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

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Message-ID: <8c3a210a-200b-eb1d-e39a-8aad15d790f4@quicinc.com>
Date:   Fri, 14 Apr 2023 16:51:52 -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 4:11 PM, Marijn Suijten wrote:
> On 2023-04-14 10:57:45, Abhinav Kumar wrote:
>> 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.
> 
> Good, because my question is specifically about *hotpluggable*
> displays/panels like the upcoming DSC support for DP.  After all there
> would be no point in me suggesting to connect and disconnect
> non-hotpluggable displays and expect something sensible to happen,
> wouldn't it?  Allow me to copy-paste the question again for convenience,
> with some minor wording changes:
> 
> 	However, if I connect a DSC DP display, then disconnect it (now the
> 	register should be non-zero, but cfg->dsc will be zero), and then
> 	connect and reconnect a non-DSC DP display multiple times, it'll get
> 	flushed every time because we never clear CTL_DSC_FLUSH after that?
> 
> And the missing part is: would multiple flushes be harmful in this case?

Well, you kept asking about "DSC panel" , that made me think you were 
asking about a non-hotpluggable MIPI DSI DSC panel and not DP DSC 
monitor. On many boards, panels can be removed/connected back to their 
daughter card. The term "panel" confused me a bit.

Now answering your question.

Yes, it will get flushed once every hotplug thats not too bad but 
importantly DSC wont be active as CTL_DSC_ACTIVE will be set to 0 so it 
wont cause any issue.


>> 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.
> 
> I don't mind when and where this lands, just want to have the semantics
> clear around persisting the value of CTL_DSC_FLUSh in the register.
> 
> Regardless, this patch doesn't sound like a fix but a workaround until
> reset_intf_cfg() is fixed to be called at the right point, and extended
> to clear CTL_DSC_ACTIVE and flush the DSCs.  Perhaps it shouldn't have a
> Fixes: tag for that reason, as you intend to reinstate this
> if (cfg->dsc) condition when that is done?
> 

Its certainly fixing the use-case of DSC to non-DSC switching. So it is 
a fix.

But yes not the best fix possible. We have to improve it by moving this 
to reset_intf_cfg() as I already committed to.


> - Marijn