Re: [PATCH v2 2/3] soundwire: SDCA: add helper macro to access controls

[Pierre-Louis Bossart]

On 9/16/20 7:35 AM, Vinod Koul wrote:
> On 14-09-20, 09:44, Pierre-Louis Bossart wrote:
> > > For LSB bits, I dont think this is an issue. I expect it to work, for example:
> > > #define CONTROL_LSB_MASK  GENMASK(2, 0)
> > >           foo |= u32_encode_bits(control, CONTROL_LSB_MASK);
> > >
> > > would mask the control value and program that in specific bitfeild.
> > >
> > > But for MSB bits, I am not sure above will work so, you may need to extract
> > > the bits and then use, for example:
> > > #define CONTROL_MSB_BITS        GENMASK(5, 3)
> > > #define CONTROL_MSB_MASK        GENMASK(17, 15)
> > >
> > >           control = FIELD_GET(CONTROL_MSB_BITS, control);
> > >           foo |= u32_encode_bits(control, CONTROL_MSB_MASK);
> > >
> > > > If you have a better suggestion that the FIELD_PREP/FIELD_GET use, I am all
> > > > ears. At the end of the day, the mapping is pre-defined and we don't have
> > > > any degree of freedom. What I do want is that this macro/inline function is
> > > > shared by all codec drivers so that we don't have different interpretations
> > > > of how the address is constructed.
> > >
> > > Absolutely, this need to be defined here and used by everyone else.
> >
> > Compare:
> >
> > #define SDCA_CONTROL_MSB_BITS        GENMASK(5, 3)
> > #define SDCA_CONTROL_MSB_MASK        GENMASK(17, 15)
> > #define SDCA_CONTROL_LSB_MASK        GENMASK(2, 0)
> >
> > foo |= u32_encode_bits(control, SDCA_CONTROL_LSB_MASK);
> > control = FIELD_GET(SDCA_CONTROL_MSB_BITS, control);
> > foo |= u32_encode_bits(control, SDCA_CONTROL_MSB_MASK);
> >
> > with the original proposal:
> >
> > foo |= FIELD_GET(GENMASK(2, 0), control))	
> > foo |= FIELD_PREP(GENMASK(17, 15), FIELD_GET(GENMASK(5, 3), control))	
> >
> > it gets worse when the LSB positions don't match, you need another variable
> > and an additional mask.
> >
> > I don't see how this improves readability? I get that hard-coding magic
> > numbers is a bad thing in general, but in this case there are limited
> > benefits to the use of additional defines.
>
> I think it would be prudent to define the masks and use them rather than
> magic values. Also it makes it future proof

I don't see your point at all. The values cannot be modified, a 
different macro would be needed for a standard change.

Anyways, I am not going to argue further, I'll use your code example as 
is and move on.