linux-kernel - Re: [Freedreno] [PATCH] drm/msm/dpu: always program dsc active bits

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Message-ID: <82bf6167-d621-1a4e-86f0-7a8567347722@quicinc.com>
Date:   Fri, 14 Apr 2023 10:57:45 -0700
From:   Abhinav Kumar <quic_abhinavk@...cinc.com>
To:     Marijn Suijten <marijn.suijten@...ainline.org>
CC:     <freedreno@...ts.freedesktop.org>, <quic_sbillaka@...cinc.com>,
        <dianders@...omium.org>, <airlied@...il.com>,
        <andersson@...nel.org>, <robdclark@...il.com>,
        <dri-devel@...ts.freedesktop.org>, <swboyd@...omium.org>,
        <vkoul@...nel.org>, <agross@...nel.org>, <daniel@...ll.ch>,
        <linux-arm-msm@...r.kernel.org>, <dmitry.baryshkov@...aro.org>,
        Kuogee Hsieh <quic_khsieh@...cinc.com>, <sean@...rly.run>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [Freedreno] [PATCH] drm/msm/dpu: always program dsc active bits



On 4/14/2023 10:34 AM, Marijn Suijten wrote:
> On 2023-04-14 08:48:43, Abhinav Kumar wrote:
>>
>> On 4/14/2023 12:35 AM, Marijn Suijten wrote:
>>> On 2023-04-12 10:33:15, Abhinav Kumar wrote:
>>> [..]
>>>>> What happens if a device boots without DSC panel connected?  Will
>>>>> CTL_DSC_FLUSH be zero and not (unnecessarily, I assume) flush any of the
>>>>> DSC blocks?  Or could this flush uninitialized state to the block?
>>>>>
>>>>
>>>> If we bootup without DSC panel connected, the kernel's cfg->dsc will be
>>>> 0 and default register value of CTL_DSC_FLUSH will be 0 so it wont flush
>>>> any DSC blocks.
>>>
>>> Ack, that makes sense.  However, if I connect a DSC panel, then
>>> disconnect it (now the register should be non-zero, but cfg->dsc will be
>>> zero), and then replug a non-DSC panel multiple times, it'll get flushed
>>> every time because we never clear CTL_DSC_FLUSH after that?
>>>
>>
>> If we remove it after kernel starts, that issue is there even today
>> without that change because DSI is not a hot-pluggable display so a
>> teardown wont happen when you plug out the panel. How will cfg->dsc be 0
>> then? In that case, its not a valid test as there was no indication to
>> DRM that display was disconnected so we cannot tear it down.
> 
> The patch description itself describes hot-pluggable displays, which I
> believe is the upcoming DSC support for DP?  You ask how cfg->dsc can
> become zero, but this is **exactly** what the patch description
> describes, and what this patch is removing the `if` for.  If we are not
> allowed to discuss that scenario because it is not currently supported,
> neither should we allow to apply this patch.
> 
> With that in mind, can you re-answer the question?
> 

I didnt follow what needs to be re-answered.

This patch is being sent in preparation of the DSC over DP support. This 
does not handle non-hotpluggable displays. I do not think dynamic switch 
between DSC and non-DSC of non-hotpluggable displays needs to be 
discussed here as its not handled at all with or without this patch.

We wanted to get early reviews on the patch. If you want this patch to 
be absorbed when rest of DSC over DP lands, I have no concerns with 
that. I wont pick this up for fixes and we will land this together with 
the rest of DP over DSC.


> - Marijn