Re: [RFC] drm/amd/amdgpu: get rid of else branch

[Nikola Pajkovsky <npajkovsky@suse.cz>]

Christian König <christian.koenig@amd.com> writes:

> Am 27.04.2017 um 18:17 schrieb Nikola Pajkovsky:
>> This is super simple elimination of else branch and I should
>> probably even use unlikely in
>>
>>   	if (ring->count_dw < count_dw) {
>>
>> However, amdgpu_ring_write() has similar if condition, but does not
>> return after DRM_ERROR and it looks suspicious. On error, we still
>> adding v to ring and keeping count_dw-- below zero.
>>
>> 	if (ring->count_dw <= 0)
>> 		DRM_ERROR("amdgpu: writing more dwords to the ring than expected!\n");
>> 	ring->ring[ring->wptr++] = v;
>> 	ring->wptr &= ring->ptr_mask;
>> 	ring->count_dw--;
>>
>> I can obviously be totaly wrong. Hmm?
>
> That's just choosing the lesser evil.
>
> When we write more DW to the ring than expected it is possible (but
> not likely) that we override stuff on the ring buffer which is still
> executed by the command processor leading to a possible CP crash.
>
> But when we completely drop the write the commands in the ring buffer
> will certainly be invalid and so the CP will certainly crash sooner or
> later.

Instead of choosing the lesser evil, is there good way to design ring
buffer right way?

> Please add the unlikely() as well and then send out the patch with a
> signed-of-by line and I will be happy to push it into our upstream
> branch.

Proper patch has been sent.

-- 
Nikola