On Fri, Nov 28, 2014 at 01:46:27PM +0000, Dave Gordon wrote: > On 28/11/14 09:29, Daniel Vetter wrote: > > We've lost the +1 required for correct timeouts in > > > > commit 5ed0bdf21a85d78e04f89f15ccf227562177cbd9 > > Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > Date: Wed Jul 16 21:05:06 2014 +0000 > > > > drm: i915: Use nsec based interfaces > > > > Use ktime_get_raw_ns() and get rid of the back and forth timespec > > conversions. > > > > Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > Acked-by: Daniel Vetter <daniel.vetter@xxxxxxxx> > > Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx> > > > > So fix this up by reinstating our handrolled _timeout function. While > > at it bother with handling MAX_JIFFIES. > > > > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82749 > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > Cc: John Stultz <john.stultz@xxxxxxxxxx> > > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx> > > --- > > drivers/gpu/drm/i915/i915_drv.h | 10 ++++++++++ > > drivers/gpu/drm/i915/i915_gem.c | 3 ++- > > 2 files changed, 12 insertions(+), 1 deletion(-) > > > > diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h > > index 02b3cb32c8a6..caae337c0199 100644 > > --- a/drivers/gpu/drm/i915/i915_drv.h > > +++ b/drivers/gpu/drm/i915/i915_drv.h > > @@ -3030,6 +3030,16 @@ static inline unsigned long msecs_to_jiffies_timeout(const unsigned int m) > > return min_t(unsigned long, MAX_JIFFY_OFFSET, j + 1); > > } > > > > +static inline unsigned long nsecs_to_jiffies_timeout(const u64 m) > > +{ > > + unsigned long j = nsecs_to_jiffies(m); > > nsecs_to_jiffies() may be (relatively) expensive (mul/div/etc), so I'd > be inclined to move the call until after the test below. It would be > nice if the test turned into a single comparison, since the RHS is a > constant for a given kernel build; but it looks like jiffies_to_usecs() > isn't expanded inline, since it's in time.c :-( In which case swapping > the lines around may also help the compiler keep 'j' live. This is only called in code that's about to sleep. Wasting a few cpu cycles is totally ok ;-) This is also the pattern the non-_timeout functions in time.c use, so I think we should be ok > > + if (m > (u64)jiffies_to_usecs(MAX_JIFFY_OFFSET) * 1000) > > I think there's a problem with this line anyway. In kernel/time/time.c: > > // Warning! Result type may be narrower than parameter type - DSG > unsigned int jiffies_to_usecs(const unsigned long j) > { > #if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ) > return (USEC_PER_SEC / HZ) * j; > #elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC) > return (j + (HZ / USEC_PER_SEC) - 1)/(HZ / USEC_PER_SEC); > #else > # if BITS_PER_LONG == 32 > return (HZ_TO_USEC_MUL32 * j) >> HZ_TO_USEC_SHR32; > # else > return (j * HZ_TO_USEC_NUM) / HZ_TO_USEC_DEN; > # endif > #endif > } > > Also, include/linux/jiffies.h: > > #define MAX_JIFFY_OFFSET ((LONG_MAX >> 1)-1) > > and include/linux/kernel.h: > > #define LONG_MAX ((long)(~0UL>>1)) > > So, on a 64-bit build we'll have LONG_MAX == 0x7fff_ffff_ffff_ffff and > MAX_JIFFY_OFFSET == 0x3fff_ffff_ffff_fffe. Multiplying that by 1000 > gives an answer that doesn't fit in an unsigned int! Oh blergh I've missed that jiffies is a long. > Even on a 32-bit build (where LONG_MAX == 0x7fff_ffff and > MAX_JIFFY_OFFSET == 0x3fff_fffe) MAX_JIFFY_OFFSET can't be multiplied by > any typical value of HZ (50, 60, 1000) without overflow! Cast operator binds tigther than *, so this case actually works. > I think the only way to get this right, give the somewhat broken nature > of the kernel function signatures and its lack of a u64 jiffies-to-nsecs > function, is to convert ONE jiffy to (unsigned int) usecs, > then widen to u64 before converting to nsecs and using that for the rest > of the calculations. I think this still overflows. I'm somewhat inclined to just tell userspace to sod off for large timeout values since we really don't care about those. But I'll see whether I can fix it up first without too much trouble. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
