Re: [PATCH 2/5] drm/i915: preserve swizzle settings if necessary v3

[Steve Aarnio <steve.j.aarnio@linux.intel.com>]

On 06/11/2014 08:41 AM, Jesse Barnes wrote:
> On Wed, 11 Jun 2014 17:39:29 +0200
> Daniel Vetter <daniel@ffwll.ch> wrote:
>
>> On Wed, Jun 11, 2014 at 5:13 PM, Jesse Barnes <jbarnes@virtuousgeek.org> wrote:
>>>> - If you have a machine which uses tiled framebuffers and enables
>>>>    swizzling in the BIOS your code will a) drop the swizzle setup in
>>>>    gem_init_hw, breaking resume b) not set the swizzle settings correctly
>>>>    in swizzle_detect, breaking swap in/out and pwrite/pread. Not sure such
>>>>    a machine exists, but still.
>>>
>>> This would affect krh's MBA, which is why I wanted testing here...
>>> anyway I'll spin a new one and ask krh to test again.
>>
>> Hm, I've thought the issue with the MBA is that it used tiled fbs, but
>> non-swizzled. And then a mess ensued when we've enabled it. But yeah,
>> unfortunately with the new logic we need to retest :(
>
> Ah yeah I think you're right, either way, need more testing.
>
> Maybe we should have just gone with the first patch to never enable
> swizzling based on Art's assertion that it didn't matter.
>

I hate to jump into the middle of a conversation that may or may not be related 
to a patch I just posted... but...

There was a very long internal discussion that the Windows guys had with H/W. 
For Gen8+ H/W recommends disabling CSX swizzle.  Technically, BDW still supports 
it, but there is a bug _somewhere_ that makes it problematic.  In any case it 
goes away for sure with Gen9+, so disabling on Gen8 doesn't hurt.

According to the other discussion, the H/W guys say that enabling actually hurts 
performance slightly, and the driver should leave the swizzle decisions to the 
memory controller.

Stevo