[virglrenderer-devel] virglrenderer on gles, format conversion

[Erik Faye-Lund]

When we try to do a vrend transfer from a renderable texture, we end up
using glReadPixels to read the data. However, OpenGL ES has
restrictions on what formats are allowed to be used for glReadPixels.
In partcular, only GL_UNSIGNED_BYTE + GL_RGBA are guaranteed to be
supported (for OpenGL ES 2.0; for OpenGL ES 3.x there's some other
variants for integer and float textures - it seems we can rely on the
destination format having at least as much range/precision as the
source format).

But vrend transfers expect no format conversion at all, they want to
read the data verbatim.

I thought the obvious choice was to modify virglrenderer to convert
back from RGBA to the source format. After a typing out a lot of code,
I realized that even though we have util_format_translate in
u_format.h, the actual implementation was deleted in d01f462[1].

OK, so that leaves me with a couple of choices:

1) Restore the format conversion code in virglrenderer.
2) Use some 3rd party pixel-format conversion code instead.
3) Do format-conversion back to the desired format in Mesa.

I gave 1) a go by reverting the removal, but this conflicted pretty
badly due to the 3 years of development that has happened since. It
might be possible to continue this effort, but I also don't really like
the idea of crash-prone format converion code in virglrenderer. You
know, for security reasons; virglrenderer runs inside the virtual
machine manager, which makes it pretty high priviledge code. If we
don't do 1), I have sent a MR[2] that removes the rest of the format
conversion, so others won't be led astray in the future.

I have relatively low expectations that we'd find a good fit for 3) as
we have a very specific and *big* set of formats we need to support.
Format conversion across APIs is often a big pain-point in my
experience.

As for 3), it's not quite as easily said as done, because we currently
only have protocol commands for reading a format verbatim. So I guess
we'd have to add a new format, and inform the Mesa driver that we're
limited in what formats we support. With this approach, we don't need
conversion code both in mesa *and* in virglrenderer, and we don't have
to actively maintain a fork of the pixel-format code separately.

I'm currently leaning towards 3) for now, but I would like input from
the list, as I feel this is a rather important directional choice.

I'm also intererested in if someone has some alternative approaches.
Perhaps we could use some lower-level API (GBM? Some sort of PBuffer or
EGLimage?) to create a lockable, linear surface of the same format,
blit to that and read the result from there? Does something like this
exist? If so, can we depend on this?

[1]: 
https://gitlab.freedesktop.org/virgl/virglrenderer/commit/d01f462c53f079e79b10617798abc56582e5124f
[2]: 
https://gitlab.freedesktop.org/virgl/virglrenderer/merge_requests/174