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! > > > > 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). -- With Best Regards, Andy Shevchenko
