From: Nils Carlson Date: Wed, 15 Jun 2011 22:08:54 +0000 (-0700) Subject: drivers/char/hpet.c: fix periodic-emulation for delayed interrupts X-Git-Tag: v3.0-rc4~47 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=273ef9509b7903e50f36aaf9f1d5dc9087fca506;p=pandora-kernel.git drivers/char/hpet.c: fix periodic-emulation for delayed interrupts When interrupts are delayed due to interrupt masking or due to other interrupts being serviced the HPET periodic-emuation would fail. This happened because given an interval t and a time for the current interrupt m we would compute the next time as t + m. This works until we are delayed for > t, in which case we would be writing a new value which is in fact in the past. This can be solved by computing the next time instead as (k * t) + m where k is large enough to be in the future. The exact computation of k is described in a comment to the code. More detail: Assuming an interval of 5 between each expected interrupt we have a normal case of t0: interrupt, read t0 from comparator, set next interrupt t0 + 5 t5: interrupt, read t5 from comparator, set next interrupt t5 + 5 t10: interrupt, read t10 from comparator, set next interrupt t10 + 5 ... So, what happens when the interrupt is serviced too late? t0: interrupt, read t0 from comparator, set next interrupt t0 + 5 t11: delayed interrupt serviced, read t5 from comparator, set next interrupt t5 + 5, which is in the past! ... counter loops ... t10: Much much later, get the next interrupt. This can happen either because we have interrupts masked for too long (some stupid driver goes on a printk rampage) or just because we are pushing the limits of the interval (too small a period), or both most probably. My solution is to read the main counter as well and set the next interrupt to occur at the right interval, for example: t0: interrupt, read t0 from comparator, set next interrupt t0 + 5 t11: delayed interrupt serviced, read t5 from comparator, set next interrupt t15 as t10 has been missed. t15: back on track. Signed-off-by: Nils Carlson Cc: John Stultz Cc: Thomas Gleixner Cc: Clemens Ladisch Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Reading git-diff-tree failed