linux-kernel - Re: Debugging Thinkpad T430s occasional suspend failure.

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Date:	Sat, 16 Feb 2013 13:45:56 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul McKenney <paul.mckenney@...aro.org>
Subject: Re: Debugging Thinkpad T430s occasional suspend failure.

On Thu, 14 Feb 2013, Linus Torvalds wrote:
> On Thu, Feb 14, 2013 at 6:09 PM, Hugh Dickins <hughd@...gle.com> wrote:
> >
> > Which won't affect my case since I never enabled it.
> 
> Well, in theory, you may have the same bug Dave just made it easier to
> trigger for himself with the forced config option.
> 
> In reality, your bug behavior differences were already big enough that
> it sounded likely that they were two different things to start with..

Yes.

I hacked around on your PM_TRACE set_magic_time() / read_magic_time()
yesterday, to save an oopsing core kernel ip there, instead of hashed
pm trace info (it makes sense in this case to invert your sequence,
putting the high order into years and the low order into minutes).

Rewarded last night by reboot to Feb 21 14:45:53 2006.  Which is
ffffffff812d60ed intel_choose_pipe_bpp_dither.isra.13+0x216/0x2d6

/home/hugh/3087X/drivers/gpu/drm/i915/intel_display.c:4159
	 * enable dithering as needed, but that costs bandwidth.  So choose
	 * the minimum value that expresses the full color range of the fb but
	 * also stays within the max display bpc discovered above.
	 */

	switch (fb->depth) {
ffffffff812d60e9:	48 8b 55 c0          	mov    -0x40(%rbp),%rdx
ffffffff812d60ed:	8b 02                	mov    (%rdx),%eax

(gcc chose to pass a pointer to fb->depth down to the function,
instead of fb itself, since that is the only use of it there.)

I expect that fb is NULL; but with an average of one failure to resume
per day, and ~26 bits of info per crash, this is not a fast procedure!

I notice that intel_pipe_set_base() allows for NULL fb,
so I'm currently running with the oops-in-rtc hackery, plus
-	switch (fb->depth) {
+	if (WARN_ON(!fb))
+		bpc = 8;
+	else switch (fb->depth) {

There's been a fair bit of change to intel_display.c since 3.7 (if
my 3.7 was indeed good), mainly splitting intel_ into haswell_ versus
ironlake_, but I've not yet spotted anything obvious; nor yet looked
to see where fb would originate from anyway.

Once I've got just a little more info out of it, I'll start another
thread addressed principally to the drm/gpu/i915 guys.

Anyway, nothing I've found yet is worth delaying v3.8.

Hugh
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