Hi, Michel,

On 08/09/2017 08:36 AM, Michel Dänzer wrote:
Hi Thomas,

first of all, would it be possible to split these patches up a bit
further? At least patch 1 seems to contain several logical changes,
which makes it a bit difficult to review.

OK. I'll try to do that.

On 08/08/17 03:48 PM, Thomas Hellstrom wrote:
Implement back-to-fake-front flips,
Implement dri3 support for GLX_SWAP_EXCHANGE_OML and GLX_SWAP_COPY_OML.

The back-to-fake-front flips will save a full buffer copy in the case of a
fake front being enabled and GLX_SWAP_UNDEFINED_OML.

Support for EGL_BUFFER_PRESERVED and GLX_SWAP_X_OML are mostly useful for
things like glretrace if traces are capured with applications relying on a
specific swapbuffer behavior.

The EGL_BUFFER_PRESERVED path previously made sure the present was done as
a copy, but there was nothing making sure that after the present,
the same back buffer was chosen.
This has now been changed so that if the previous back buffer is
idle, we reuse it. Otherwise we grab a new and copy the contents and
buffer age from the previous back buffer. Server side flips are allowed.


GLX_SWAP_EXCHANGE_OML will behave similarly, except that we try to reuse the
previous fake front as the new back buffer if it's idle. If not, we grab
a new back buffer and copy the contents and buffer age from the old fake front.
I'm not sure it's worth copying the contents of the desired next back
buffer to a different one and using that instead. There might be cases
where doing so results in lower performance than simply using the
desired back buffer anyway. Have you made any measurements WRT this?

Agreed. I haven't done any measurements but my reasoning was that probably any performance loss would be mostly associated with the allocating itself, hence a short-lived problem once enough buffers are allocated. The copying would, at least on modern tiling hardware, probably not be a big loss since we don't flush.

With EGL_BUFFER_PRESERVED/GLX_SWAP_COPY_OML, always re-using the same
back buffer means that the client only needs to allocate one back
buffer, resulting in lower graphics memory consumption.

True. But there are some implications:

First, with GLX_SWAP_COPY_OML, reusing the back buffer means we need to disable server-side page-flipping, otherwise the buffer would never be idle. Second, if we have a fake front, there will be at least one copy anyway. So in essence we need to disable server-side page-flipping and might not gain anything in the end if using a fake front.

With GLX_SWAP_EXCHANGE_OML, we'd be reusing the old fake front which will, after a delay, always be idle. Also we don't need an extra copy so the only loss will be a delay which might impact latency-sensitive applications. I don't expect SWAP_EXCHANGE will be used much anyway and even if we enable it in st/dri, it won't be advertised until the server side AIGLX starts to use the image loader extension or we rewrite the GLX fbconfig selection.

What do you think of adding a driConf option to select "wait for backbuffer idle in swapbuffers"


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