Re: [PATCH] drm/i915/bxt: Fix wrongly placed ')' in I915_READ()

[Dave Gordon <david.s.gordon@intel.com>]

On 17/09/15 14:20, Damien Lespiau wrote:
> Not the first time! not the last time?
>
> There is a possibility to use gcc 5's -Wbool-compare to try and compare
> (reg) in those macros to a constant and gcc will warn that the
> comparison between a boolean expression and a constant is always either
> true or false. Maybe.

Since boolean true (1) cannot be a valid argument to this macro, it 
could contain a compile-time check that the parameter is not 1; if 
boolean false (0) happens not to be a valid register address (BSpec says 
MMIO 0 is reserved) the macro could check that the argument is neither 
of these values, and the compiler might then detect that all possible 
paths lead to a compile-time error. Something like this?

#define I915_READ(reg)                                                 \
         ({                                                             \
             if (__builtin_constant_p(reg)) {                           \
                 BUILD_BUG_ON((reg) == false);                          \
                 BUILD_BUG_ON((reg) == true);                           \
             }                                                          \
             dev_priv->uncore.funcs.mmio_readl(dev_priv, (reg), true);  \
         })

Interestingly, that reported three errors, all in intel_dsi.c where the 
port-selection macros use 0s to fill in dummy elements when less than 3 
ports are being used.

In function ‘intel_dsi_get_hw_state’
In function ‘intel_dsi_port_disable’
In function ‘intel_dsi_port_enable’

Other than that, there weren't any cases where a bool constant was being 
passed to this specific macro (as of today).

.Dave.

> Cc: Imre Deak <imre.deak@intel.com>
> Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
> ---
>   drivers/gpu/drm/i915/intel_ddi.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_ddi.c b/drivers/gpu/drm/i915/intel_ddi.c
> index 4823184..5b600bf 100644
> --- a/drivers/gpu/drm/i915/intel_ddi.c
> +++ b/drivers/gpu/drm/i915/intel_ddi.c
> @@ -2882,7 +2882,7 @@ static bool bxt_ddi_pll_get_hw_state(struct drm_i915_private *dev_priv,
>   	 * here just read out lanes 0/1 and output a note if lanes 2/3 differ.
>   	 */
>   	hw_state->pcsdw12 = I915_READ(BXT_PORT_PCS_DW12_LN01(port));
> -	if (I915_READ(BXT_PORT_PCS_DW12_LN23(port) != hw_state->pcsdw12))
> +	if (I915_READ(BXT_PORT_PCS_DW12_LN23(port)) != hw_state->pcsdw12)
>   		DRM_DEBUG_DRIVER("lane stagger config different for lane 01 (%08x) and 23 (%08x)\n",
>   				 hw_state->pcsdw12,
>   				 I915_READ(BXT_PORT_PCS_DW12_LN23(port)));


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