[Intel-gfx] [PATCH 1/3] drm/i915: Use readl/writel for ring buffer access

[Tvrtko Ursulin]

On 14/04/16 12:16, Chris Wilson wrote:
> On Thu, Apr 14, 2016 at 11:59:29AM +0100, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>
>> We know ringbuffers are memory and not ports so if we use readl
>> and writel instead of ioread32 and iowrite32 (which dispatch to
>> the very same functions after checking the address range) we
>> avoid generating functions calls and branching on every access.
>
> We don't need to use readl/write at all, since they are normal memory
> on llc, and on x86 we can pretend that iomaps (!llc/stolen) are as well.

It is fine to use readl/writel since it translates to a single mov 
instruction anyway on x86.

> This patch is in the queue along with killing the incorrect spare iomem
> annotation.

Ok did not spot them. Don't mind either way, thought this is quick, easy 
and obvious improvement when I spotted the ugly code generated for ring 
buffer writing.

Mind you it is still not completely pretty with this patch since it is 
full of reloads and adds for ringbuf->virtual_start and tail which I 
can't figure how to help GCC optimize. Unless we make being, emit and 
advance functions return the current tail pointer and also accept it. In 
that case it all shrinks by half.

Regards,

Tvrtko