Re: [PATCH 5/6] drm/i915/gt: Serialize GRDOM access between multiple engine resets

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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On 30/06/2022 08:32, Mauro Carvalho Chehab wrote:
> Em Wed, 29 Jun 2022 17:02:59 +0100
> Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> escreveu:
> 
>> On 29/06/2022 16:30, Mauro Carvalho Chehab wrote:
>>> On Tue, 28 Jun 2022 16:49:23 +0100
>>> Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> wrote:
>>>    
>>>> .. which for me means a different patch 1, followed by patch 6 (moved
>>>> to be patch 2) would be ideal stable material.
>>>>
>>>> Then we have the current patch 2 which is open/unknown (to me at least).
>>>>
>>>> And the rest seem like optimisations which shouldn't be tagged as fixes.
>>>>
>>>> Apart from patch 5 which should be cc: stable, but no fixes as agreed.
>>>>
>>>> Could you please double check if what I am suggesting here is feasible
>>>> to implement and if it is just send those minimal patches out alone?
>>>
>>> Tested and porting just those 3 patches are enough to fix the Broadwell
>>> bug.
>>>
>>> So, I submitted a v2 of this series with just those. They all need to
>>> be backported to stable.
>>
>> I would really like to give even a smaller fix a try. Something like, although not even compile tested:
>>
>> commit 4d5e94aef164772f4d85b3b4c1a46eac9a2bd680
>> Author: Chris Wilson <chris.p.wilson@intel.com>
>> Date:   Wed Jun 29 16:25:24 2022 +0100
>>
>>       drm/i915/gt: Serialize TLB invalidates with GT resets
>>       
>>       Avoid trying to invalidate the TLB in the middle of performing an
>>       engine reset, as this may result in the reset timing out. Currently,
>>       the TLB invalidate is only serialised by its own mutex, forgoing the
>>       uncore lock, but we can take the uncore->lock as well to serialise
>>       the mmio access, thereby serialising with the GDRST.
>>       
>>       Tested on a NUC5i7RYB, BIOS RYBDWi35.86A.0380.2019.0517.1530 with
>>       i915 selftest/hangcheck.
>>       
>>       Cc: stable@vger.kernel.org
>>       Fixes: 7938d61591d3 ("drm/i915: Flush TLBs before releasing backing store")
>>       Reported-by: Mauro Carvalho Chehab <mchehab@kernel.org>
>>       Tested-by: Mauro Carvalho Chehab <mchehab@kernel.org>
>>       Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
>>       Signed-off-by: Chris Wilson <chris.p.wilson@intel.com>
>>       Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
>>       Acked-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>>       Reviewed-by: Andi Shyti <andi.shyti@intel.com>
>>       Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
>>       Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> diff --git a/drivers/gpu/drm/i915/gt/intel_gt.c b/drivers/gpu/drm/i915/gt/intel_gt.c
>> index 8da3314bb6bf..aaadd0b02043 100644
>> --- a/drivers/gpu/drm/i915/gt/intel_gt.c
>> +++ b/drivers/gpu/drm/i915/gt/intel_gt.c
>> @@ -952,7 +952,23 @@ void intel_gt_invalidate_tlbs(struct intel_gt *gt)
>>           mutex_lock(&gt->tlb_invalidate_lock);
>>           intel_uncore_forcewake_get(uncore, FORCEWAKE_ALL);
>>    
>> +       spin_lock_irq(&uncore->lock); /* serialise invalidate with GT reset */
>> +
>> +       for_each_engine(engine, gt, id) {
>> +               struct reg_and_bit rb;
>> +
>> +               rb = get_reg_and_bit(engine, regs == gen8_regs, regs, num);
>> +               if (!i915_mmio_reg_offset(rb.reg))
>> +                       continue;
>> +
>> +               intel_uncore_write_fw(uncore, rb.reg, rb.bit);
>> +       }
>> +
>> +       spin_unlock_irq(&uncore->lock);
>> +
>>           for_each_engine(engine, gt, id) {
>> +               struct reg_and_bit rb;
>> +
>>                   /*
>>                    * HW architecture suggest typical invalidation time at 40us,
>>                    * with pessimistic cases up to 100us and a recommendation to
>> @@ -960,13 +976,11 @@ void intel_gt_invalidate_tlbs(struct intel_gt *gt)
>>                    */
>>                   const unsigned int timeout_us = 100;
>>                   const unsigned int timeout_ms = 4;
>> -               struct reg_and_bit rb;
>>    
>>                   rb = get_reg_and_bit(engine, regs == gen8_regs, regs, num);
>>                   if (!i915_mmio_reg_offset(rb.reg))
>>                           continue;
>>    
>> -               intel_uncore_write_fw(uncore, rb.reg, rb.bit);
>>                   if (__intel_wait_for_register_fw(uncore,
>>                                                    rb.reg, rb.bit, 0,
>>                                                    timeout_us, timeout_ms,
>>
> 
> This won't work, as it is not serializing TLB cache invalidation with
> i915 resets. Besides that, this is more or less merging patches 1 and 3,

Could you explain why you think it is not doing exactly that? In both 
versions end result is TLB flush requests are under the uncore lock and 
waits are outside it.

> placing patches with different rationales altogether. Upstream rule is
> to have one logical change per patch.

I don't think it applies in this case. It is simply splitting into two 
loops so lock can be held across all mmio writes. I think of it this way 
- what is the rationale for sending only the first patch to stable? What 
does it _fix_ on it's own?

>> If this works it would be least painful to backport. The other improvements can then be devoid of the fixes tag.
> 
>  From backport PoV, it wouldn't make any difference applying one patch
> or two. See, intel_gt_invalidate_tlbs() function doesn't exist before
> changeset 7938d61591d3 ("drm/i915: Flush TLBs before releasing backing store"),
> so, it shouldn't have merge conflicts while backporting it, maybe except
> if some functions it calls (or parameters) have changed. On such case,
> the backport fix should be trivial, and the end result of backporting
> one folded patch or two would be the same.

Yes a lot of things changed. Not least engine and GT pm code. Note that 
TLB flushing was backported all the way to 4.4 so any hunk you don't 
strictly need can and will bite you. I have attached a tarball of 
patches for you to explore. :)
Regards,

Tvrtko

> If any conflict happens, I can help doing the backports.
> 
>>> I still think that other TLB patches are needed/desired upstream, but
>>> I'll submit them on a separate series. Let's fix the regression first ;-)
>>
>> Yep, that's exactly right.
>>
>> Regards,
>>
>> Tvrtko

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