Andy Shevchenko <andy.shevchenko@xxxxxxxxx> writes: > On Mon, Jun 20, 2022 at 10:08 PM Aidan MacDonald > <aidanmacdonald.0x0@xxxxxxxxx> 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);
