[Intel-gfx] [PATCH] drm/i915: encourage BIT() macro usage in register definitions

[Michal Wajdeczko]

On Wed, 27 Jun 2018 16:41:13 +0200, Jani Nikula <jani.nikula at intel.com>  
wrote:

> There's already some BIT() usage here and there, embrace it.
>
> Cc: Paulo Zanoni <paulo.r.zanoni at intel.com>
> Signed-off-by: Jani Nikula <jani.nikula at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_reg.h | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_reg.h  
> b/drivers/gpu/drm/i915/i915_reg.h
> index 476118f46cf3..64b9c270045d 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -65,9 +65,10 @@
>   * but do note that the macros may be needed to read as well as write  
> the
>   * register contents.
>   *
> - * Define bits using ``(1 << N)`` instead of ``BIT(N)``. We may change  
> this in
> - * the future, but this is the prevailing style. Do **not** add  
> ``_BIT`` suffix
> - * to the name.
> + * Define bits using ``BIT(N)`` instead of ``(1 << N)``. Do **not** add  
> ``_BIT``
> + * suffix to the name. Exception to ``BIT()`` usage: Value 1 for a bit  
> field
> + * should be defined using ``(1 << N)`` to be in line with other values  
> such as
> + * ``(2 << N)`` for the same field.
>   *
>   * Group the register and its contents together without blank lines,  
> separate
>   * from other registers and their contents with one blank line.
> @@ -105,7 +106,7 @@
>   *  #define _FOO_A                      0xf000
>   *  #define _FOO_B                      0xf001
>   *  #define FOO(pipe)                   _MMIO_PIPE(pipe, _FOO_A, _FOO_B)
> - *  #define   FOO_ENABLE                (1 << 31)
> + *  #define   FOO_ENABLE                BIT(31)

hmm, this breaks nice consistency between one- and multi-bit fields ..

>   *  #define   FOO_MODE_MASK             (0xf << 16)

.. but if you want to use macro for single bit, then maybe you should
also consider other existing macro for the mask definition:

        #define   FOO_MODE_MASK             GENMASK(19, 16)

>   *  #define   FOO_MODE_SHIFT            16
>   *  #define   FOO_MODE_BAR              (0 << 16)

.. but we still don't have any macro for defining multi-bit values
so I'm not sure if this change will make code really easier to read

Michal