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

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 14 Oct 2015, Jani Nikula <jani.nikula@linux.intel.com> wrote:
> On Wed, 14 Oct 2015, Dave Gordon <david.s.gordon@intel.com> wrote:
>> 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?
>
> The past errors have also been of the form
>
> 	if (I915_READ(SOME_REG(pipe) & MASK) == val)
>
> which isn't caught by your check. See [1].
>
>
> BR,
> Jani.
>
>
> [1] http://mid.gmane.org/1442595836-23981-1-git-send-email-ville.syrjala@linux.intel.com
>
>
>
>
>
>>
>> #define I915_READ(reg)                                                 \
>>          ({                                                             \
>>              if (__builtin_constant_p(reg)) {                           \

Also, what Damien fixed wouldn't have been caught because
"BXT_PORT_PCS_DW12_LN23(port) != hw_state->pcsdw12" is not a builtin
constant.

Daniel, I don't think this is interim solution material, sorry.

BR,
Jani.


>>                  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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>
> -- 
> Jani Nikula, Intel Open Source Technology Center

-- 
Jani Nikula, Intel Open Source Technology Center
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