3, 2022 at 1:11 PM Aidan MacDonald
> <aidanmacdonald.0x0@...il.com> wrote:
>>
>> Some devices use a multi-bit register field to change the GPIO
>> input/output direction. Add the ->reg_field_xlate() callback to
>> support such devices in gpio-regmap.
>>
>> ->reg_field_xlate() builds on ->reg_mask_xlate() by allowing the
>> driver to return a mask and values to describe a register field.
>> gpio-regmap will use the mask to isolate the field and compare or
>> update it using the values to implement GPIO level and direction
>> get and set ops.
>
> Thanks for the proposal. My comments below.
>
> ...
>
>> +static void
>> +gpio_regmap_simple_field_xlate(struct gpio_regmap *gpio,
>> +                              unsigned int base, unsigned int offset,
>> +                              unsigned int *reg, unsigned int *mask,
>> +                              unsigned int *values)
>> +{
>> +       gpio->reg_mask_xlate(gpio, base, offset, reg, mask);
>> +       values[0] = 0;
>> +       values[1] = *mask;
>
> This is a fragile and less compile-check prone approach. If you know
> the amount of values, make a specific data type for that, or pass the
> length of the output buffer..
>
>> +}
>
> ...
>
>> +       unsigned int values[2];
>
>> +       return (val & mask) == values[1];
>
>> +       unsigned int values[2];
>
> How will the callee know that it's only 2 available?
>
>
>> +       regmap_update_bits(gpio->regmap, reg, mask, values[!!val]);
>
> If we have special meaning of the values, perhaps it needs to follow
> an enum of some definitions, so everybody will understand how indices
> are mapped to the actual data in the array.
>
>> +       unsigned int values[2];
>
>> +       regmap_update_bits(gpio->regmap, reg, mask, values[1]);
>
>> +       unsigned int values[2];
>
>> +       if ((val & mask) == values[invert])
>
> How do you guarantee this won't overflow? (see above comment about
> indices mapping)
>
>> +       unsigned int values[2];
>
> As per above comments.

The documentation states that ->reg_field_xlate returns values[0] and
values[1] for low/high level or input/output direction. IOW, 0 is low
level / input direction and 1 is high level / output direction.

Embedding the array in a struct seems like a better idea though, thanks.