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

Michael Walle
2021-05-30T21:22:27Z

Am 2021-05-30 20:16, schrieb Andy Shevchenko:
> On Sun, May 30, 2021 at 7:51 PM Hans de Goede <hdegoede@redhat.com>
> 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