linux-kernel - Re: [PATCH v4 1/1] drm/mediatek: Adjust bandwidth limit for DP

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Message-ID: <667a201e7893d6d4bf9ae0267a5afaf06faa5db8.camel@mediatek.com>
Date: Fri, 27 Jun 2025 01:32:18 +0000
From: CK Hu (胡俊光) <ck.hu@...iatek.com>
To: Project_Global_Chrome_Upstream_Group
	<Project_Global_Chrome_Upstream_Group@...iatek.com>,
	"chunkuang.hu@...nel.org" <chunkuang.hu@...nel.org>, "simona@...ll.ch"
	<simona@...ll.ch>, AngeloGioacchino Del Regno
	<angelogioacchino.delregno@...labora.com>, "airlied@...il.com"
	<airlied@...il.com>, Mac Shen (沈俊)
	<Mac.Shen@...iatek.com>, "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
	"matthias.bgg@...il.com" <matthias.bgg@...il.com>, Nicolas Prado
	<nfraprado@...labora.com>, Peng Liu (刘鹏)
	<Peng.Liu@...iatek.com>, LIANKUN YANG (杨连坤)
	<Liankun.Yang@...iatek.com>
CC: "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/1] drm/mediatek: Adjust bandwidth limit for DP

On Wed, 2025-06-25 at 14:49 -0400, Nícolas F. R. A. Prado wrote:
> External email : Please do not click links or open attachments until you have verified the sender or the content.
> 
> 
> On Wed, 2025-06-25 at 17:54 +0800, Liankun Yang wrote:
> > By adjusting the order of link training and relocating it to HPD,
> > link training can identify the usability of each lane in the current
> > link.
> > 
> > It also supports handling signal instability and weakness due to
> > environmental issues, enabling the acquisition of a stable bandwidth
> > for the current link. Subsequently, DP work can proceed based on
> > the actual maximum bandwidth.
> > 
> > It should training in the hpd event thread.
> > Check the mode with lane count and link rate of training.
> > 
> > If we're eDP and capabilities were already parsed we can skip
> > reading again because eDP panels aren't hotpluggable hence the
> > caps and training information won't ever change in a boot life
> > 
> > Therefore, bridge typec judgment is required for edp training in
> > atomic_enable function.
> > 
> > Signed-off-by: Liankun Yang <liankun.yang@...iatek.com>
> > ---
> > Change in V4:
> > - Tested the internal eDP display on MT8195 Tomato and it is fine.
> > Per suggestion from the previous thread:
> > https://urldefense.com/v3/__https://patchwork.kernel.org/project/linux-mediatek/patch/20250318140236.13650-2-liankun.yang@mediatek.com/__;!!CTRNKA9wMg0ARbw!g0Ii2UaT9Wu1FktoGneG4gHV_zwIAwcqEihEA0W4XHKOW2vv7i5MLlekFB9AbbBJvWKvbyaiQFkF-JpPcq0$
> 
> Hi,
> 
> I tested this patch on MT8195 Tomato, on top of next-20250625. Indeed
> the internal eDP display is unaffected by this commit: it still works
> fine.
> 
> The external displays though not so much. I tested 3 different
> displays, using 2 different USBC-to-HDMI adapters, and in all cases the
> behavior was the same:
> - Before the patch, the image on the display is completely corrupted
> and unusable. The only discernible element on the display is the mouse
> cursor, which shows perfectly fine. Occasionally no image will be shown
> at all, but most of the times, the behavior is as described.
> - After the patch, nothing ever shows at all on the display. It is
> always black.
> 
> So while the external display support on Tomato is basically broken as
> of the latest next, this patch seems to regress the support even
> further.

Hi, Nicolas:

Have external display worked normally in previous version?
If no, I think this patch just let external display change from one bad state to another bad state.
If yes, I think we should fix this problem first, and then apply this patch to test.

Regards,
CK

> 
> --
> Thanks,
> 
> Nícolas