Re: 5.15-rc1 i915 blank screen booting on ThinkPads

[Hugh Dickins]

On Thu, 16 Sep 2021, Jani Nikula wrote:
> On Thu, 16 Sep 2021, Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
> > On 16/09/2021 05:37, Hugh Dickins wrote:
> >> Two Lenovo ThinkPads, old T420s (2011), newer X1 Carbon 5th gen (2017):
> >> i915 working fine on both up to 5.14, but blank screens booting 5.15-rc1,
> >> kernel crashed in some way.
...
> > Kernel logs with drm.debug=0xe, with the broken black screen state, 
> > would probably answer a lot of questions if you could gather it from 
> > both machines?
> 
> And for that, I think it's best to file separate bugs at [1] and attach
> the logs there. It helps keep the info in one place. Thanks.
> 
> BR,
> Jani.
> 
> [1] https://gitlab.freedesktop.org/drm/intel/issues/new

Thanks for the quick replies: but of course, getting kernel logs was
the difficult part, this being bootup, with just a blank screen, and
no logging to disk at this stage.  I've never needed it before, but
netconsole to the rescue.

Problem then obvious, both machines now working,
please let me skip the bug reports, here's a patch:

[PATCH] drm/i915: fix blank screen booting crashes

5.15-rc1 crashes with blank screen when booting up on two ThinkPads
using i915.  Bisections converge convincingly, but arrive at different
and surprising "culprits", none of them the actual culprit.

netconsole (with init_netconsole() hacked to call i915_init() when
logging has started, instead of by module_init()) tells the story:

kernel BUG at drivers/gpu/drm/i915/i915_sw_fence.c:245!
with RSI: ffffffff814d408b pointing to sw_fence_dummy_notify().
I've been building with CONFIG_CC_OPTIMIZE_FOR_SIZE=y, and that
function needs to be 4-byte aligned.

Fixes: 62eaf0ae217d ("drm/i915/guc: Support request cancellation")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---

 drivers/gpu/drm/i915/gt/intel_context.c |    1 +
 1 file changed, 1 insertion(+)

--- a/drivers/gpu/drm/i915/gt/intel_context.c
+++ b/drivers/gpu/drm/i915/gt/intel_context.c
@@ -362,6 +362,7 @@ static int __intel_context_active(struct
 	return 0;
 }
 
+__aligned(4)	/* Respect the I915_SW_FENCE_MASK */
 static int sw_fence_dummy_notify(struct i915_sw_fence *sf,
 				 enum i915_sw_fence_notify state)
 {