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

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Date:	Wed, 20 Mar 2013 21:15:23 -0400
From:	Douglas Gilbert <dgilbert@...erlog.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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 13-03-20 05:50 PM, Andrew Morton wrote:
> On Fri, 15 Mar 2013 18:37:12 +0100 Nicolas Ferre <nicolas.ferre@...el.com> 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.
>
> This description doesn't really allow me or others to work out whether
> the fix should be included in 3.9 or backported into earlier kernels.
>
> So please, when fixing a bug do include a full description of the
> user-visible effects of that bug.  And your opinion regarding the
> -mainline and -stable decision is always useful.

The interrupt mask register (IMR) for the RTC is broken
on the AT91SAM9x5 sub-family of SoCs (good overview of the
members here: http://www.eewiki.net/display/linuxonarm/AT91SAM9x5 ).
The "user visible effect" is the RTC doesn't work.

That sub-family is less than two years old and only has devicetree
(DT) support and came online circa lk 3.7 . The dust is yet to
settle on the DT stuff at least for AT91 SoCs (translation:
lots of stuff is still broken, so much that it is hard to know
where to start).

The fix in the patch is pretty simple: just shadow the silicon
IMR register with a variable in the driver. Some older SoCs (pre-DT)
use the the rtc-at91rm9200 driver (e.g. obviously the AT91RM9200)
and they should not be impacted by the change. There shouldn't
be a large volume of interrupts associated with a RTC.

Compared to a relatively stable kernel subsystem like SCSI, what
is happening in the ARM architecture with DT is huge and ongoing.
So I think you either need new rules or suspend some of the stricter
rules applied to more stable subsystems. Just my two cents worth.

Doug Gilbert

who hasn't seen that frill-necked lizard for a while
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