Subject: Re: USB mass storage and ARM cache coherency
From: Catalin Marinas <catalin.marinas@arm.com>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Matthew Dharm <mdharm-kernel@one-eyed-alien.net>,
       linux-usb@vger.kernel.org, "Mankad, Maulik Ojas" <x0082077@ti.com>,
       Sergei Shtylyov <sshtylyov@ru.mvista.com>,
       Ming Lei <tom.leiming@gmail.com>,
       Sebastian Siewior <bigeasy@linutronix.de>,
       Oliver Neukum <oliver@neukum.org>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       "Shilimkar, Santosh" <santosh.shilimkar@ti.com>,
       Pavel Machek <pavel@ucw.cz>, Greg KH <greg@kroah.com>,
       linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
In-Reply-To: <20100226220351.GE23933@n2100.arm.linux.org.uk>
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Organization: ARM Limited
Date: Sun, 28 Feb 2010 23:20:22 +0000
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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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