Re: [PATCH 2/2] drm/i915: Serialise updates to GGTT with access through GGTT on Braswell

[Jesse Barnes <jbarnes@virtuousgeek.org>]

On 11/19/2015 01:35 AM, Chris Wilson wrote:
> On Thu, Nov 19, 2015 at 10:14:08AM +0100, Daniel Vetter wrote:
>> On Wed, Nov 18, 2015 at 03:08:47PM -0800, Jesse Barnes wrote:
>>> On 11/17/2015 08:37 AM, Daniel Vetter wrote:
>>>> On Fri, Oct 30, 2015 at 04:58:41PM +0000, Chris Wilson wrote:
>>>>> On Fri, Oct 30, 2015 at 05:14:21PM +0100, Daniel Vetter wrote:
>>>>>> On Fri, Oct 23, 2015 at 06:43:32PM +0100, Chris Wilson wrote:
>>>>>>> When accessing through the GTT from one CPU whilst concurrently updating
>>>>>>> the GGTT PTEs in another thread, the hardware likes to return random
>>>>>>> data. As we have strong serialisation prevent us from modifying the PTE
>>>>>>> of an active GTT mmapping, we have to conclude that it whilst modifying
>>>>>>> other PTE's that error occurs. (I have not looked for any pattern such
>>>>>>> as modifying PTE within the same page or cacheline as active PTE -
>>>>>>> though checking whether revoking neighbouring objects should be enough
>>>>>>> to test that theory.) The corruption also seems restricted to Braswell
>>>>>>> and disappears with maxcpus=0. This patch stops all access through the
>>>>>>> GTT by other CPUs when we update any PTE by stopping the machine around
>>>>>>> the GGTT update.
>>>>>>>
>>>>>>> Testcase: igt/gem_concurrent_blit
>>>>>>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89079
>>>>>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>>>>>
>>>>>> Wild guess, since it wouldn't be the first time hw engineers screwed this
>>>>>> up.
>>>>>>
>>>>>> Cheers, Daniel
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
>>>>>> index d1c5cf89fe77..de983c8e6e54 100644
>>>>>> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
>>>>>> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
>>>>>> @@ -2337,12 +2337,8 @@ int i915_gem_gtt_prepare_object(struct drm_i915_gem_object *obj)
>>>>>>  
>>>>>>  static void gen8_set_pte(void __iomem *addr, gen8_pte_t pte)
>>>>>>  {
>>>>>> -#ifdef writeq
>>>>>> -	writeq(pte, addr);
>>>>>> -#else
>>>>>>  	iowrite32((u32)pte, addr);
>>>>>>  	iowrite32(pte >> 32, addr + 4);
>>>>>> -#endif
>>>>>
>>>>> Tried:
>>>>>  static void gen8_set_pte(void __iomem *addr, gen8_pte_t pte)
>>>>>   {
>>>>>   -#ifdef writeq
>>>>>   -       writeq(pte, addr);
>>>>>   -#else
>>>>>   -       iowrite32((u32)pte, addr);
>>>>>   -       iowrite32(pte >> 32, addr + 4);
>>>>>   -#endif
>>>>>   +       iowrite32(0, addr);
>>>>>   +       wmb();
>>>>>   +       iowrite32(upper_32_bits(pte), addr + 4);
>>>>>   +       iowrite32(lower_32_bits(pte), addr);
>>>>>   +       wmb();
>>>>>    }
>>>>>     
>>>>> and just the plain iowrite(lower), iowrite(upper), neither helps.
>>>>
>>>> Added a note about this and applied to dinq. Yay for awesome hw.
>>>
>>> I thought Ville explained how this wasn't necessary?
>>
>> Ville can't repro, Chris claims it fixes something, I don't have a
>> system. We probably should dig into this more, but since I didn't see
>> anything going on I figured I can just pull it in for now.
> 
> Both myself, old QA (when they finally got around to running some of the
> coherency tests), new QA and VPG have reported coherency issues with
> access through the GGTT.

I can believe it; it would be good to find the root cause the hw issue
though.  Obviously we're not understanding something fully...

Jesse

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