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

[Sander Vanheule <sander@svanheule.net>]

Hi Michael, Andy,

On Fri, 2021-05-28 at 08:37 +0200, Michael Walle wrote:
> Am 2021-05-24 13:41, schrieb Sander Vanheule:
> > Hi Andy, Andrew,
> > 
> > On Mon, 2021-05-24 at 10:53 +0300, Andy Shevchenko wrote:
> > > On Mon, May 24, 2021 at 4:11 AM Andrew Lunn <andrew@lunn.ch> wrote:
> > > > 
> > > > > Changes since v2:
> > > > >   - MDIO regmap support was merged, so patch is dropped here
> > > > 
> > > > Do you have any idea how this will get merged. It sounds like one of
> > > > the Maintainers will need a stable branch of regmap.
> > > 
> > > This is not a problem if Mark provides an immutable branch to pull 
> > > from.
> > 
> > Mark has a tag (regmap-mdio) for this patch:
> > https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git/tag/?h=regmap-mdio
> > 
> > > 
> > > > >   - Introduce GPIO regmap quirks to set output direction first
> > > > 
> > > > I thought you had determined it was possible to set output before
> > > > direction?
> > > 
> > > Same thoughts when I saw an updated version of that patch. My
> > > anticipation was to not see it at all.
> > 
> > The two devices I've been trying to test the behaviour on are:
> >  * Netgear GS110TPP: has an RTL8231 with three LEDs, each driven via a 
> > pin
> >    configured as (active-low) GPIO. The LEDs are easy for a quick 
> > visual check.
> >  * Zyxel GS1900-8: RTL8231 used for the front panel button, and an 
> > active-low
> >    GPIO used to hard reset the main SoC (an RTL8380). I've modified 
> > this board
> >    to change some of the strapping pin values, but testing with the 
> > jumpers and
> >    pull-up/down resistors is a bit more tedious.
> > 
> > On the Netgear, I tested the following with and without the quirk:
> > 
> >    # Set as OUT-LOW twice, to avoid the quirk. Always turns the LED on
> >    gpioset 1 32=0; gpioset 1 32=0
> >    # Get value to change to input, turns the LED off (high impedance)
> >    # Will return 1 due to (weak) internal pull-up
> >    gpioget 1 32
> >    # Set as OUT-HIGH, should result in LED off
> >    # When the quirk is disabled, the LED turns on (i.e. old OUT-LOW 
> > value)
> >    # When the quirk is enabled, the LED remains off (i.e. correct
> > OUT-HIGH value)
> >    gpioset 1 32=1
> > 
> > Now, what's confusing (to me) is that the inverse doesn't depend on the 
> > quirk:
> > 
> >    # Set as OUT-HIGH twice
> >    gpioset 1 32=1; gpioset 1 32=1
> >    # Change to high-Z
> >    gpioget 1 32
> >    # Set to OUT-LOW, always results in LED on, with or without quirk
> >    gpioset 1 32=0
> > 
> > Any idea why this would be (or appear) broken on the former case, but 
> > not on the
> > latter?
> 
> Before reading this, I'd have guessed that they switch the internal 
> register
> depending on the GPIO direction; I mean there is only one register 
> address
> for both the input and the output register. Hm.
> 
> Did you try playing around with raw register accesses and see if the 
> value
> of the GPIO data register is changing when you switch GPIOs to 
> input/output.
> 
> Eg. you could try https://github.com/kontron/miitool to access the 
> registers
> from userspace (your ethernet controller has to have support for the 
> ioctl's
> though, see commit a613bafec516 ("enetc: add ioctl() support for 
> PHY-related
> ops") for an example).

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?


Best,
Sander