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

From: Christian KÃnig
Date: Thu May 04 2017 - 09:19:26 EST

Am 04.05.2017 um 14:57 schrieb Nikola Pajkovsky:
Christian KÃnig <christian.koenig@xxxxxxx> 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;

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

It is designed the right way.

See this only happens when a developer increases the number of dw written, but forgets to reserve ring buffer space before starting the write.

When the function recognizes that something is wrong it is to late to actually reserve more space, so the only option left is printing a message for the dev to fix the code.

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
Proper patch has been sent.

Seen and reviewed, thanks for the help.