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

From: Douglas Gilbert
Date: Wed Mar 20 2013 - 21:16:05 EST

On 13-03-20 05:50 PM, Andrew Morton wrote:
On Fri, 15 Mar 2013 18:37:12 +0100 Nicolas Ferre <nicolas.ferre@xxxxxxxxx> 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: ).
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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