On Thu, Jun 26, 2008 at 11:18 AM, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
Keith Packard wrote:
The graphics memory BAR is generally fairly good sized; on Intel chips,Usually people use ioremap to map device memory. Wouldn't that work in this
it's between 256M and 1G (and growing). I want to write data into this
region from kernel space, but it's really too big to map the whole thing
into kernel address space, especially on 32-bit systems. ioremap is not
a good option here -- it's way too slow.
With CONFIG_HIGHMEM enabled, I can use kmap_atomic_pfn (well, actually
the kmap_atomic_proc_pfn included in the DRM tree) and things work quite
well -- performance is good, with barely any measurable time spent in
the PTE whacking (~1%).
However, with CONFIG_HIGHMEM disabled, there aren't any PTEs reserved
for this kind of mapping fun. This makes me suspect that abusing
kmap_atomic for this operation would not be appreciated.
Should I use kmap_atomic_pfn to reach my PCI BAR like this?
Would it be reasonable to supply a patch that made this work even
without CONFIG_HIGHMEM?
case?
"but it's really too big to map the whole thing
into kernel address space, especially on 32-bit systems. ioremap is not
a good option here -- it's way too slow."
From the original mail.
doing tlb flush for iounmap is slow as all hell if you do it a lot,
and we can't afford to mmap the whole aperture it can 1GB.