Re: 5.15-rc1 i915 blank screen booting on ThinkPads

[Matthew Brost <matthew.brost@intel.com>]

On Fri, Sep 17, 2021 at 02:26:48PM -0700, Hugh Dickins wrote:
> On Thu, 16 Sep 2021, Jani Nikula wrote:
> > On Thu, 16 Sep 2021, Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> 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:
> 

Thanks for finding / fixing this Hugh. I will post this patch in a way
our CI system can understand.

Matt 

> [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@google.com>
> ---
> 
>  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)
>  {