Re: [PATCH v2 07/15] drm/i915: Protect the request->global_seqno with the engine->timeline lock

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

On 22/02/2017 12:45, Chris Wilson wrote:
> On Wed, Feb 22, 2017 at 12:29:00PM +0000, Tvrtko Ursulin wrote:
>>
>> On 22/02/2017 11:46, Chris Wilson wrote:
>>> A request is assigned a global seqno only when it is on the hardware
>>> execution queue. The global seqno can be used to maintain a list of
>>> requests on the same engine in retirement order, for example for
>>> constructing a priority queue for waiting. Prior to its execution, or
>>> if it is subsequently removed in the event of preemption, its global
>>> seqno is zero. As both insertion and removal from the execution queue
>>> may operate in IRQ context, it is not guarded by the usual struct_mutex
>>> BKL. Instead those relying on the global seqno must be prepared for its
>>> value to change between reads. Only when the request is complete can
>>> the global seqno be stable (due to the memory barriers on submitting
>>> the commands to the hardware to write the breadcrumb, if the HWS shows
>>> that it has passed the global seqno and the global seqno is unchanged
>>> after the read, it is indeed complete).
>>
>> Missed some questions I've raised on this one in the previous round.
>
> Just missed it when scanning for opens from the threads.
>
>> I never got round re-reading it if you were waiting for that by any chance.
>
> Looks like it is one you have to read after seeing where the series
> goes.

Yes I did not peek ahead when reading it the first time. Looks fine.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko

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