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

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Date:	Tue, 5 Jul 2016 12:25:38 +0100
From:	Jakub Kicinski <jakub.kicinski@...ronome.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	"kvalo@...eaurora.org" <kvalo@...eaurora.org>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"hannes@...essinduktion.org" <hannes@...essinduktion.org>,
	"nbd@....name" <nbd@....name>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv3 wl-drv-next 1/2] add basic register-field manipulation
 macros

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.