Andy Shevchenko <andy.shevchenko@xxxxxxxxx> writes: > 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? Maybe there needs to be some explanation of what the typical use case is and why you'd choose write_bits() over update_bits(), because the more I think about it the less clear it is. You're right that the read could be served from a cache. But I'm not sure if a cache would be safe if even one bit in the register is volatile, and I can't really see a use case for write_bits() that doesn't involve volatile behavior of some sort. In any event, I'm just going to drop this patch and the related driver patches in favor of removing mask_writeonly entirely, since it looks like it was never used, and after thinking about it I'm not sure what I did helps much. If some driver needs write_bits() for mask registers down the road it's not a big deal to add this back. >> 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);
