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

Dave Gordon
2015-10-14T12:59:01Z

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)));