[Intel-gfx] [PATCH] drm/i915: Allow fuzzy matching in intel_compare_link_m_n

[Daniel Vetter]

On Wed, Jan 06, 2016 at 01:54:43PM +0100, Maarten Lankhorst wrote:
> This prevents a unnecessary modeset on a dell XPS 13 (2016).
> 
> N is always a power of 2, which means that for fuzzy matching we should
> compare for inequality on the n values, then do fuzzy matching on the m
> values.
> 
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Tested-by: 

Wasn't sure about overflow risks, but as long as the value we compute
originally computed didn't overflow this should be safe.

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

> --
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 7afbdc45a278..aa4f1e69b92e 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -12638,19 +12638,22 @@ intel_compare_m_n(unsigned int m, unsigned int n,
>  
>  	BUILD_BUG_ON(DATA_LINK_M_N_MASK > INT_MAX);
>  
> -	if (m > m2) {
> -		while (m > m2) {
> +	if (n > n2) {
> +		while (n > n2) {
>  			m2 <<= 1;
>  			n2 <<= 1;
>  		}
> -	} else if (m < m2) {
> -		while (m < m2) {
> +	} else if (n < n2) {
> +		while (n < n2) {
>  			m <<= 1;
>  			n <<= 1;
>  		}
>  	}
>  
> -	return m == m2 && n == n2;
> +	if (n != n2)
> +		return false;
> +
> +	return intel_fuzzy_clock_check(m, m2);
>  }
>  
>  static bool
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch