On Sun, Mar 13, 2005 at 12:35:43PM +1100, Benjamin Herrenschmidt wrote:

Hi !

I'm currently rewriting radeonfb to implement support for dual head, and
ultimately, to make it more friendly to be hooked on DRM for mesa-solo
style setups.

I have some issues however related to the way memory is mapped and
dealing with apertures. Here is the story, suggestions welcome:

The radeon card exposes to the system 2 separate apertures. That is, the
PCI region is actually cut by the hardware in two halves, each of them
beeing an "aperture". Each aperture can have different configuration for
the endian swappers (and possibly the surface tiling registers).

I can configure the apertures to both map to the same bit of video
memory (both covering the framebuffer from 0), or to be "split", that is
aperture 0 covering the framebuffer from 0 to CONFIG_APER_SIZE (size of
an aperture, that is half of the PCI BAR allocation), and aperture 1
covering the framebuffer from CONFIG_APER_SIZE to CONFIG_APER_SIZE*2.

However, I can't change anything to CONFIG_APER_SIZE itself, it's
decided by straps, either HW or in the ROM. So we end up with different
setups depending on how the BIOS has configured things. I know that
Apple chips are usually wired so that CONFIG_APER_SIZE is half the video
memory, so if I use the first mode, I can only access half of the video
RAM from PCI, if I use the second, each aperture maps a different half
of video memory with possibly different endian swapping.

But I think the setups in real life are more diverse and some BIOSes
will have CONFIG_APER_SIZE at least as big as the entire video memory,
thus forcing me to use the "overlapped" setup. In fact, CONFIG_APER_SIZE
may even be smaller than half of the vram and thus limiting the CPU to
part of the VRAM anyway.

I have toyed with all sort of setups, and I have +/- decided to not
bother, and always do this, please tell me what you think:

Always setup HOST_PATH_CNTL.HDP_APER_CNTL to 0. That is, both apertures
are always overlapping. On Macs, or other machines that strap
CONFIG_APER_SIZE to half of VRAM, that means only half of vram can be
directly accessed by the CPU. I think this is fine because of these:

 - We only really need to bother about CPU access for the framebuffer
itself (and possibly the cursor). That is normal non-accelerated fbdev
operations an mmap'ing of the framebuffer in user space. This is not
really a problem if that is limited to some part of vram. It puts a
small constraint on the allocation of video memory: the framebuffer has
to be near the beginning.

It will limit DirectFB to access only CONFIG_APER_SIZE. DirectFB needs CPU 
access to offscreen memory for software fallbacks and explicit user 
access. Any other compositing window system would need to do the same. If 
the video memory manager ever gets done then it shouldn't be a major 
problem because the kernel could blit the data to/from the inaccesible 
part without the application even realizing it. Although direct access 
might be useful in that case also since it could reduce pressure on the 
GART address space.

But my opinion is that a mode switch will
pretty much always invalidate everything that is cached in video memory,

I don't see any reason for such a sledgehammer approach. If the new mode 
doesn't overlap with any offscreen data then there is no need to 
invalidate anything.

-- 
Ville Syrjälä
syrjala@sci.fi
http://www.sci.fi/~syrjala/


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