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

[Hans de Goede]

Hi,

On 5/30/21 6:19 PM, Sander Vanheule wrote:
> 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@xxxxxxx> 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?

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.

Regards,

Hans