Re: ioctl32 on amd64

[Egbert E. <ei...@su...>]

Eric Anholt writes:
 > 
 > I've taken a look, and started writing down my understanding of handles
 > and offsets here:
 > http://dri.freedesktop.org/wiki/DrmMapHandling

Thanks for putting together this page!

 > 
 > As far as I could tell, math was not being done on handles.  This makes
 > sense, since the DRM mmap handler is comparing whether the passed in
 > offset is exactly equal to (not just in the range of) a map's
 > offset/length.  But I'm really concerned about the mixing of handles and

Right. However I came across one place where map handles were used
as base address of a physical range. Then an offset was added. This
had nothing to do with using handles in mmap().
The 'handles' seemed to be some multi purpose thing.

Please take a look at 
       https://bugs.freedesktop.org/attachment.cgi?id=2363
and the following change:

diff -rNu ../unpatched/Mesa/src/mesa/drivers/dri/mga/mga_xmesa.c Mesa/src/mesa/drivers/dri/mga/mga_xmesa.c
--- ../unpatched/Mesa/src/mesa/drivers/dri/mga/mga_xmesa.c	2004-12-16 01:23:47.000000000 +0100
+++ Mesa/src/mesa/drivers/dri/mga/mga_xmesa.c	2005-04-05 21:26:38.000000000 +0200
@@ -612,8 +612,7 @@
    _tnl_allow_pixel_fog( ctx, GL_FALSE );
    _tnl_allow_vertex_fog( ctx, GL_TRUE );
 
-   mmesa->primary_offset = mmesa->mgaScreen->primary.handle;
-
+   mmesa->primary_offset = drmAgpBase(sPriv->fd);
    ctx->DriverCtx = (void *) mmesa;
    mmesa->glCtx = ctx;
 
If you look at mgaioctl.c you will find code that does arithmetic
with mmesa->primary_offset.
In my opinion the assignment:

  mmesa->primary_offset = mmesa->mgaScreen->primary.handle;

is a bug.
I've made sure that the kernel code still works even without my changes
however there is the small but >0 possibility that it doesn't as this
address might have been taken when when squeezing a 64bit address into
32 bit, in which case the HandleID() macro creates a different handle.

One could argue that the physical address spaces are usually added 
before SHMs are so this chance never exists.



 > offsets that I'm seeing, which I really can't reconcile with the code
 > actually working.  But if we could confirm that all DRI drivers were
 > using handles to map from, then I think we've got a lot of freedom in
 > deciding what type a handle is.

Right. 
 > 
 > If someone who's looked at this as well could read that page and correct
 > anything I've misunderstood, that would be great.
 > 

Will do.

Egbert.