Re: [PATCH igt 1/3] lib/igt_fb: also call __gem_set_tiling for Y tiling

From: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
To: "Zanoni, Paulo R" <paulo.r.zanoni@intel.com>,
	"ville.syrjala@linux.intel.com" <ville.syrjala@linux.intel.com>
Cc: "intel-gfx@lists.freedesktop.org" <intel-gfx@lists.freedesktop.org>
Subject: Re: [PATCH igt 1/3] lib/igt_fb: also call __gem_set_tiling for Y tiling
Date: Mon, 1 Feb 2016 17:44:42 +0000	[thread overview]
Message-ID: <56AF998A.7040101@linux.intel.com> (raw)
In-Reply-To: <1454346984.2538.14.camel@intel.com>

On 01/02/16 17:16, Zanoni, Paulo R wrote:
> Em Sex, 2016-01-29 às 21:06 +0200, Ville Syrjälä escreveu:
>> On Fri, Jan 29, 2016 at 04:46:30PM -0200, Paulo Zanoni wrote:
>>> The interesting thing is that if we don't do this, we still get a
>>> Y tiled framebuffer, but there won't be a fence around it, which
>>> makes
>>> the GTT mmaps less interesting. Is this a Kernel bug?
>>
>> I think some tests currently depend on not having a fence for Y tiled
>> fbs. So this could break stuff.
>
> Do you have any additional information that could help me discover
> which ones? A quick look on the IGT tests mentioning tiling didn't
> point anything obvious.
>
> Besides, I think it's probably not a good idea to have such a high
> level helper function behaving differently depending on the tiling
> type, I'd vote to either call set_tiling on both or on none.

Noticed the thread by accident. :)

I can't help with the question of which tests might be affected by this. 
Some low level ones like kms_addfb don't use the fb helpers so they 
shouldn't be. Can't remember if any other would be.

But just a little bit of background:

Basically with the introduction of Y tiled (and Yf) scanout in Gen9 we 
have forked the path and destroyed the coupling between obj->tiling and 
framebuffer tiling.

The X special casing in create_bo_for_fb is for compatibility with old 
userspace, but going forward it was decided fb  modifiers should be used 
to tell the driver about tiling and get/set_tiling ioctl is about 
fencing and only that.

Paths implemented in IGT back then were rendering to Y and Yf tiling fbs 
via a temporary linear surface which is then blitted (blit?) to the real 
fb obj. (With the blitter doing the appropriate transformation.)

So in that respect adding Y tiling to create_bo_for_fb would be wrong 
because it is not aligned with the above, and also you cannot support Yf 
this way at all.

But I do agree this creates a problem for some use cases within the IGT 
since the fb and backing obj are created atomically and once that is 
done you cannot fiddle with obj->tiling (aka fencing).

Regards,

Tvrtko
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