From: Andy Shevchenko andriy.shevchenko@linux.intel.com
commit b6da197a2e9670df6f07e6698629e9ce95ab614e upstream.
As reported by Guilherme G. Piccoli:
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The rtc-cmos interrupt setting was changed in the commit 079062b28fb4 ("rtc: cmos: prevent kernel warning on IRQ flags mismatch") in order to allow shared interrupts; according to that commit's description, some machine got kernel warnings due to the interrupt line being shared between rtc-cmos and other hardware, and rtc-cmos didn't allow IRQ sharing that time.
After the aforementioned commit though it was observed a huge increase in lost HPET interrupts in some systems, observed through the following kernel message:
[...] hpet1: lost 35 rtc interrupts
After investigation, it was narrowed down to the shared interrupts usage when having the kernel option "irqpoll" enabled. In this case, all IRQ handlers are called for non-timer interrupts, if such handlers are setup in shared IRQ lines. The rtc-cmos IRQ handler could be set to hpet_rtc_interrupt(), which will produce the kernel "lost interrupts" message after doing work - lots of readl/writel to HPET registers, which are known to be slow.
Although "irqpoll" is not a default kernel option, it's used in some contexts, one being the kdump kernel (which is an already "impaired" kernel usually running with 1 CPU available), so the performance burden could be considerable. Also, the same issue would happen (in a shorter extent though) when using "irqfixup" kernel option.
In a quick experiment, a virtual machine with uptime of 2 minutes produced
300 calls to hpet_rtc_interrupt() when "irqpoll" was set, whereas without
sharing interrupts this number reduced to 1 interrupt. Machines with more hardware than a VM should generate even more unnecessary HPET interrupts in this scenario.
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After looking into the rtc-cmos driver history and DSDT table from the Microsoft Surface 3, we may notice that Hans de Goede submitted a correct fix (see dependency below). Thus, we simply revert the culprit commit.
Fixes: 079062b28fb4 ("rtc: cmos: prevent kernel warning on IRQ flags mismatch") Depends-on: a1e23a42f1bd ("rtc: cmos: Do not assume irq 8 for rtc when there are no legacy irqs") Reported-by: Guilherme G. Piccoli gpiccoli@canonical.com Cc: Hans de Goede hdegoede@redhat.com Signed-off-by: Andy Shevchenko andriy.shevchenko@linux.intel.com Tested-by: Guilherme G. Piccoli gpiccoli@canonical.com Reviewed-by: Hans de Goede hdegoede@redhat.com Link: https://lore.kernel.org/r/20200123131437.28157-1-andriy.shevchenko@linux.int... Signed-off-by: Alexandre Belloni alexandre.belloni@bootlin.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- drivers/rtc/rtc-cmos.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/drivers/rtc/rtc-cmos.c +++ b/drivers/rtc/rtc-cmos.c @@ -806,7 +806,7 @@ cmos_do_probe(struct device *dev, struct rtc_cmos_int_handler = cmos_interrupt;
retval = request_irq(rtc_irq, rtc_cmos_int_handler, - IRQF_SHARED, dev_name(&cmos_rtc.rtc->dev), + 0, dev_name(&cmos_rtc.rtc->dev), cmos_rtc.rtc); if (retval < 0) { dev_dbg(dev, "IRQ %d is already in use\n", rtc_irq);