Am 15.12.22 um 00:08 schrieb Robin Murphy:
On 2022-12-14 22:02, Alex Deucher wrote:
On Wed, Dec 14, 2022 at 4:54 PM Robin Murphy <robin.murphy@xxxxxxx>
wrote:
On 2022-12-12 02:08, Luben Tuikov wrote:
Fix screen corruption on older 32-bit systems using AGP chips.
On older systems with little memory, for instance 1.5 GiB, using an
AGP chip,
the device's DMA mask is 0xFFFFFFFF, but the memory mask is
0x7FFFFFF, and
subsequently dma_addressing_limited() returns 0xFFFFFFFF < 0x7FFFFFFF,
false. As such the result of this static inline isn't suitable for
the last
argument to ttm_device_init()--it simply needs to now whether to
use GFP_DMA32
when allocating DMA buffers.
This sounds wrong to me. If the issues happen on systems without PAE it
clearly can't have anything to with the actual DMA address size. Not to
mention that AFAICS 32-bit x86 doesn't even have ZONE_DMA32, so
GFP_DMA32 would be functionally meaningless anyway. Although the
reported symptoms initially sounded like they could be caused by DMA
going to the wrong place, that is also equally consistent with a
loss of
cache coherency.
My (limited) understanding of AGP is that the GART can effectively
alias
memory to a second physical address, so I could well believe that
something somewhere in the driver stack needs to perform some cache
maintenance to avoid coherency issues, and that in these particular
setups whatever that is might be assuming the memory is direct-mapped
and thus going wrong for highmem pages.
So as I said before, I really think this is not about using
GFP_DMA32 at
all, but about *not* using GFP_HIGHUSER.
One of the wonderful features of AGP is that it has to be used with
uncached memory. The aperture basically just provides a remapping of
physical pages into a linear aperture that you point the GPU at. TTM
has to jump through quite a few hoops to get uncached memory in the
first place, so it's likely that that somehow isn't compatible with
HIGHMEM. Can you get uncached HIGHMEM?
I guess in principle yes, if you're careful not to use regular
kmap()/kmap_atomic(), and always use pgprot_noncached() for
userspace/vmalloc mappings, but clearly that leaves lots of scope for
slipping up.
I theory we should do exactly that in TTM, but we have very few users
who actually still exercise that functionality.
Working backwards from primitives like set_memory_uc(), I see various
paths in TTM where manipulating the caching state is skipped for
highmem pages, but I wouldn't even know where to start looking for
whether the right state is propagated to all the places where they
might eventually be mapped somewhere.
The tt object has the caching state for the pages and
ttm_prot_from_caching() then uses pgprot_noncached() and co for the
userspace/vmalloc mappings.
Regards,
Christian.
Cheers,
Robin.
