linux-kernel - Re: [PATCH 2/3] drm/format-helper: Add drm_fb_xrgb8888_to_xrgb2101010

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Message-ID: <20211122123938.47fcbef0@eldfell>
Date:   Mon, 22 Nov 2021 12:39:38 +0200
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Hector Martin <marcan@...can.st>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] drm/format-helper: Add
 drm_fb_xrgb8888_to_xrgb2101010_dstclip()

On Mon, 22 Nov 2021 19:05:16 +0900
Hector Martin <marcan@...can.st> wrote:

> On 22/11/2021 18.52, Pekka Paalanen wrote:
> > On Wed, 17 Nov 2021 23:58:28 +0900
> > Hector Martin <marcan@...can.st> wrote:
> >   
> >> Add XRGB8888 emulation support for devices that can only do XRGB2101010.
> >>
> >> This is chiefly useful for simpledrm on Apple devices where the
> >> bootloader-provided framebuffer is 10-bit, which already works fine with
> >> simplefb. This is required to make simpledrm support this too.
> >>
> >> Signed-off-by: Hector Martin <marcan@...can.st>
> >> ---
> >>   drivers/gpu/drm/drm_format_helper.c | 64 +++++++++++++++++++++++++++++
> >>   include/drm/drm_format_helper.h     |  4 ++
> >>   2 files changed, 68 insertions(+)  
> > 
> > Hi Hector,
> > 
> > I'm curious, since the bootloader seems to always set up a 10-bit mode,
> > is there a reason for it that you can guess? Is the monitor in WCG or
> > even HDR mode?  
> 
> My guess is that Apple prefer to use 10-bit framebuffers for seamless 
> handover with their graphics stack, which presumably uses 10-bit 
> framebuffers these days. It seems to be unconditional; I've never seen 
> anything but 10 bits across all Apple devices, both with the internal 
> panels on laptops and with bog standard external displays on the Mac 
> Mini via HDMI. HDR is not necessary, even very dumb capture cards and 
> old screens get a 10-bit framebufer in memory.

That makes perfect sense, thanks!

Switching between sRGB and WCG or HDR mode is not a modeset, it's just
HDMI/DP/whatever metadata/infoframe.

> The only time I see an 8-bit framebuffer is with *no* monitor connected 
> on the Mini, in which case you get an 8-bit 640x1136 dummy framebuffer 
> (that's the iPhone 5 screen resolution... :-) )
> 

Thanks,
pq

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