linux-kernel - Re: [PATCH] rtc: rtc-at91rm9200: use a variable for storing IMR

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Date:	Tue, 26 Mar 2013 17:09:59 -0400
From:	Douglas Gilbert <dgilbert@...erlog.com>
To:	Johan Hovold <jhovold@...il.com>
CC:	Nicolas Ferre <nicolas.ferre@...el.com>,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	Ludovic Desroches <ludovic.desroches@...el.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rtc: rtc-at91rm9200: use a variable for storing IMR

On 13-03-26 03:27 PM, Johan Hovold wrote:
> On Fri, Mar 15, 2013 at 06:37:12PM +0100, Nicolas Ferre wrote:
>> On some revisions of AT91 SoCs, the RTC IMR register is not working.
>> Instead of elaborating a workaround for that specific SoC or IP version,
>> we simply use a software variable to store the Interrupt Mask Register and
>> modify it for each enabling/disabling of an interrupt. The overhead of this
>> is negligible anyway.
>
> The patch does not add any memory barriers or register read-backs when
> manipulating the interrupt-mask variable. This could possibly lead to
> spurious interrupts both when enabling and disabling the various
> RTC-interrupts due to write reordering and bus latencies.
>
> Has this been considered? And is this reason enough for a more targeted
> work-around so that the SOCs with functional RTC_IMR are not affected?

The SoCs in question use a single embedded ARM926EJ-S and
according to the Atmel documentation, that CPU's instruction
set contains no barrier (or related) instructions.

In the arch/arm/mach-at91 sub-tree of the kernel source
I can find no use of the wmb() call. Also checked all drivers
in the kernel containing "at91" and none called wmb().

Doug Gilbert

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