Re: 5.15-rc1 i915 blank screen booting on ThinkPads

[Jani Nikula]

On Thu, 16 Sep 2021, Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
> Hi,
>
> 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.
>
> T420s could be SandyBridge and X1 Carbon KabyLake.
>
>> I wanted to say what i915 generations these are, but don't know where
>> to look - I don't see it in dmesg, even when DRM_I915_DEBUG enabled.
>> 
>> Possibly relevant: builtin kernels, CONFIG_MODULES off, no initrd.
>> 
>> On the older laptop:
>> 
>> First bisection showed first bad commit
>> 41e5c17ebfc2 "drm/i915/guc/slpc: Sysfs hooks for SLPC"
>> 
>> But reverting that still crashed boot with blank screen (and
>> reverting the two related commits after it made no difference).
>> 
>> Second bisection, starting from 5.15-rc1 bad and 41e5c17ebfc2 "good",
>> but patching it out each time before building, showed first bad commit
>> 3ffe82d701a4 "drm/i915/xehp: handle new steering options"
>> 
>> That one did not revert cleanly from 5.15-rc1, but reverting
>> 927dfdd09d8c "drm/i915/dg2: Add SQIDI steering" then
>> 1705f22c86fb "drm/i915/dg2: Update steering tables" then
>> 768fe28dd3dc "drm/i915/xehpsdv: Define steering tables" then
>> 3ffe82d701a4 "drm/i915/xehp: handle new steering options"
>> worked (there was one very easy fixup needed somewhere).
>> 
>> And 5.15-rc1 with those five reversions boots and runs fine...
>> on that older laptop.  But reverting those from the kernel on the
>> newer laptop did not help at all, still booting with blank screen
>> (or no more lines shown after the switch from VGA).  Put them back.
>
> Bisect results sound suspicious since the steering patches do not come 
> into play on SandyBridge.
>
>> On the newer laptop, bisection showed first bad commit
>> 62eaf0ae217d "drm/i915/guc: Support request cancellation"
>> 
>> And 5.15-rc1 with that reverted boots and runs fine on the newer.
> But not on the older laptop?
>
> Given bisect points to this, it may be worth trying to build both 
> kernels with CONFIG_DRM_I915_REQUEST_TIMEOUT=0 (no reverts) to see what 
> happens. But first the logs which I'll ask next.
>
>> I am hoping that there will be some i915 fixups to come in a later rc!
>> May be nothing more than uninitialized variables or NULL pointers.
>> You'll probably want more info from me: please ask, but I'm slow.
>
> 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


>
> Regards,
>
> Tvrtko

-- 
Jani Nikula, Intel Open Source Graphics Center