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.
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.
Cheers,
Robin.
