Messages in this thread | ![/](/images/icornerl.gif) | | Subject | mapping PCI registers with write combining (and PAT on x86)... | From | Roland Dreier <> | Date | Tue, 12 Dec 2006 14:05:32 -0800 |
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Suppose that I would like to map some PIO registers (in a PCI BAR) to userspace, and I would like to enable write combining if possible.
I have two problems. First, there's no generic interface for requesting write combining if possible when doing io_remap_pfn_range(). Would it make sense to define pageprot_writecombine for all architectures (and make it fall back to doing non-cached access if write combining is not possible)? And it seems that making pgprot_noncached() universal wouldn't hurt either.
Second, given the extreme shortage of MTRRs, it seems that write combining is not really possible in general on x86 without some interface to use PATs instead. What is holding up something like Eric Biederman's patches from going in?
Right now we end up with stuff like the absolutely hair-raising code in drivers/video/fbmem.c shown below. I really would like to make progress towards having a better interface for doing this stuff, and I'm more than willing to work on getting something mergable.
#if defined(__mc68000__) #if defined(CONFIG_SUN3) pgprot_val(vma->vm_page_prot) |= SUN3_PAGE_NOCACHE; #elif defined(CONFIG_MMU) if (CPU_IS_020_OR_030) pgprot_val(vma->vm_page_prot) |= _PAGE_NOCACHE030; if (CPU_IS_040_OR_060) { pgprot_val(vma->vm_page_prot) &= _CACHEMASK040; /* Use no-cache mode, serialized */ pgprot_val(vma->vm_page_prot) |= _PAGE_NOCACHE_S; } #endif #elif defined(__powerpc__) vma->vm_page_prot = phys_mem_access_prot(file, off >> PAGE_SHIFT, vma->vm_end - vma->vm_start, vma->vm_page_prot); #elif defined(__alpha__) /* Caching is off in the I/O space quadrant by design. */ #elif defined(__i386__) || defined(__x86_64__) if (boot_cpu_data.x86 > 3) pgprot_val(vma->vm_page_prot) |= _PAGE_PCD; #elif defined(__mips__) || defined(__sparc_v9__) vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot); #elif defined(__hppa__) pgprot_val(vma->vm_page_prot) |= _PAGE_NO_CACHE; #elif defined(__arm__) || defined(__sh__) || defined(__m32r__) vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot); #elif defined(__ia64__) if (efi_range_is_wc(vma->vm_start, vma->vm_end - vma->vm_start)) vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot); else vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot); #else #warning What do we have to do here?? #endif if (io_remap_pfn_range(vma, vma->vm_start, off >> PAGE_SHIFT, vma->vm_end - vma->vm_start, vma->vm_page_prot)) return -EAGAIN; return 0;
Thanks! Roland - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.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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