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From:   Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
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Subject: Re: [PATCH 15/49] regmap-irq: Change the behavior of mask_writeonly
In-reply-to: <CAHp75Vd7Sq9RMqin_y-8qUEAJLaGfuqxAbe+qcMB22=bqkyZqg@mail.gmail.com>
Date:   Tue, 21 Jun 2022 22:13:03 +0100
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Andy Shevchenko <andy.shevchenko@gmail.com> writes:

> On Mon, Jun 20, 2022 at 10:08 PM Aidan MacDonald
> <aidanmacdonald.0x0@gmail.com> wrote:
>>
>> No drivers currently use mask_writeonly, and in its current form
>> it seems a bit misleading. When set, mask registers will be
>> updated with regmap_write_bits() instead of regmap_update_bits(),
>> but regmap_write_bits() still does a read-modify-write under the
>> hood. It's not a write-only operation.
>>
>> Performing a simple regmap_write() is probably more useful, since
>> it can be used for chips that have separate set & clear registers
>> for controlling mask bits. Such registers are normally volatile
>> and read as 0, so avoiding a register read minimizes bus traffic.
>
> Reading your explanations and the code, I would rather think about
> fixing the regmap_write_bits() to be writeonly op.

That's impossible without special hardware support.

> Otherwise it's unclear what's the difference between
> regmap_write_bits() vs. regmap_update_bits().

This was not obvious to me either. They're the same except in how they
issue the low-level write op -- regmap_update_bits() will only do the
write if the new value differs from the current one. regmap_write_bits()
will always do a write, even if the new value is the same.

I think the problem is lack of documentation. I only figured this out
by reading the implementation.

>>         if (d->chip->mask_writeonly)
>> -               return regmap_write_bits(d->map, reg, mask, val);
>> +               return regmap_write(d->map, reg, val & mask);
>>         else
>>                 return regmap_update_bits(d->map, reg, mask, val);