Em Qua, 2016-09-14 às 10:22 +0100, Chris Wilson escreveu:
> On Tue, Sep 13, 2016 at 08:40:19PM +0100, Chris Wilson wrote:
> >
> > I was looking at some wait_for() timeouts on a slow system, with
> > lots of
> > debug enabled (KASAN, lockdep, mmio_debug). Thinking that we were
> > mishandling the timeout, I tried to ensure that we loop at least
> > once
> > after first testing COND. However, the double test of COND either
> > side
> > of the timeout check makes that unlikely. But we can do an
> > equivalent
> > loop, that keeps the COND check after testing for timeout (required
> > so
> > that we are not preempted between testing COND and then testing for
> > a
> > timeout) without expanding COND twice.
> >
> > The advantage of only expanding COND once is a dramatic reduction
> > in
> > code size:
> >
> > text data bss dec hex
> > 1308733 5184 1152 1315069 1410fd
> > before
> > 1305341 5184 1152 1311677 1403bd
> > after
> >
> > Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursu...@intel.com>
> > Cc: Ville Syrjälä <ville.syrj...@linux.intel.com>
> > Cc: Daniel Vetter <daniel.vet...@ffwll.ch>
> > ---
> > drivers/gpu/drm/i915/intel_drv.h | 13 ++++++++-----
> > 1 file changed, 8 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_drv.h
> > b/drivers/gpu/drm/i915/intel_drv.h
> > index cb99a2540863..597899d71df9 100644
> > --- a/drivers/gpu/drm/i915/intel_drv.h
> > +++ b/drivers/gpu/drm/i915/intel_drv.h
> > @@ -52,13 +52,16 @@
> > */
> > #define _wait_for(COND, US, W) ({ \
> > unsigned long timeout__ = jiffies + usecs_to_jiffies(US) +
> > 1; \
> > - int ret__ = 0;
> > \
> > - while (!(COND)) {
> > \
> > - if (time_after(jiffies, timeout__)) {
> > \
> > - if (!(COND))
> > \
> > - ret__ = -ETIMEDOUT;
> > \
> > + int ret__;
> > \
>
> Ok, this is the magic. Missed initializer, gcc goes wild trimming
> undefined code. Patch is completely bogus.
IMHO, expanding a macro argument only once is an improvement on its
own, even if the resulting binary is not smaller, since it makes the
code a little safer.
> -Chris
>
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