Re: [RFC PATCH 0/2] Clause-22/Clause-45 MDIO regmap support

From: Andy Shevchenko
Date: Mon Jun 07 2021 - 08:16:14 EST

On Mon, Jun 7, 2021 at 2:55 PM Mark Brown <broonie@xxxxxxxxxx> wrote:
> On Fri, Jun 04, 2021 at 08:16:53PM +0200, Sander Vanheule wrote:
> > On Fri, 2021-06-04 at 18:25 +0100, Mark Brown wrote:
> > > I think these registers are in practice going to either need to be
> > > volatile (how most of them work at the minute) or otherwise handled in
> > > regmap (eg, the page support we've got). Having two different names for
> > > the same register feels like it's asking for bugs if any of the higher
> > > level functions of regmap get used.
> > This is actually an issue with a GPIO chip that I'm trying to implement [1]. To
> > set an output, data is written to the register. To get an input value, data is
> > read from the register. Since a register contains data for 16 GPIO lines, a
> > regular read-modify-write could erroneously overwrite output values. A pin
> > outside of the RMW mask could've changed to an input, and may now be reading a
> > different value. The issue I was running into, had to do with a RMW not being
> > written because the pin value apparently hadn't changed.
> If the hardware isn't able to read back the status of the pins in output
> mode (even if it's always reading back from the input circuit where is
> it getting other inputs from?) you're probably better off with just
> having an open coded cache separately than trying to make up fake
> registers that rely on current implementation details to work.

Isn't it a disadvantage of regmap APIs? The hardware that uses the
same offset for R and W with different semantics is quite normal. I
think it is a good exercise to implement regmap-8250 as an example of
how to deal with such hardware.

With Best Regards,
Andy Shevchenko