linux-kernel - Re: drm/mgag200: doesn't work in panic context

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Message-ID: <20150629080914.GB12383@pd.tnic>
Date:	Mon, 29 Jun 2015 10:09:14 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Daniel Vetter <daniel.vetter@...ll.ch>
Cc:	"Luck, Tony" <tony.luck@...el.com>,
	"Wang, Rui Y" <rui.y.wang@...el.com>,
	Dave Airlie <airlied@...hat.com>,
	"Clark, Rob" <robdclark@...il.com>,
	"Roper, Matthew D" <matthew.d.roper@...el.com>,
	"Chen, Gong" <gong.chen@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: drm/mgag200: doesn't work in panic context

On Sat, Jun 27, 2015 at 07:56:19PM +0200, Daniel Vetter wrote:

<snip very useful explanation for !GPU people>

> Which could all happen very much after the kernel made it's dying
> sigh. Display hw has long stopped being this simple and display
> drivers also.

Thanks for the explanation. I was fearing that it would go in such
direction though. :\ Oh well.

So there are two observations to be ventured here:

* there's still no robust and simple way to save/show oops messages on
laptops. When a laptop crashes and the user has only this one machine,
catching the oops can be a serious PITA. Laptop most likely doesn't have
serial, modeset is hard to do, as you explained, and saving it into
persistent storage (UEFI) might brick it in some cases or whatever the
firmware will do to it. I guess we can keep wishing here for something
better.

* The server aspect might not really need GPU modesetting IMHO because
realistically, servers have serial so the oops can go out there. Which
begs the other question:

Shouldn't drm_fb_helper_panic() and that path below it simply be
removed?

I mean, if it needs to do all that hw-reconf on the panic path and if
that reconfiguration is not completely reliable, maybe we shouldn't do
it all?

This way, we will basically remain as quiet as possible in order not
to muddle the system unreasonably when we're about to die and the only
thing we want to catch is an oops or maybe kexec the crash kernel...

Oops will go out on serial anyway and server will kexec - GPU/fb stuff
doesn't do anything. Yes, no?

Hmmm.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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