linux-kernel - Re: [PATCH 1/8] fbdev: sm712fb: fix white screen of death on reboot, don't set CR3B-CR3F.

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Message-ID: <20190331121851.h3j2pud6yhelsknr@debian>
Date:   Sun, 31 Mar 2019 13:18:51 +0100
From:   Sudip Mukherjee <sudipm.mukherjee@...il.com>
To:     Yifeng Li <tomli@...li.me>
Cc:     Teddy Wang <teddy.wang@...iconmotion.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/8] fbdev: sm712fb: fix white screen of death on reboot,
 don't set CR3B-CR3F.

On Sun, Mar 17, 2019 at 06:24:57AM +0800, Yifeng Li wrote:
> On a Thinkpad s30 (Pentium III / i440MX, Lynx3DM), rebooting with
> sm712fb framebuffer driver would cause a white screen of death on
> the next POST, presumably the proper timings for the LCD panel was
> not reprogrammed properly by the BIOS.
> 
> Experiments showed a few CRTC Scratch Registers, including CRT3D,
> CRT3E and CRT3F may be used internally by BIOS as some flags. CRT3B is
> a hardware testing register, we shouldn't mess with it. CRT3C has
> blanking signal and line compare control, which is not needed for this
> driver.
> 
> Stop writing to CR3B-CR3F (a.k.a CRT3B-CRT3F) registers. Even if these
> registers don't have side-effect on other systems, writing to them is
> also highly questionable.
> 
> Signed-off-by: Yifeng Li <tomli@...li.me>
> Cc: stable@...r.kernel.org  # v4.4+

Tested-by: Sudip Mukherjee <sudipm.mukherjee@...il.com>

--
Regards
Sudip