On Tuesday, June 21, 2022, Aidan MacDonald <aidanmacdonald.0x0@xxxxxxxxx> wrote: > > 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. Okay, it makes a lot of sense for W1C type of bits in the register. Also, “reading” might imply to restore last value from cache, no? > > 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); > -- With Best Regards, Andy Shevchenko
