linux-kernel - RE: Mapping PCI BAR through /sys/devices/pci* sets cache-disable andwrite-through

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Date:	Wed, 16 Apr 2008 15:23:25 -0700
From:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
To:	"Keith Packard" <keithp@...thp.com>,
	"linux-kernel" <linux-kernel@...r.kernel.org>
Cc:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Subject: RE: Mapping PCI BAR through /sys/devices/pci* sets cache-disable andwrite-through

 

>-----Original Message-----
>From: linux-kernel-owner@...r.kernel.org 
>[mailto:linux-kernel-owner@...r.kernel.org] On Behalf Of Keith Packard
>Sent: Wednesday, April 16, 2008 10:40 AM
>To: linux-kernel
>Subject: Mapping PCI BAR through /sys/devices/pci* sets 
>cache-disable andwrite-through
>
>The X server recently (within the last year) switched from mapping the
>frame buffer from /dev/mem to using /sys/devices/pci*. This caused a
>significant memory bandwidth reduction for writes, similar to 
>the effect
>caused when the associated MTRR is set to UC instead of WC.
>
>Looking at the code path, we find, in i386:pci_mmap_page_range:
>
>        prot = pgprot_val(vma->vm_page_prot);
>        if (boot_cpu_data.x86 > 3)
>                prot |= _PAGE_PCD | _PAGE_PWT;
>        vma->vm_page_prot = __pgprot(prot);
>
>Which is to say, on any CPU which supports it, force the cache-disable
>and write-through bits on.
>
>Is there some reason this is there? Surely applications should be
>expected to set these attributes correctly, currently using MTRR and in
>the future, using PAT directly.
>

Setting flags "_PAGE_PCD | _PAGE_PWT" maps to PAT_3 which is "UC". So,
the mapping X server will get is UC and if X sets MTRR to WC, it will
not have any effect on this mapping, which will be still UC. There may
be apps which depend on this /sys/ interface giving UC mapping always...

With PAT patches, we now have /sys/devices/pci*resource_wc (in addition
to resource) for any PREFETCHABLE region. So, X has to change to use
that new interface to get WC mapping.

Thanks,
Venki
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