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

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Date:   Fri, 9 Jun 2023 09:42:13 +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 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:
>>>> 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
>>>> @@ -16,6 +16,7 @@
>>>>    #include <media/v4l2-mem2mem.h>
>>>>    #include <media/videobuf2-dma-contig.h>
>>>>    #include <media/v4l2-device.h>
>>>> +#include <linux/clk-provider.h>
>>>
>>>                      ^^^^^^^^^^^^^^
>>>
>>> This seems like a violation of the API separation.
> 
> Yes.
> 
>>>
>>>>    #include "mtk_vcodec_drv.h"
>>>>    #include "mtk_vcodec_dec.h"
>>>> @@ -38,22 +39,29 @@ static int mtk_vcodec_get_hw_count(struct mtk_vcodec_dev *dev)
>>>>           }
>>>>    }
>>>>
>>>> +static bool mtk_vcodec_is_hw_active(struct mtk_vcodec_dev *dev)
>>>> +{
>>>> +       u32 cg_status = 0;
>>>> +
>>>> +       if (!dev->reg_base[VDEC_SYS])
>>>> +               return __clk_is_enabled(dev->pm.vdec_active_clk);
>>>
>>> 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.

That's *synthetically* the whole story...

>>
>> As for the syscon, that's something that we've been discussing as well... the
>> thing is: we're really *really* checking if a clock is enabled, so we should
>> be using clock related calls... reading from a syscon means that we'd have to
>> perform a register read (of.. again.. a clock) outside of the clock framework
>> which, in my opinion, wouldn't be clean; I'd expect that to become a bit messy
>> in the future too, should more MediaTek SoCs (I think MT8192/95 are already in
>> the list, Nicolas please correct me if I'm wrong here) need the same thing, as
>> we'd be adding more definitions around.
>>