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

From: Stephen Boyd <sboyd@kernel.org>
To: "AngeloGioacchino Del Regno"
	<angelogioacchino.delregno@collabora.com>,
	"Chen-Yu Tsai" <wenst@chromium.org>,
	"Nícolas F. R. A. Prado" <nfraprado@collabora.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>,
	Hans Verkuil <hverkuil-cisco@xs4all.nl>,
	kernel@collabora.com,
	Andrew-CT Chen <andrew-ct.chen@mediatek.com>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Tiffany Lin <tiffany.lin@mediatek.com>,
	Yunfei Dong <yunfei.dong@mediatek.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
	linux-mediatek@lists.infradead.org
Subject: Re: [PATCH v2 3/5] media: mediatek: vcodec: Read HW active status from clock
Date: Thu, 15 Jun 2023 10:40:55 -0700	[thread overview]
Message-ID: <eee9a51a9fbe44f028ef33cd92c18a51.sboyd@kernel.org> (raw)
In-Reply-To: <76a641e6-64ef-a973-422c-de50d498e52f@collabora.com>

Quoting AngeloGioacchino Del Regno (2023-06-15 00:30:56)
> 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@collabora.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

You don't have to add the power domain in DT, do you? You can populate a
power domain in software directly?

> really looks like being the only viable option.
> 
> Stephen, many thanks for the valuable suggestions and the nice conversation.
>

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