Re: [PATCH] usb: ehci: make HC see up-to-date qh/qtd descriptor ASAP

From: Nicolas Pitre <nico@fluxnic.net>
To: Will Deacon <will.deacon@arm.com>
Cc: Mark Salter <msalter@redhat.com>,
	Russell King <linux@arm.linux.org.uk>,
	Chen Peter-B29397 <B29397@freescale.com>,
	"ming.lei@canonical.com" <ming.lei@canonical.com>,
	"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
	"stern@rowland.harvard.edu" <stern@rowland.harvard.edu>,
	Greg KH <greg@kroah.com>,
	"linux-omap@vger.kernel.org" <linux-omap@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] usb: ehci: make HC see up-to-date qh/qtd descriptor ASAP
Date: Wed, 31 Aug 2011 13:46:50 -0400 (EDT)	[thread overview]
Message-ID: <alpine.LFD.2.00.1108311332410.20358@xanadu.home> (raw)
In-Reply-To: <20110831152137.GG8777@e102144-lin.cambridge.arm.com>

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