On Tue, 5 Jul 2016 10:56:13 +0000, David Laight wrote: > 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).
I think creating a standard set of macros in a global header file is actually helping the problems you raise. One is much more likely to know exactly what a common macro is doing than some driver-specific ad hoc macro. Common macros are also much more likely to be well tested making "unexpected issues" less likely to appear. Defining constants is fine in 20% of cases when you have only a small set of meaningful values (e.g. what to do for a 8 bit delay or priority field?) Besides when fields are not aligned to 4 bits it's hard to tell from the shifted value what the base was.
