linux-kernel - Re: [PATCH v2] drm/msm/dpu: Filter modes based on adjusted mode clock

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Message-ID: <caf44ce9-48ec-45b4-b633-3a49b7705b1e@packett.cool>
Date: Mon, 12 Jan 2026 04:54:28 -0300
From: Val Packett <val@...kett.cool>
To: Xilin Wu <sophon@...xa.com>,
 Jessica Zhang <jessica.zhang@....qualcomm.com>,
 Rob Clark <robdclark@...il.com>, Dmitry Baryshkov <lumag@...nel.org>,
 Sean Paul <sean@...rly.run>, Marijn Suijten <marijn.suijten@...ainline.org>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>
Cc: Abhinav Kumar <quic_abhinavk@...cinc.com>, linux-arm-msm@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, Neil Armstrong <neil.armstrong@...aro.org>,
 Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Subject: Re: [PATCH v2] drm/msm/dpu: Filter modes based on adjusted mode clock


On 1/12/26 3:31 AM, Xilin Wu wrote:
> On 5/7/2025 9:38 AM, Jessica Zhang wrote:
>> Filter out modes that have a clock rate greater than the max core clock
>> rate when adjusted for the perf clock factor
>>
>> This is especially important for chipsets such as QCS615 that have lower
>> limits for the MDP max core clock.
>>
>> Since the core CRTC clock is at least the mode clock (adjusted for the
>> perf clock factor) [1], the modes supported by the driver should be less
>> than the max core clock rate.
>>
>> [1] 
>> https://elixir.bootlin.com/linux/v6.12.4/source/drivers/gpu/drm/msm/disp/dpu1/dpu_core_perf.c#L83
>>
>> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
>> Signed-off-by: Jessica Zhang <jessica.zhang@....qualcomm.com>
>> ---
>
> Hi. This patch effectively filters out the 3840x2160@...Hz mode on 
> SC8280XP CRD. The calculated adjusted_mode_clk is 623700, which 
> slightly exceeds the supported max core clock of 600000.
>
> However, 4K 120Hz works flawlessly with the limit removed on this 
> platform. I even tried connecting two 4K 120Hz displays, and they can 
> work properly simultaneously. Is it possible to bring back support for 
> this mode, or adjust the limits? 

hm, interestingly on X1E80100 we didn't hit *that* limit, 
the adjusted_mode_clk (576318) was only above what disp_cc_mdss_mdp_clk 
was set to (575000), and reducing the clk_inefficiency_factor from 105 
to 104 was enough to lower it.

https://gitlab.freedesktop.org/drm/msm/-/issues/38#note_3216051

I guess it's also sink dependent, like if the mode for some monitors has 
much more front/back porch etc.? What's the entire modeline that 
resulted in 623700?

~val