Re: [PATCHv4 wl-drv-next 0/2] register-field manipulation macros

Jakub Kicinski [off-list ref] writes:

This set moves to a global header file macros which I find
very useful and worth popularising.  The basic problem is
that since C bitfields are not very dependable accessing
subfields of registers becomes slightly inconvenient.
It is nice to have the necessary mask and shift operations
wrapped in a macro.  It is also nice to have that macro
compute the shift amount based on the mask automatically.

My implementation follows what Felix Fietkau has done in
mt76.  Hannes Frederic Sowa suggested more use of standard
Linux/GCC functions.  Since the RFC I've also added a 
compile-time check to validate that the value passed to
setters fits in the mask.

I attempted the use of static inlines instead of macros
but it makes GCC < 6.0 barf at the BUILD_BUG_ON()s.
I also noticed that forcing arguments to be u32 for inlines
makes the compiler use 32bit arithmetic where it could
get away with 64bit before (on 64bit machines, obviously).
That's a potential performance concern but probably not
a very practical one today.  Apart from looking "cleaner"
static inlines would have the advantage that we could #undef
the auxiliary macros at the end of the header.

This information about various compiler problems is good to have in the
commit log as the cover letter itself is not stored in the git log.

v3:
Build bot caught a build failure with -Os set.  AFAICT gcc
did not handle temporary variable I put in the macro
expression too well.  I work around that by defining
__BUILD_BUG_ON_NOT_POWER_OF_2 and using it instead of
BUILD_BUG_ON(!tmp || is_power_of_2(tmp)).

Please review and advise on improvements.

If accepted I think would be best to push this through
Kalle's tree, since the only existing user is in
drivers/net/wireless/.

I like it. But I feel much more comfortable to take these if I get an
ack from someone like Dave Miller or Andrew Morton (CCed).

Full patches here:

https://patchwork.kernel.org/patch/9214129/
https://patchwork.kernel.org/patch/9214135/

-- 
Kalle Valo
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