xf86-video-armada + etnaviv (Was: Re: I.MX6 HDMI support in v4.2)

[Russell King - ARM Linux]

On Mon, Sep 28, 2015 at 05:40:13PM +0200, Lucas Stach wrote:
> Am Montag, den 28.09.2015, 16:24 +0100 schrieb Russell King - ARM Linux:
> > On Mon, Sep 28, 2015 at 04:48:08PM +0200, Lucas Stach wrote:
> > > I've looked at that again and the issue here seems to be that GStreamer
> > > is handing us a pointer to a buffer that isn't aligned to a page. The
> > > buffers size is properly rounded up to a pagesize, as requested by
> > > QueryImageAttributes, but if the buffer start pointer isn't aligned to a
> > > page boundary we end up with an non-mappable buffer anyway.
> > > 
> > > Unfortunately there is no obvious way for a driver to request a minimum
> > > alignment for the buffer. The only possible fix is for the client to
> > > always align the buffer to a page boundary in hopes that this is enough
> > > for the hardware to map it directly and allow to skip any unwanted
> > > copying.
> > 
> > We can't just "round up" the size.  We've no idea whether the buffer
> > came from shmem (for XvShmPutImage), or whether it's part of an internal
> > X buffer (for XvPutImage).  In the latter case, we've no idea whether
> > data in the remainder of the page will be read or written by the CPU
> > when, eg, a signal occurs - and X does use signals.
> > 
> I'm not talking about the driver rounding up the size. That is obviously
> (contrary to what my patch did) the wrong thing to do.
> 
> I was talking of the client (as in VLC, GStreamer, whatever) aligning
> the buffer to a page boundary. As there is no way for the client to
> query the alignment restrictions, we may still need a fallback path in
> the driver, so that if an unaligned buffer comes in we don't do a
> buffer_from_userptr but actually copy client memory to a new buffer.

You really do _not_ want to do be copying image data.  With large images
(1080p) that will consume lots of CPU, and tie up the X server doing not
much other than copying data.  You might as well manually convert and
copy the data to the screen at that point.

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