linux-kernel - Re: [PATCH v2 3/5] media: mediatek: vcodec: Read HW active status from clock

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Message-ID: <76a641e6-64ef-a973-422c-de50d498e52f@collabora.com>
Date:   Thu, 15 Jun 2023 09:30:56 +0200
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     Stephen Boyd <sboyd@...nel.org>, Chen-Yu Tsai <wenst@...omium.org>,
        Nícolas F. R. A. Prado 
        <nfraprado@...labora.com>
Cc:     Matthias Brugger <matthias.bgg@...il.com>,
        Hans Verkuil <hverkuil-cisco@...all.nl>, kernel@...labora.com,
        Andrew-CT Chen <andrew-ct.chen@...iatek.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tiffany Lin <tiffany.lin@...iatek.com>,
        Yunfei Dong <yunfei.dong@...iatek.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH v2 3/5] media: mediatek: vcodec: Read HW active status
 from clock

Il 15/06/23 02:40, Stephen Boyd ha scritto:
> Quoting AngeloGioacchino Del Regno (2023-06-14 01:13:43)
>> Il 12/06/23 21:19, Stephen Boyd ha scritto:
>>> Quoting AngeloGioacchino Del Regno (2023-06-09 00:42:13)
>>>> Il 09/06/23 01:56, Stephen Boyd ha scritto:
>>>>> Quoting AngeloGioacchino Del Regno (2023-06-08 02:01:58)
>>>>>> Il 08/06/23 10:12, Chen-Yu Tsai ha scritto:
>>>>>>> On Thu, Jun 8, 2023 at 4:57 AM Nícolas F. R. A. Prado
>>>>>>> <nfraprado@...labora.com> wrote:
>>>>
>>>> The firmware gives an indication of "boot done", but that's for the "core" part
>>>> of the vcodec... then it manages this clock internally to enable/disable the
>>>> "compute" IP of the decoder.
>>>>
>>>> As far as I know (and I've been researching about this) the firmware will not
>>>> give any "decoder powered, clocked - ready to get data" indication, and the
>>>> only way that we have to judge whether it is in this specific state or not is
>>>> to check if the "VDEC_ACTIVE" clock got enabled by the firmware.
>>>
>>> Is Linux ever going to use clk consumer APIs like clk_enable/clk_disable
>>> on this VDEC_ACTIVE clk? If the answer is no, then there isn't any
>>> reason to put it in the clk framework, and probably syscon is the way to
>>> go for now.
>>>
>>
>> Not for the current platform, but that may change in future SoCs... we're not sure.
> 
> If you're not using the clk consumer APIs then it shouldn't be a clk.
> 
>>
>>> Another approach could be to wait for some amount of time after telling
>>> firmware to power up and assume the hardware is active.
>>>
>>
>> That would be highly error prone though. Expecting that the HW is alive means that
>> we're 100% sure that both firmware and driver are doing the right thing at every
>> moment, which is something that we'd like to assume but, realistically, for safety
>> reasons we just don't.
>>
>> Should we anyway go for a syscon *now* and then change it to a clock later, if any
>> new platform needs this as a clock?
> 
> Yeah. Or implement this as a power domain and have it read the register
> directly waiting to return from the power_on()?

A power domain would force us to incorrectly describe the hardware in the bindings
though, I think... so, Nícolas, please, let's go for a syscon at this point, as it
really looks like being the only viable option.

Stephen, many thanks for the valuable suggestions and the nice conversation.

Cheers!
Angelo