On Wed, 31 Aug 2011, Will Deacon wrote:
> On Wed, Aug 31, 2011 at 02:43:33PM +0100, Mark Salter wrote:
> > On Wed, 2011-08-31 at 09:49 +0100, Will Deacon wrote:
> > > On Wed, Aug 31, 2011 at 01:23:47AM +0100, Chen Peter-B29397 wrote:
> > > > One question: why this write buffer issue did not happen at UP ARM V7 platform, whose dma buffer
> > > > also uncache, but bufferable?
> > >
> > > Which CPU was on this platform?
> >
> > Using a 3.1.0-rc4+ kernel on a Pandaboard, and running 'hdparm -t' on a
> > usb disk drive, I see ~5.8MB/s read speed. Same kernel, but passing
> > nosmp on the commandline, I see 20.3MB/s.
> >
> > Can someone explain why nosmp would make such a difference?
>
> Oh gawd, that's horrible. I have a feeling it's probably a separate issue
> though, caused by:
>
> omap_modify_auxcoreboot0(0x200, 0xfffffdff);
>
> in boot_secondary for OMAP. Unfortunately I have no idea what that line is
> doing because it ends up talking to the secure monitor.
Well, this issue is apparently affecting other ARMv9 implementations
too. In which case this code in arch/arm/mm/mmu.c could be responsible:
if (is_smp()) {
/*
* Mark memory with the "shared" attribute
* for SMP systems
*/
user_pgprot |= L_PTE_SHARED;
kern_pgprot |= L_PTE_SHARED;
vecs_pgprot |= L_PTE_SHARED;
mem_types[MT_DEVICE_WC].prot_sect |= PMD_SECT_S;
mem_types[MT_DEVICE_WC].prot_pte |= L_PTE_SHARED;
mem_types[MT_DEVICE_CACHED].prot_sect |= PMD_SECT_S;
mem_types[MT_DEVICE_CACHED].prot_pte |= L_PTE_SHARED;
mem_types[MT_MEMORY].prot_sect |= PMD_SECT_S;
mem_types[MT_MEMORY].prot_pte |= L_PTE_SHARED;
mem_types[MT_MEMORY_NONCACHED].prot_sect |= PMD_SECT_S;
mem_types[MT_MEMORY_NONCACHED].prot_pte |= L_PTE_SHARED;
}
However I don't see the nosmp kernel argument having any effect on the
result from is_smp().
Nicolas
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