linux-kernel - Re: [PATCH] drm/mediatek: Add no pending_planes flag checking for mtk_crtc_finish_page

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Message-ID: <a92217eb88afae8ce947be8f064e9c96ca568497.camel@mediatek.com>
Date: Tue, 19 Nov 2024 06:33:00 +0000
From: Jason-JH Lin (林睿祥) <Jason-JH.Lin@...iatek.com>
To: CK Hu (胡俊光) <ck.hu@...iatek.com>,
	"chunkuang.hu@...nel.org" <chunkuang.hu@...nel.org>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>,
	Singo Chang (張興國) <Singo.Chang@...iatek.com>,
	Shawn Sung (宋孝謙) <Shawn.Sung@...iatek.com>,
	Nancy Lin (林欣螢) <Nancy.Lin@...iatek.com>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	Project_Global_Chrome_Upstream_Group
	<Project_Global_Chrome_Upstream_Group@...iatek.com>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "fshao@...omium.org"
	<fshao@...omium.org>, AngeloGioacchino Del Regno
	<angelogioacchino.delregno@...labora.com>
Subject: Re: [PATCH] drm/mediatek: Add no pending_planes flag checking for
 mtk_crtc_finish_page_flip()

On Mon, 2024-11-18 at 08:20 +0000, CK Hu (胡俊光) wrote:
> Hi, Jason:
> 
> On Mon, 2024-11-18 at 11:36 +0800, Jason-JH.Lin wrote:
> > mtk_crtc_finish_page_flip() is used to notify the page flip that
> > has
> > been completed to the userspace, so userspace can free the frame
> > buffer
> > of the last frame and commit the next frame.
> > 
> > However, mtk_crtc_finish_page_flip() cannot guarantee the GCE has
> > configured all the display hardware settings of the last frame.
> > This may cause the display hardware to still access the last frame
> > buffer released by the userspace.
> > 
> > So add the checking condition of no pending_plens flags to make
> > sure
> > GCE has executed all the pending planes configuration.
> 
> What's the probability?
> If it's low probability, we could drop calling
> mtk_crtc_finish_page_flip().
> If it's not low probability, I think drop calling
> mtk_crtc_finish_page_flip() would reduce the fps and we should find
> another way.

It won't happen in display normal flow, which uses the
mbox_send_message() in IRQ context.
But it's easy to reproduce in display SVP flow, which uses the
mbox_send_message() to queue a work to the workqueue out of the IRQ
context for communicating with TEE world.

In mediaTek hardware design of configuring display hardware by GCE, I
think we should use mtk_crtc_finish_page_flip() to notify 
`DRM_EVENT_FLIP_COMPLETED` to userspace when GCE finished configuring
all display hardware settings for each atomic_commit().

Although it's still no problem in display normal flow without this
modification.
But I believe we can increase the accuracy of the timing to notify
`DRM_EVENT_FLIP_COMPLETED` to userspace, if we called
mtk_crtc_finished_page_flip() when the all the display hardware
settings is configured.

Maybe this patch is not quite straightforward to express the meaning of
calling mtk_crtc_finish_page_flip() after GCE completing all the
display configuration.
So after I verifying it in my local environment with SVP flow, I'll
move mtk_crtc_finish_page_flip() to the ddp_cmdq_cb() directly and send
the v2.

Regards,
Jason-JH.Lin

> 
> Regards,
> CK
>