by Kalle Valo

[permalink] [raw]

Subject: Re: [PATCHv8,1/4] add basic register-field manipulation macros

Jakub Kicinski <[email protected]> wrote:
> Common approach to accessing register fields is to define
> structures or sets of macros containing mask and shift pair.
> Operations on the register are then performed as follows:
>
> field = (reg >> shift) & mask;
>
> reg &= ~(mask << shift);
> reg |= (field & mask) << shift;
>
> Defining shift and mask separately is tedious. Ivo van Doorn
> came up with an idea of computing them at compilation time
> based on a single shifted mask (later refined by Felix) which
> can be used like this:
>
> #define REG_FIELD 0x000ff000
>
> field = FIELD_GET(REG_FIELD, reg);
>
> reg &= ~REG_FIELD;
> reg |= FIELD_PREP(REG_FIELD, field);
>
> FIELD_{GET,PREP} macros take care of finding out what the
> appropriate shift is based on compilation time ffs operation.
>
> GENMASK can be used to define registers (which is usually
> less error-prone and easier to match with datasheets).
>
> This approach is the most convenient I've seen so to limit code
> multiplication let's move the macros to a global header file.
> Attempts to use static inlines instead of macros failed due
> to false positive triggering of BUILD_BUG_ON()s, especially with
> GCC < 6.0.
>
> Signed-off-by: Jakub Kicinski <[email protected]>
> Reviewed-by: Dinan Gunawardena <[email protected]>

Thanks, 4 patches applied to wireless-drivers-next.git:

3e9b3112ec74 add basic register-field manipulation macros
faad5433b722 mt7601u: remove redefinition of GENMASK
adcc710d0a9e mt7601u: remove unnecessary include
d43af50566b4 mt7601u: use linux/bitfield.h

--
Sent by pwcli
https://patchwork.kernel.org/patch/9306999/

2016-09-03 11:05:37

by Kalle Valo

[permalink] [raw]

Subject: Re: [PATCHv8 0/4] register-field manipulation macros

Jakub Kicinski <[email protected]> writes:

> Small improvement suggested by Daniel Borkmann - use
> two underscore prefix.
>
> https://www.mail-archive.com/[email protected]/msg125423.html
>
> -- v6 blurb:
>
> 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 and have that macro compute shift
> at compilation time using FFS.
>
> 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.
>
> 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)).
>
> I'm planning to use those macros in another driver soon,
> Felix may also use them when upstreaming mt76 if he chooses
> to do so.
>
> IMHO if accepted it would be best to push these through
> Kalle's wireless drivers' tree, since the only existing
> user is in drivers/net/wireless/.
>
> v8:
> - use two underscores for auxiliary macros (Daniel B).
> v7:
> - drop the explicit type marking (u32/u64) - depend on the type
> of the mask instead;
> - only allow compilation time constant masks;
> - barf at "type of register too small to ever match mask" on get;
> - rename PUT -> PREP.
> v6:
> - do a full rename in patch 2;
> - CC many people.
> v5:
> - repost.
> v4:
> - add documentation in the header.
> v3:
> - don't use variables in statement expressions;
> - use __BUILD_BUG_ON_NOT_POWER_OF_2.
> v2:
> - change Felix's email address.
>
> Jakub Kicinski (4):
> add basic register-field manipulation macros
> mt7601u: remove redefinition of GENMASK
> mt7601u: remove unnecessary include
> mt7601u: use linux/bitfield.h

I applied these now to the pending branch of my wireless-drivers-next
tree. Let's see if the kbuild bot finds any problems.

Please also CC lkml, did that now.

--
Kalle Valo