svn commit: r273963 - head/sys/dev/agp

[Nathan Whitehorn]

On 11/02/14 08:07, Tijl Coosemans wrote:
> On Sun, 02 Nov 2014 07:27:24 -0800 Nathan Whitehorn <nwhitehorn at freebsd.org> wrote:
>> There is actually a write-combining memory type on PowerPC. You can set
>> it the same way as on x86 (with VM_MEMATTR_WRITE_COMBINING).
> Yes, that's what the patch does, but in sys/powerpc (aim/mmu_oea.c,
> aim/mmu_oea64.c and booke/pmap.c) VM_MEMATTR_WRITE_COMBINING becomes
> PTE_I meaning Caching Inhibited.  Does powerpc do write-combining for
> this type?


It is write-combining on all north bridges that support write combining. 
Regular caching-inhibited is PTE_I | PTE_G. PTE_I means there is no 
caching, which you want, and PTE_G means that every read/write 
instruction is a read/write operation of exactly that length exactly 
right then on the bus. Removing the PTE_G (guarded) flag means the bus 
is free to do write-combining if it wants to and is the standard way to 
signal this.

> That code also maps VM_MEMATTR_WRITE_BACK to PTE_I by the way.  I think
> that should be PTE_M (write-back caching + cache coherency).
>

I think you're right but would like to consider for a few days before 
changing it.
-Nathan