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

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

On 14/04/16 12:30, Chris Wilson wrote:
> On Thu, Apr 14, 2016 at 12:24:20PM +0100, Tvrtko Ursulin wrote:
>>
>> 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@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.
>
> We figured out how to help gcc with that in userspace using:
>
> out = ring_begin(num_dwords);
> out[0] = cmd;
> out[N] = dwN
>
> GCC will then do
>
> mov $imm0, 0x0($eax)
> mov $imm1, 0x4($eax)
> mov $edx, 0x8($eax)
> etc

Would be nice, hope it happens soon. :)

Regards,

Tvrtko
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