RE: [PATCHv3 wl-drv-next 1/2] add basic register-field manipulation macros

From: Jakub Kicinski

Sent: 01 July 2016 22:27

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 REG_FIELD 0x000ff000

 field = FIELD_GET(REG_FIELD, reg);

 reg &= ~REG_FIELD;
 reg |= FIELD_PUT(REG_FIELD, field);

My problem with these sort of 'helpers' is that they make it much harder
to read code unless you happen to know exactly what the helpers do.
Unexpected issues (like values being sign extended) can be hard to spot.

A lot of the time you can make things simpler by not doing the shifts
(ie define shifted constants).

	David