linux-kernel - Re: USB mass storage and ARM cache coherency

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Date:	Wed, 17 Feb 2010 20:44:04 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Oliver Neukum <oliver@...kum.org>,
	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	Matthew Dharm <mdharm-kernel@...-eyed-alien.net>,
	Ming Lei <tom.leiming@...il.com>,
	"Mankad, Maulik Ojas" <x0082077@...com>,
	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Pavel Machek <pavel@....cz>, Greg KH <greg@...ah.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: USB mass storage and ARM cache coherency

On Thu, Feb 18, 2010 at 07:37:00AM +1100, Benjamin Herrenschmidt wrote:
> On Wed, 2010-02-17 at 15:27 +0000, Catalin Marinas wrote:
> > We do the same on ARM. The problem with most (all) HCD drivers that do
> > PIO is that they copy the data to the transfer buffer but there is no
> > call in this driver to flush_dcache_page(). The upper mass storage or
> > filesystem layers don't call this function either, so there isn't
> > anything that would set the PG_arch1 bit. 
> 
> Actually, clear it :-)
> 
> I suppose that's one thing that needs to be fixed in the drivers.

No, because that'd probably bugger up the Sparc64 method of delaying
flush_dcache_page.

This method works as follows:

- a page cache page is allocated - this has PG_arch_1 clear.

- IO happens on it and it's placed into the page cache.  PG_arch_1 is
  still clear.

- someone calls read()/write() which accesses the page.  The generic
  file IO layers call flush_dcache_page() in response to read()/write()
  fs calls.  flush_dcache_page() spots that the page is not yet mapped
  into userspace, and sets PG_arch_1 to mark the fact that the kernel
  mapping is dirty.

- when someone maps the page, we check PG_arch_1 in update_mmu_cache.
  If PG_arch_1 is set, we flush the kernel mapping.

Clearly, if we go around having drivers clearing PG_arch_1, this is going
to break horribly.
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