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Date: Thu, 23 Jun 2022 21:54:37 +0100 From: Aidan MacDonald <aidanmacdonald.0x0@...il.com> To: Andy Shevchenko <andy.shevchenko@...il.com> Cc: Mark Brown <broonie@...nel.org>, Andy Gross <agross@...nel.org>, Bjorn Andersson <bjorn.andersson@...aro.org>, Srinivas Kandagatla <srinivas.kandagatla@...aro.org>, Banajit Goswami <bgoswami@...eaurora.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, Chanwoo Choi <cw00.choi@...sung.com>, Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>, Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>, MyungJoo Ham <myungjoo.ham@...sung.com>, Michael Walle <michael@...le.cc>, Linus Walleij <linus.walleij@...aro.org>, Bartosz Golaszewski <brgl@...ev.pl>, Thomas Gleixner <tglx@...utronix.de>, Marc Zyngier <maz@...nel.org>, Lee Jones <lee.jones@...aro.org>, Manivannan Sadhasivam <mani@...nel.org>, Cristian Ciocaltea <cristian.ciocaltea@...il.com>, Chen-Yu Tsai <wens@...e.org>, "tharvey@...eworks.com" <tharvey@...eworks.com>, "rjones@...eworks.com" <rjones@...eworks.com>, Matti Vaittinen <mazziesaccount@...il.com>, "orsonzhai@...il.com" <orsonzhai@...il.com>, "baolin.wang7@...il.com" <baolin.wang7@...il.com>, "zhang.lyra@...il.com" <zhang.lyra@...il.com>, Jernej Skrabec <jernej.skrabec@...il.com>, Samuel Holland <samuel@...lland.org>, Liam Girdwood <lgirdwood@...il.com>, Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>, "linux-actions@...ts.infradead.org" <linux-actions@...ts.infradead.org>, linux-arm-msm <linux-arm-msm@...r.kernel.org>, linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>, "linux-sunxi@...ts.linux.dev" <linux-sunxi@...ts.linux.dev>, ALSA Development Mailing List <alsa-devel@...a-project.org> Subject: Re: [PATCH 15/49] regmap-irq: Change the behavior of mask_writeonly Andy Shevchenko <andy.shevchenko@...il.com> writes: > On Tuesday, June 21, 2022, Aidan MacDonald <aidanmacdonald.0x0@...il.com> wrote: >> Andy Shevchenko <andy.shevchenko@...il.com> writes: >> >> > On Mon, Jun 20, 2022 at 10:08 PM Aidan MacDonald >> > <aidanmacdonald.0x0@...il.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. > > 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);
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