linux-kernel - Re: [PATCH 1/2] regmap: add regmap_might

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Message-ID: <20210506124342.GC4642@sirena.org.uk>
Date:   Thu, 6 May 2021 13:43:42 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Michael Walle <michael@...le.cc>
Cc:     linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: Re: [PATCH 1/2] regmap: add regmap_might_sleep()

On Sat, May 01, 2021 at 12:10:16AM +0200, Michael Walle wrote:
> Am 2021-04-30 19:26, schrieb Mark Brown:

> > But that's a driver for a specific device AFAICT which looks like it's
> > only got an I2C binding on the MFD so the driver knows that it's for a
> > device that's on a bus that's going to sleep and doesn't need to infer
> > anything?  This looks like the common case I'd expect where there's no
> > variation.

> You are right, at the moment this driver only has an I2C binding. But
> the idea was that this IP block and driver can be reused behind any
> kind of bridge; I2C, SPI or MMIO. Actually, I had the impression

Is this actually a way people are building hardware though?

> that all you need to do to convert it to MMIO is to replace the
> "kontron,sl28cpld" compatible with a "syscon" compatible. But it isn't
> that easy. Anyway, the idea is that you don't need to change anything
> in the gpio-sl28cpld driver, just change the parent. But if we can't
> ask the regmap what type it is, then we'll have to modify the
> gpio-sl28cpld driver and we will have to figure it out by some other
> means.

Well, you don't need to change anything at all - the driver will work
perfectly fine if it's flagging up the GPIOs as potentially sleeping
even if they end up not actually sleeping.

> > If users happen to end up with a map flagged as fast they can work on
> > the whatever driver uses this stuff and not realise they're breaking
> > other users of the same driver that end up with slow I/O.  The whole
> > point of the flag in GPIO is AIUI warnings to help with that case.

> Hm, but as of now, the only thing which makes the gpio-regmap driver
> slow i/o is the regmap itself.

Surely it's just a case of the device that's creating the gpio regmap
setting a flag when it instantiates it?  It's just one more thing that
the parent knows about the device.  This doesn't seem insurmountable.

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