Re: [PATCH 2/3] linux/bits.h: Add fixed-width GENMASK and BIT macros

Lucas De Marchi
2023-05-12T16:31:00Z

On Fri, May 12, 2023 at 02:14:19PM +0300, Andy Shevchenko wrote:
>On Mon, May 08, 2023 at 10:14:02PM -0700, Lucas De Marchi wrote:
>> Add GENMASK_U32(), GENMASK_U16() and GENMASK_U8() macros to create
>> masks for fixed-width types and also the corresponding BIT_U32(),
>> BIT_U16() and BIT_U8().
>
>Why?

to create the masks/values for device registers that are
of a certain width, preventing mistakes like:

#define REG1 0x10
#define REG1_ENABLE BIT(17)
#define REG1_FOO GENMASK(16, 15);

register_write(REG1_ENABLE, REG1);

... if REG1 is a 16bit register for example. There were mistakes in the
past in the i915 source leading to the creation of the REG_* variants on
top of normal GENMASK/BIT (see last patch and commit 09b434d4f6d2
("drm/i915: introduce REG_BIT() and REG_GENMASK() to define register
contents")

We are preparing another driver (xe), still to be merged but already
open (https://gitlab.freedesktop.org/drm/xe/kernel), that has
similar requirements.

>
>> All of those depend on a new "U" suffix added to the integer constant.
>> Due to naming clashes it's better to call the macro U32. Since C doesn't
>> have a proper suffix for short and char types, the U16 and U18 variants
>> just use U32 with one additional check in the BIT_* macros to make
>> sure the compiler gives an error when the those types overflow.
>> The BIT_U16() and BIT_U8() need the help of GENMASK_INPUT_CHECK(),
>> as otherwise they would allow an invalid bit to be passed. Hence
>> implement them in include/linux/bits.h rather than together with
>> the other BIT* variants.
>
>So, we have _Generic() in case you still wish to implement this.

humn... how would a _Generic() help here? The input is 1 or 2 integer
literals (h and l) so the compiler can check it is correct at build
time. See example above.

Lucas De Marchi