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

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 5/27/21 12:38 PM, Andy Shevchenko wrote:
> +Cc: Hans
> 
> Hans, sorry for disturbing you later too much. Here we have "nice"
> hardware which can't be used in a glitch-free mode (somehow it reminds
> me lynxpoint, baytrail, cherryview designs). If you have any ideas to
> share (no need to dive deep or look at it if you have no time), you're
> welcome.

I'm afraid I've no ideas how to solve this nicely. Documenting the
issue might be the best we can do.

Regards,

Hans



> 
> On Thu, May 27, 2021 at 12:02 AM Sander Vanheule <sander@svanheule.net> wrote:
>>
>> On Tue, 2021-05-25 at 20:11 +0300, Andy Shevchenko wrote:
>>> On Mon, May 24, 2021 at 7:30 PM Andy Shevchenko
>>> <andy.shevchenko@gmail.com> wrote:
>>>> On Mon, May 24, 2021 at 6:03 PM Sander Vanheule <sander@svanheule.net>
>>>> wrote:
>>>>> On Mon, 2021-05-24 at 15:54 +0300, Andy Shevchenko wrote:
>>>
>>> ...
>>>
>>>>> Sadly, I don't. Most of the info we have comes from code archives of
>>>>> switch
>>>>> vendors (Zyxel, Cisco etc). Boards need to be reverse engineered, and the
>>>>> few
>>>>> leaked datasheets that can be found on the internet aren't exactly thick
>>>>> in
>>>>> information.
>>>>>
>>>>> The RTL8231 datasheet is actually quite useful, but makes no mention of
>>>>> the
>>>>> output value isse. Since this isn't an official resource, I don't think it
>>>>> would
>>>>> be appropriate to link it via a Datasheet: tag.
>>>>> https://github.com/libc0607/Realtek_switch_hacking/blob/files/RTL8231_Datasheet_
>>>>> 1.2.pdf
>>>>>
>>>>> Looking at the datasheet again, I came up with a... terrible hack to work
>>>>> around
>>>>> the output value issue.
>>>>>
>>>>> The chip also has GPIO_INVERT registers that I hadn't used until now,
>>>>> because
>>>>> the logical inversion is handled in the kernel. However, these inversion
>>>>> registers only apply to the output values. So, I could implement glitch-
>>>>> free
>>>>> output behaviour in the following way:
>>>>>  * After chip reset, and before enabling the output driver (MFD
>>>>> initialisation):
>>>>>     - Mux all pins as GPIO
>>>>>     - Change all pins to outputs,
>>>>
>>>> No. no, no. This is much worse than the glitches. You never know what
>>>> the hardware is connected there and it's potential breakage (on hw
>>>> level) possible.
>>>>
>>>>>  so the data registers (0x1c-0x1e) become writable
>>>>>     - Write value 0 to all pins
>>>>>     - Change all pins to GPI to change them into high-Z
>>>>>  * In the pinctrl/gpio driver:
>>>>>     - Use data registers as input-only
>>>>>     - Use inversion register to determine output value (can be written any
>>>>> time)
>>>>>
>>>>> The above gives glitch-free outputs, but the values that are read back
>>>>> (when
>>>>> configured as output), come from the data registers. They should now be
>>>>> coming
>>>>> from the inversion (reg_set_base) registers, but the code prefers to use
>>>>> the
>>>>> data registers (reg_dat_base).
>>>>
>>>> Lemme read the datasheet and see if I find any clue for the hw behaviour.
>>>
>>> Thank you for your patience!
>>>
>>> Have you explored the possibility of using En_Sync_GPIO?
>>
>> Got around to testing things.
>>
>> If En_Sync_GPIO is enabled, it's still possible to change the pin direction
>> without also writing the Sync_GPIO bit. So even with the latching, glitches are
>> still produced.
>>
>> As long as Sync_GPIO is not set to latch the new values, it also appears that
>> reads of the data registers result in the current output value, not the new one.
>>
>> As a different test, I've added a pull-down, to make the input level low. Now I
>> see the opposite behaviour as before (with set-value-before-direction):
>>  * OUT-HIGH > IN (low) > OUT-LOW: results in a high level (i.e. old value)
>>  * OUT-HIGH > IN (low) > OUT-HIGH: results in a high level (new/old value)
>>  * OUT-LOW > IN (low) > OUT-HIGH: results in a high level (new value, or toggled
>>    old value?)
>>  * OUT-LOW > IN (low) > OUT-LOW: results in a low level (new/old value)
>>
>> For reference, with a pull-up:
>>  * OUT-HIGH > IN (high) > OUT-HIGH: high result
>>  * OUT-HIGH > IN (high) > OUT-LOW: low result
>>  * OUT-LOW > IN (high) > OUT-HIGH: low result
>>  * OUT-LOW > IN (high) > OUT-LOW: low result
>>
>> I've only tested this with the sysfs interface, so I don't know what the result
>> would be on multiple writes to the data register (during input, but probably not
>> very relevant). Nor have I tested direction changes if the input has changed
>> between two output values.
>>
>> I may have some time tomorrow for more testing, but otherwise it'll have to wait
>> until the weekend. Any other ideas in the meantime?
> 
> No ideas so far. In x86 we used to have something similar (baytrail,
> cherryview, lynxpoint), but it's firmware assisted. I think that this
> hardware (realtek) is supposed either
> - to be firmware / bootloader assisted, so in a way that platform is
> preconfigured when Linux starts and any GPIO request won't be harmful
> as long as it doesn't change direction on the pins (which is usually
> guaranteed by DT and corresponding drivers to do the correct things)
> - be used for glitch-tolerant hardware (LEDs, for example, where
> nobody usually will noticed 1ms blink)
> 
> That said, I have not been convinced we have to quirk gpio-regmap for
> this one. Just describe the issues with hardware in the accompanying
> documentation.
> 
> But if maintainers or somebody comes with a better / different
> approach I am all ears.
>