Messages in this thread | | | Date | Fri, 12 May 2023 09:29:23 -0700 | From | Lucas De Marchi <> | Subject | Re: [PATCH 2/3] linux/bits.h: Add fixed-width GENMASK and BIT macros |
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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
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