[Intel-gfx] [PATCH v2 01/13] drm/i915: Avoid negative shift due to bigjoiner_pipes==0

[Navare, Manasi]

On Wed, Feb 23, 2022 at 03:13:03PM +0200, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> bigjoiner_pipes==0 leads bigjoiner_master_pipe() to
> do BIT(ffs(0)-1) which is undefined behaviour. The code should
> actually still work fine since the only place we provoke
> that is intel_crtc_bigjoiner_slave_pipes() and it'll bitwise
> AND the result with 0, so doesn't really matter what we get
> out of bigjoiner_master_pipe(). But best not provoke undefined
> behaviour anyway.
> 
> Reported-by: kernel test robot <oliver.sang at intel.com>
> Fixes: a6e7a006f5d5 ("drm/i915: Change bigjoiner state tracking to use the pipe bitmask")
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

Reviewed-by: Manasi Navare <manasi.d.navare at intel.com>

Manasi

> ---
>  drivers/gpu/drm/i915/display/intel_display.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> index 80b19c304c43..f3f5f11a5abf 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.c
> +++ b/drivers/gpu/drm/i915/display/intel_display.c
> @@ -346,7 +346,10 @@ static enum pipe bigjoiner_master_pipe(const struct intel_crtc_state *crtc_state
>  
>  u8 intel_crtc_bigjoiner_slave_pipes(const struct intel_crtc_state *crtc_state)
>  {
> -	return crtc_state->bigjoiner_pipes & ~BIT(bigjoiner_master_pipe(crtc_state));
> +	if (crtc_state->bigjoiner_pipes)
> +		return crtc_state->bigjoiner_pipes & ~BIT(bigjoiner_master_pipe(crtc_state));
> +	else
> +		return 0;
>  }
>  
>  bool intel_crtc_is_bigjoiner_slave(const struct intel_crtc_state *crtc_state)
> -- 
> 2.34.1
>