The /dev/mem mmap constraints apply.
- However, since this is for mapping legacy MMIO space, WB access
- does not make sense. This matters on machines without legacy
- VGA support: these machines may have WB memory for the entire
- first megabyte (or even the entire first granule).
+ mmap of /proc/bus/pci/.../??.?
- On these machines, we could mmap legacy_mem as WB, which would
- be safe in terms of attribute aliasing, but X has no way of
- knowing that it is accessing regular memory, not a frame buffer,
- so the kernel should fail the mmap rather than doing it with WB.
+ This is an MMIO mmap of PCI functions, which additionally may or
+ may not be requested as using the WC attribute.
+
+ If WC is requested, and the region in kern_memmap is either WC
+ or UC, and the EFI memory map designates the region as WC, then
+ the WC mapping is allowed.
+
+ Otherwise, the user mapping must use the same attribute as the
+ kernel mapping.
read/write of /dev/mem
ioremap()
- This returns a kernel identity mapping for use inside the
- kernel.
+ This returns a mapping for use inside the kernel.
If the region is in kern_memmap, we should use the attribute
- specified there. Otherwise, if the EFI memory map reports that
- the entire granule supports WB, we should use that (granules
- that are partially reserved or occupied by firmware do not appear
- in kern_memmap). Otherwise, we should use a UC mapping.
+ specified there.
+
+ If the EFI memory map reports that the entire granule supports
+ WB, we should use that (granules that are partially reserved
+ or occupied by firmware do not appear in kern_memmap).
+
+ If the granule contains non-WB memory, but we can cover the
+ region safely with kernel page table mappings, we can use
+ ioremap_page_range() as most other architectures do.
+
+ Failing all of the above, we have to fall back to a UC mapping.
PAST PROBLEM CASES
succeed. It may create either WB or UC user mappings, depending
on whether the region is in kern_memmap or the EFI memory map.
- mmap of 0x0-0xA0000 /dev/mem by "hwinfo" on HP sx1000 with VGA enabled
+ mmap of 0x0-0x9FFFF /dev/mem by "hwinfo" on HP sx1000 with VGA enabled
See https://bugzilla.novell.com/show_bug.cgi?id=140858.
so it is safe to use WB mappings.
The kernel VGA driver may ioremap the VGA frame buffer at 0xA0000,
- which will use a granule-sized UC mapping covering 0-0xFFFFF. This
- granule covers some WB-only memory, but since UC is non-speculative,
- the processor will never generate an uncacheable reference to the
- WB-only areas unless the driver explicitly touches them.
+ which uses a granule-sized UC mapping. This granule will cover some
+ WB-only memory, but since UC is non-speculative, the processor will
+ never generate an uncacheable reference to the WB-only areas unless
+ the driver explicitly touches them.
mmap of 0x0-0xFFFFF legacy_mem by "X"
- If the EFI memory map reports this entire range as WB, there
- is no VGA MMIO hole, and the mmap should fail or be done with
- a WB mapping.
+ If the EFI memory map reports that the entire range supports the
+ same attributes, we can allow the mmap (and we will prefer WB if
+ supported, as is the case with HP sx[12]000 machines with VGA
+ disabled).
- There's no easy way for X to determine whether the 0xA0000-0xBFFFF
- region is a frame buffer or just memory, so I think it's best to
- just fail this mmap request rather than using a WB mapping. As
- far as I know, there's no need to map legacy_mem with WB
- mappings.
+ If EFI reports the range as partly WB and partly UC (as on sx[12]000
+ machines with VGA enabled), we must fail the mmap because there's no
+ safe attribute to use.
- Otherwise, a UC mapping of the entire region is probably safe.
- The VGA hole means the region will not be in kern_memmap. The
- HP sx1000 chipset doesn't support UC access to the memory surrounding
- the VGA hole, but X doesn't need that area anyway and should not
- reference it.
+ If EFI reports some of the range but not all (as on Intel firmware
+ that doesn't report the VGA frame buffer at all), we should fail the
+ mmap and force the user to map just the specific region of interest.
mmap of 0xA0000-0xBFFFF legacy_mem by "X" on HP sx1000 with VGA disabled
This is a special case of the previous case, and the mmap should
fail for the same reason as above.
+ read of /sys/devices/.../rom
+
+ For VGA devices, this may cause an ioremap() of 0xC0000. This
+ used to be done with a UC mapping, because the VGA frame buffer
+ at 0xA0000 prevents use of a WB granule. The UC mapping causes
+ an MCA on HP sx[12]000 chipsets.
+
+ We should use WB page table mappings to avoid covering the VGA
+ frame buffer.
+
NOTES
[1] SDM rev 2.2, vol 2, sec 4.4.1.
git.openpandora.org / pandora-kernel.git / blobdiff