[Intel-gfx] [PATCH] drm/i915: Only expand COND once in wait_for()

[Chris Wilson]

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 <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Cc: Daniel Vetter <daniel.vetter at 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.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre