[Mesa-dev] [PATCH 02/10] dri_interface: add __DRI_IMAGE_TRANSFER_USER_STRIDE

[Marek Olšák]

On Mon, Apr 30, 2018 at 3:11 PM, Eric Anholt <eric at anholt.net> wrote:

> Marek Olšák <maraeo at gmail.com> writes:
>
> > From: Nicolai Hähnle <nicolai.haehnle at amd.com>
> >
> > Allow the caller to specify the row stride (in bytes) with which an image
> > should be mapped. Note that completely ignoring USER_STRIDE is a valid
> > implementation of mapImage.
> >
> > This is horrible API design. Unfortunately, cros_gralloc does indeed have
> > a horrible API design -- in that arbitrary images should be allowed to be
> > mapped with the stride that a linear image of the same width would have.
> >
> > There is no separate capability bit because it's unclear how stricter
> > requirements should be defined.
>
> I think that this is a bad interface, and you should just push an extra
> copy into the caller so that they're encouraged to fix their bad API
> instead of pushing their mistake down into Mesa ABI that we get stuck
> with forever.
>

I agree that it's a very bad interface. I would also like cros_gralloc not
to do silly things. I don't know what kind of hardware gralloc is designed
around - it's certainly not designed around modern GPUs, that part is
clear. The emulation of the user-specified stride increases memory usage
(especially if the stride is larger than necessary), so it's a bad idea by
definition. I don't know if it's still possible to change gralloc not to
use it.

The reason the feature is pushed into the driver is that only the driver
can efficiently force a certain stride where the stride is much larger than
box.width*bpp of the mapping, and still only tile/detile box.width*bpp.

Marek
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