Re: [PATCH 1/2] drm/i915/guc: Simplify code by keeping kmap of guc_client object

[Dave Gordon <david.s.gordon@intel.com>]

On 12/02/16 13:03, Tvrtko Ursulin wrote:
>
> On 11/02/16 23:09, yu.dai@intel.com wrote:
>> From: Alex Dai <yu.dai@intel.com>
>>
>> GuC client object is always pinned during its life cycle. We cache
>> the kmap of its first page, which includes guc_process_desc and
>> doorbell. By doing so, we can simplify the code where we read from
>> this page to get where GuC is progressing on work queue; and the
>> code where driver program doorbell to send work queue item to GuC.

[snip]

>>
>> -    /* Finally, update the cached copy of the GuC's WQ head */
>> -    gc->wq_head = desc->head;
>
> Did you mean to remove the above?

I wondered that too at first, but the answer is "yes" -- see below.

>>
>> +    client->client_base = kmap(i915_gem_object_get_dirty_page(obj, 0));
>
> Was this another bug, that the page/object wasn't dirtied before?

It wouldn't have made any difference; the object is pinned in the GTT 
forever, so it can't be swapped out or reclaimed.

>> -    uint32_t wq_head;
>
> Hm ok I don't get why kmap caching means removing this as well?

'wq_head' was an optimisation so that we could check whether there was 
known to be space in the workqueue without kmapping and reading the 
process descriptor. Now that the client (which includes the process 
descriptor) is permanently mapped, there's no advantage to caching the 
head; we might just as well read the current value from 'desc->head' 
each time.

> Btw I don't see patch 2/2 ?
>
> Regards,
>
> Tvrtko
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