On 2016-06-13 15:29, Jakub Kicinski wrote: > C bitfields are problematic and best avoided. Developers > interacting with hardware registers find themselves searching > for easy-to-use alternatives. Common approach 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 X_REG_FIELD 0x000ff000 > > field = FIELD_GET(X_REG_FIELD, reg); > > reg &= ~X_REG_FIELD; > reg |= FIELD_PUT(X_REG_FIELD, field); > > FIELD_{GET,PUT} 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. > > Compared to Felix Fietkau's implementation from mt76 this one > uses standard Linux and GCC functions such as is_power_of_2() > and __builtin_ffsll(). > > Signed-off-by: Jakub Kicinski <jakub.kicin...@netronome.com> > --- > include/linux/bitfield.h | 58 > ++++++++++++++++++++++++++++++++++++++++++++++++ > include/linux/log2.h | 6 +++++ > 2 files changed, 64 insertions(+) > create mode 100644 include/linux/bitfield.h > > diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h > new file mode 100644 > index 000000000000..ae2224464523 > --- /dev/null > +++ b/include/linux/bitfield.h > @@ -0,0 +1,58 @@ > +/* > + * Copyright (C) 2014 Felix Fietkau <n...@openwrt.org> Please change my email address to n...@nbd.name here
- Felix