Re: [PATCH v3 0/6] RTL8231 GPIO expander support

[Michael Walle]

Am 2021-05-30 20:16, schrieb Andy Shevchenko:
> On Sun, May 30, 2021 at 7:51 PM Hans de Goede <hdegoede@xxxxxxxxxx> 
> wrote:
> > On 5/30/21 6:19 PM, Sander Vanheule wrote:
> > > On Fri, 2021-05-28 at 08:37 +0200, Michael Walle wrote:
>
> ...
>
> > > I think I found a solution!

nice!

> > > As Michael suggested, I tried raw register reads and writes, to eliminate any
> > > side effects of the intermediate code. I didn't use the ioctls (this isn't a
> > > netdev), but I found regmap's debugfs write functionality, which allowed me to
> > > do the same.
> > >
> > > I was trying to reproduce the behaviour I reported earlier, but couldn't. The
> > > output levels were always the intended ones. At some point I realised that the
> > > regmap_update_bits function does a read-modify-write, which might shadow the
> > > actual current output value.
> > > For example:
> > >  * Set output low: current out is low
> > >  * Change to input with pull-up: current out is still low, but DATAx reads high
> > >  * Set output high: RMW reads a high value (the input), so assumes a write is
> > >    not necessary, leaving the old output value (low).
> > >
> > > Currently, I see two options:
> > >  * Use regmap_update_bits_base to avoid the lazy RMW behaviour
> > >  * Add a cache for the output data values to the driver, and only use these
> > >    values to write to the output registers. This would allow keeping lazy RMW
> > >    behaviour, which may be a benefit on slow busses.
> > >
> > > With either of these implemented, if I set the output value before the
> > > direction, everything works! :-)
> > >
> > > Would you like this to be added to regmap-gpio, or should I revert back to a
> > > device-specific implementation?
> >
> > Regmap allows you to mark certain ranges as volatile, so that they 
> > will not
> > be cached, these GPIO registers containing the current pin value seems 
> > like
> > a good candidate for this. This is also necessary to make reading the 
> > GPIO
> > work without getting back a stale, cached value.
>
> After all it seems a simple missed proper register configuration in
> the driver for regmap.
> Oh, as usual something easy-to-solve requires tons of time to find it. 
> :-)
>
> Sander, I think you may look at gpio-pca953x.c to understand how it
> works (volatility of registers).

But as far as I see is the regmap instantiated without a cache?

-michael