Messages in this thread | | | Date | Thu, 28 Mar 2013 10:57:18 +0100 | From | Johan Hovold <> | Subject | Re: [PATCH] rtc: rtc-at91rm9200: use a variable for storing IMR |
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On Tue, Mar 26, 2013 at 05:09:59PM -0400, Douglas Gilbert wrote: > 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.
The ARM926EJ-S actually does have a Drain Write Buffer instruction but it's not used by the ARM barrier-implementation unless CONFIG_ARM_DMA_MEM_BUFFERABLE or CONFIG_SMP is set.
However, wmb() always implies a compiler barrier which is what is needed in this case.
> 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().
I/O-operations are normally not reordered, but this patch is faking a hardware register and thus extra care needs to be taken.
To repeat:
> @@ -198,9 +203,12 @@ static int at91_rtc_alarm_irq_enable(struct device *dev, unsigned int enabled) > > if (enabled) { > at91_rtc_write(AT91_RTC_SCCR, AT91_RTC_ALARM); > + at91_rtc_imr |= AT91_RTC_ALARM;
Here a barrier is needed to prevent the compiler from reordering the two writes (i.e., mask update and interrupt enable).
> at91_rtc_write(AT91_RTC_IER, AT91_RTC_ALARM); > - } else > + } else { > at91_rtc_write(AT91_RTC_IDR, AT91_RTC_ALARM);
Here a barrier is again needed to prevent the compiler from reordering, but we also need a register read back (of some RTC-register) before updating the mask. Without the register read back, there could be a window where the mask does not match the hardware state due to bus latencies.
Note that even with a register read back there is a (theoretical) possibility that the interrupts have not yet been disabled when the fake mask is updated. The only way to know for sure is to poll RTC_IMR but that is the very register you're trying to emulate.
> + at91_rtc_imr &= ~AT91_RTC_ALARM; > + } > > return 0; > }
In the worst-case scenario ignoring the shared RTC-interrupt could lead to the disabling of the system interrupt and thus also PIT, DBGU, ...
I think this patch should be reverted and a fix for the broken SoCs be implemented which does not penalise the other SoCs. That is, only fall-back to faking IMR on the SoCs where it is actually broken.
Nicolas, should I send a revert patch and follow up with a fix for the broken SoCs which includes the required barriers and read-backs?
Note that the patch is already being picked up for some stable trees. The fix I'm proposing would require adding minimal DT-support to the driver and is not really stable material. Therefore, a revert followed by a patch for 3.10 seems like the way to go.
Johan
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