[Mesa-dev] [PATCH v3] i965: Fix ETC2/EAC GetCompressed* functions on Gen7 GPUs

[Eleni Maria Stea]

Hi Eero,

Thanks for your feedback,

On Thu, 3 May 2018 13:30:38 +0300
Eero Tamminen <eero.t.tamminen at intel.com> wrote:

> Hi,
> 
> On 02.05.2018 20:19, Matt Turner wrote:
> > On Wed, May 2, 2018 at 9:13 AM, Eleni Maria Stea <estea at igalia.com>
> > wrote:  
> >> Gen 7 GPUs store the compressed EAC/ETC2 images in other
> >> non-compressed formats that can render. When GetCompressed*
> >> functions are called, the pixels are returned in the
> >> non-compressed format that is used for the rendering.
> >>
> >> With this patch we store both the compressed and non-compressed
> >> versions of the image, so that both rendering commands and
> >> GetCompressed* commands work.
> >>
> >> Also, the assertions for GL_MAP_WRITE_BIT and
> >> GL_MAP_INVALIDATE_RANGE_BIT in intel_miptree_map_etc function have
> >> been removed because when the miptree is mapped for reading (for
> >> example from a GetCompress* function) the GL_MAP_WRITE_BIT won't
> >> be set (and shouldn't be set).
> >>
> >> Fixes: the following test in CTS for gen7:
> >> KHR-GL45.direct_state_access.textures_compressed_subimage test
> >>
> >> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104272  
> > 
> > I think you can add
> > 
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=81843
> > 
> > as well :)  
> 
> This is really lovely feature.
> 
> Compressed texture formats are used to:
> 1. Reduce disk / network usage for the application install
> 2. Increase run-time performance (by reducing required bandwidth)
> 3. Reduce program memory usage
> 
> At the cost of worse texture quality.
> 
> Mesa transparently converting ETC to uncompressed data on platforms
> that don't support ETC in HW, means that application doesn't get 2),
> just worse texture quality, although some applications would have
> capability to fall back to another (HW supported) texture compression
> format.
> 
> And this new patch means that instead of 3), memory usage actually
> _increases_ compared to application using non-compressed textures
> directly.
> 

You are right about the memory usage and about the purpose of the patch.

> Some (many?) applications might fail to run if ETC isn't supported, so
> I  understand why this feature is done, but it would be nice to have
> some better way to handle it.
> 
> Maybe some new extension that can be used by future game engines &
> application toolkits to query which of the compressed texture formats
> are faked, so that they can instead select a compression format that
> actually provides run-time benefits?
> 
> 
> 	- Eero

After having a look I think that we already have an extension for that
type of query: the ARB_internalformat_query2. We are not sure if the
existing queries would fit, but in case they don't we think that the
new extension would rather add more queries to this one than introducing
new methods. What do you think?

Thank you,
Eleni