Re: kmap_atomic_pfn for PCI BAR access?

[Jeremy Fitzhardinge]

Dave Airlie wrote:
> 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,
> > > 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?
> > >
> > >       
> > Usually people use ioremap to map device memory.  Wouldn't that work in this
> > 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.
>   

Uh, yep.

> 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.
>   

Maybe Nick's vmap reimplementation would help here.  It effectively 
allows you to map stuff into the vmalloc space, and do lazy tlb flushes 
to mitigate the cost of map/unmap.  He posted the patches week or so ago.

   J
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/