[Intel-gfx] [PATCH] drm/i915: use variadic macros and arrays to choose port/pipe based registers

[Chris Wilson]

On Fri, Jan 27, 2017 at 05:57:06PM +0200, Jani Nikula wrote:
> This allows the use of more than 3 ports/pipes/whatever without tricks,
> even if the register offsets are not evenly spaced.
> 
> There's the risk of out of bounds access if we're not careful; currently
> that would "just" lead to the wrong register offset being used. It might
> be possible to add build bug ons for build time constant indexing.
> 
> We already have ports defined up to E, not sure if we might have bugs
> related to them and the current _PORT3() macro.
> 
>    text	   data	    bss	    dec	    hex	filename
> 1239868	  46199	   4096	1290163	 13afb3	drivers/gpu/drm/i915/i915.ko
> 1238828	  46199	   4096	1289123	 13aba3	drivers/gpu/drm/i915/i915.ko
> 
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Cc: Tvrtko Ursulin <tursulin at ursulin.net>
> Signed-off-by: Jani Nikula <jani.nikula at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_reg.h | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index 672cb102f477..c6435a447300 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -48,6 +48,8 @@ static inline bool i915_mmio_reg_valid(i915_reg_t reg)
>  	return !i915_mmio_reg_equal(reg, INVALID_MMIO_REG);
>  }
>  
> +#define _PICK(__index, ...) (((const u32 []){ __VA_ARGS__ })[__index])

Very neat. The danger is that for a variable index, the compiler will
plonk the array on the stack, for each invocation. Though for a constant
the compiler will see through it and generate the right constant.

https://godbolt.org/g/YCK1od

Given that it looks like the compiler will get smarter, this looks like
a much less error prone way of writing these.

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre