Re: [PATCH 1/2] add basic register-field manipulation macros

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 <redacted>
---
 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 <nbd@openwrt.org>