linux-kernel - Re: [PATCH 1/6] ASoC: wcd938x: switch to using gpiod API

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Date:   Thu, 20 Apr 2023 16:16:59 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Mark Brown <broonie@...nel.org>
Cc:     Vinod Koul <vkoul@...nel.org>,
        Bard Liao <yung-chuan.liao@...ux.intel.com>,
        Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        Sanyog Kale <sanyog.r.kale@...el.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Patrick Lai <quic_plai@...cinc.com>
Subject: Re: [PATCH 1/6] ASoC: wcd938x: switch to using gpiod API

On 20/04/2023 15:00, Mark Brown wrote:
> On Thu, Apr 20, 2023 at 02:30:17PM +0200, Krzysztof Kozlowski wrote:
>> On 20/04/2023 13:58, Mark Brown wrote:
>>> On Thu, Apr 20, 2023 at 12:16:12PM +0200, Krzysztof Kozlowski wrote:
> 
>>>> -	gpio_direction_output(wcd938x->reset_gpio, 0);
>>>> -	/* 20us sleep required after pulling the reset gpio to LOW */
>>>> +	gpiod_set_value_cansleep(wcd938x->reset_gpio, 1);
>>>> +	/* 20us sleep required after asserting the reset gpio */
> 
>>> This is inverting the sense of the GPIO in the API from active low to
>>> active high which will mean we're introducing a new reliance on having
>>> the signal described as active low in DT.  That's an ABI concern.
> 
>> It's bringing it to the correct level. Old code was not respecting the
>> DTS thus if such DTS came with inverted design, the driver would not work.
> 
> Sure, but OTOH if the user didn't bother specifying as active low it
> would work.  I suspect it's more likely that someone missed a flag that
> had no practical impact in DT than that someone would add an inverter to
> their design.
> 
>> We were already fixing the upstream DTS users and I thought all of them
>> are fixed since long time (half a year) or even correct from the
>> beginning. Now I found one more case with incorrect level, which I will fix.
> 
> That's just upstream, what about any downstream users?

Life of downstream. We all know the drill: merge your DTS or suffer. The
WCD938x codecs are moderately new, so I do not expect many downstream
users. They are in theory possible, because driver was merged in
v5.14-rc1 and for the newest products Qualcomm uses v5.15. Although now
it is v5.15, but the time driver was merged, maybe it was v5.10.

I could rework this patch to provide backwards compatible solution like
I did for WSA:
https://lore.kernel.org/all/20230102114152.297305-4-krzysztof.kozlowski@linaro.org/

There are downsides of it, but as you pointed out - it's actually very
rare to have the signal inverted in hardware.

> 
>>> I remain deeply unconvinced that remapping active low outputs like this
>>> in the GPIO API is helping.
> 
>> The code is mapping them to correct state. The previous state was
>> incorrect and did not allow to handle active high (which can happen).
>> This is the effort to make code correct - driver and DTS.
> 
> We could handle inversions through an explicit property if that were
> needed, that would be a less problematic transition and clearer in the
> consumer code.

I am not sure if it is worth. The DTS is supposed to describe hardware,
so even if reset pin flag was not effective, it is a mistake to describe
it as ACTIVE_HIGH. Do we care about keeping broken code happy? If yes,
then property is the way to go. If partially, then I can add
backwards-compatible approach like I mentioned above.

Best regards,
Krzysztof