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Date: Wed, 25 Jun 2008 21:36:03 -0700 From: Arjan van de Ven <arjan@...radead.org> To: "Dave Airlie" <airlied@...il.com> Cc: "Jeremy Fitzhardinge" <jeremy@...p.org>, "Keith Packard" <keithp@...thp.com>, "Dave Airlie" <airlied@...ux.ie>, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: kmap_atomic_pfn for PCI BAR access? On Thu, 26 Jun 2008 11:23:11 +1000 "Dave Airlie" <airlied@...il.com> wrote: > On Thu, Jun 26, 2008 at 11:18 AM, Jeremy Fitzhardinge > <jeremy@...p.org> 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. > > 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. well kmap does a tlb flush as well... you can't get away from doing a flush if you change cpu mapping. What you CAN do is play tricks and flush only once in a while and make sure you don't recycle mappings in the mean time (like kmap does). I can totally see doing an iounmap_lazy() and then have an iounmap_flush_lazy() thing or something like that.... -- If you want to reach me at my work email, use arjan@...ux.intel.com For development, discussion and tips for power savings, visit http://www.lesswatts.org -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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