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

Il 12/06/23 21:19, Stephen Boyd ha scritto:

Quoting AngeloGioacchino Del Regno (2023-06-09 00:42:13)

quoted

Il 09/06/23 01:56, Stephen Boyd ha scritto:

quoted

Quoting AngeloGioacchino Del Regno (2023-06-08 02:01:58)

quoted

Il 08/06/23 10:12, Chen-Yu Tsai ha scritto:

quoted

On Thu, Jun 8, 2023 at 4:57 AM Nícolas F. R. A. Prado
[off-list ref] wrote:

quoted

diff --git a/drivers/media/platform/mediatek/vcodec/mtk_vcodec_dec_drv.c b/drivers/media/platform/mediatek/vcodec/mtk_vcodec_dec_drv.c
index 9c652beb3f19..8038472fb67b 100644
--- a/drivers/media/platform/mediatek/vcodec/mtk_vcodec_dec_drv.c
+++ b/drivers/media/platform/mediatek/vcodec/mtk_vcodec_dec_drv.c

AFAIK this is still around for clk drivers that haven't moved to clk_hw.
It shouldn't be used by clock consumers. Would it be better to just pass
a syscon?

This is a legit usage of __clk_is_enabled().... because that's what we're really
doing here, we're checking if a clock got enabled by the underlying MCU (as that
clock goes up after the VDEC boots).

If this is *not* acceptable as it is, we will have to add a clock API call to
check if a clock is enabled... but it didn't seem worth doing since we don't
expect anyone else to have any legit usage of that, or at least, we don't know
about anyone else needing that...

The design of the clk.h API has been that no clk consumer should need to
find out if a clk is enabled. Instead, the clk consumer should enable
the clk if they want it enabled. Is there no other way to know that the
vcodec hardware is active?

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.

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?

I'm in doubt now on how to proceed.

----

I see that the __clk_is_enabled() API is being used in some other
consumer drivers. I think at one point we were down to one or two users.
I'll try to remove this function entirely, but it will still be possible
to get at the clk_hw for a clk with __clk_get_hw() and then call
clk_hw_is_enabled().

Makes sense.

Regards,
Angelo

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