Re: [PATCH 06/12] drm/i915: Do not lie about atomic wait granularity

[Chris Wilson <chris@chris-wilson.co.uk>]

On Tue, Feb 02, 2016 at 11:06:24AM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> 
> Currently the wait_for_atomic_us only allows for a millisecond
> granularity which is not nice towards callers requesting small
> micro-second waits.
> 
> Re-implement it so micro-second granularity is really supported
> and not just in the name of the macro.
> 
> v2:
>   * Warn when used from non-atomic context (if possible).
>   * Warn on too long atomic waits.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>

This also warns if we attempt to use a variable timeout as well, which
is a bonus.

>  drivers/gpu/drm/i915/intel_drv.h | 27 ++++++++++++++++++++++++---
>  1 file changed, 24 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
> index 779d17a9fcce..af6c1539106f 100644
> --- a/drivers/gpu/drm/i915/intel_drv.h
> +++ b/drivers/gpu/drm/i915/intel_drv.h
> @@ -63,11 +63,32 @@
>  	ret__;								\
>  })
>  
> +#if defined(CONFIG_DRM_I915_DEBUG) && defined(CONFIG_PREEMPT_COUNT)

Ok, that took a little bit of digging to verify.

/* If CONFIG_PREEMPT_COUNT is disabled, in_atomic() always reports false. */

> +  #define _WAIT_FOR_ATOMIC_CHECK WARN_ON_ONCE(!in_atomic())
> +#else
> +  #define _WAIT_FOR_ATOMIC_CHECK do { } while(0)
> +#endif

Hmm, not often that we do indent inside #if, but when it is, it is
more typically '# define'

> +#define _wait_for_atomic(COND, US) ({ \
> +	unsigned long end__; \
> +	int ret__ = 0; \
> +	_WAIT_FOR_ATOMIC_CHECK; \
> +	BUILD_BUG_ON((US) > 50000); \
> +	end__ = (local_clock() >> 10) + (US) + 1; \
> +	while (!(COND)) { \
> +		if (time_after((unsigned long)(local_clock() >> 10), end__)) { \
> +			ret__ = -ETIMEDOUT; \
> +			break; \
> +		} \
> +		cpu_relax(); \
> +	} \
> +	ret__; \
> +})

Ok,
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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