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

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Message-ID: <1267399222.4485.26.camel@e102109-lin.cambridge.arm.com>
Date:	Sun, 28 Feb 2010 23:20:22 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Matthew Dharm <mdharm-kernel@...-eyed-alien.net>,
	linux-usb@...r.kernel.org, "Mankad, Maulik Ojas" <x0082077@...com>,
	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Ming Lei <tom.leiming@...il.com>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	Oliver Neukum <oliver@...kum.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	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 Fri, 2010-02-26 at 22:03 +0000, Russell King - ARM Linux wrote:
> On Sat, Feb 27, 2010 at 08:49:40AM +1100, Benjamin Herrenschmidt wrote:
> > It will deadlock if you use normal IRQs. I don't see a good way around
> > that other than using a higher-level type of IRQs. I though ARM has
> > something like that (FIQs ?). Can you use those guys for IPIs ?
[...]
> The other problem we'd encounter using FIQs for IPIs is that some IPIs
> need to take locks - and in order to make that safe, we'd either need
> another class of locks which disable IRQs and FIQs together, or we'd
> need to disable FIQs everywhere we disable IRQs - at which point FIQs
> become utterly pointless.

You could use the FIQ only for the DMA cache maintenance operations and
not as a generic IPI mechanism. But the hardware needs to be modified.


-- 
Catalin

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