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

[Vinod Koul <vkoul@kernel.org>]

On 10-09-20, 08:53, Pierre-Louis Bossart wrote:
> 
> 
> On 9/10/20 1:22 AM, Vinod Koul wrote:
> > On 09-09-20, 08:48, Pierre-Louis Bossart wrote:
> > > 
> > > > > > > + *	25		0 (Reserved)
> > > > > > > + *	24:22		Function Number [2:0]
> > > > > > > + *	21		Entity[6]
> > > > > > > + *	20:19		Control Selector[5:4]
> > > > > > > + *	18		0 (Reserved)
> > > > > > > + *	17:15		Control Number[5:3]
> > > > > > > + *	14		Next
> > > > > > > + *	13		MBQ
> > > > > > > + *	12:7		Entity[5:0]
> > > > > > > + *	6:3		Control Selector[3:0]
> > > > > > > + *	2:0		Control Number[2:0]
> > > > > > > + */
> > > > > > > +
> > > > > > > +#define SDW_SDCA_CTL(fun, ent, ctl, ch)						\
> > > > > > > +	(BIT(30)							|	\
> > > > > > 
> > > > > > Programmatically this is fine, but then since we are defining for the
> > > > > > description above, IMO it would actually make sense for this to be defined
> > > > > > as FIELD_PREP:
> > > > > > 
> > > > > >            FIELD_PREP(GENMASK(30, 26), 1)
> > > > > > 
> > > > > > or better
> > > > > > 
> > > > > >            u32_encode_bits(GENMASK(30, 26), 1)
> > > > > > 
> > > > > > > +	FIELD_PREP(GENMASK(24, 22), FIELD_GET(GENMASK(2, 0), (fun)))	|	\
> > > > > > 
> > > > > > Why not use u32_encode_bits(GENMASK(24, 22), (fun)) instead for this and
> > > > > > below?
> > > > > 
> > > > > Because your comment for the v1 review was to use FIELD_PREP/FIELD_GET, and
> > > > > your other patches for bitfield access only use FIELD_PREP/FIELD_GET.
> > > > 
> > > > yes and looking at this, I feel u32_encode_bits(GENMASK(24, 22), (fun))
> > > > would look better than FIELD_PREP(GENMASK(24, 22), FIELD_GET(GENMASK(2, 0), (fun)))
> > > > 
> > > > Do you agree?
> > > 
> > > The Function (fun) case is the easy one: the value is not split in two.
> > > 
> > > But look at the entity case, it's split in two:
> > > 
> > > FIELD_PREP(BIT(21), FIELD_GET(BIT(6), (ent)))			FIELD_PREP(GENMASK(12, 7),
> > > FIELD_GET(GENMASK(5, 0), (ent)))
> > > 
> > > same for control
> > > 
> > > FIELD_PREP(GENMASK(20, 19), FIELD_GET(GENMASK(5, 4), (ctl)))	|	
> > > FIELD_PREP(GENMASK(6, 3), FIELD_GET(GENMASK(3, 0), (ctl)))	|	
> > > 
> > > and same for channel number
> > > 
> > > FIELD_PREP(GENMASK(17, 15), FIELD_GET(GENMASK(5, 3), (ch)))	|	
> > > FIELD_PREP(GENMASK(2, 0), FIELD_GET(GENMASK(2, 0), (ch))))
> > > 
> > > I don't see how we can avoid using the FIELD_GET to extract the relevant
> > > bits from entity, control, channel number values.
> > 
> > No, you dont need FIELD_GET, that would be pointless for this helper if
> > that was the case
> 
> I don't get how one would specify which parts of the original value are
> extracted?
> 
> > 
> > > 
> > > Or I am missing your point completely.
> > 
> > Correct
> > 
> > It should be:
> > 
> >          foo |= u32_encode_bits(val, FOO_MASK_A);
> > 
> > which would write val into bits represented by FOO_MASK_A by
> > appropriately shifting val and masking it with FOO_MASK_A
> > 
> > So net result is bits in FOO_MASK_A are modified with val, rest of the
> > bits are not touched
> 
> Vinod, please see the explanation below [1], we need to split the original
> value in two and insert the bits in two separate locations.
> 
> You only considered the simple case for the functions, your proposal will
> not work for entities, controls and channel numbers.
> 
> > > 
> > > 
> > > > > > And while at it, consider defining masks for various fields rather than
> > > > > > using numbers in GENMASK() above, that would look better, be more
> > > > > > readable and people can reuse it.
> > > > > 
> > > > > Actually on this one I disagree. These fields are not intended to be used by
> > > > > anyone, the goal is precisely to hide them behind regmap, and the use of raw
> > > > > numbers makes it easier to cross-check the documentation and the code.
> > > > > Adding a separate set of definitions would not increase readability.
> > > > 
> > > > Which one would you prefer:
> > > > 
> > > >           #define SDCA_FUN_MASK           GENMASK(24, 22)
> > > > 
> > > >           foo |= u32_encode_bits(SDCA_FUN_MASK, fun)
> > > > 
> > > > Or the one proposed...?
> > > 
> > > Same as above, let's see what this does with the control case where we'd
> > > need to have four definitions:
> 
> [1]
> 
> > > 
> > > #define SDCA_CONTROL_DEST_MASK1 GENMASK(20, 19)
> > > #define SDCA_CONTROL_ORIG_MASK1 GENMASK(5, 4)
> > > #define SDCA_CONTROL_DEST_MASK2 GENMASK(6, 3)
> > > #define SDCA_CONTROL_ORIG_MASK2 GENMASK(3, 0)

I think I missed ORIG and DEST stuff, what does this mean here?

Relooking at the bit definition, for example 'Control Number' is defined
in both 17:15 as well as 2:0, why is that. Is it split?

How does one program a control number into this?

> > > 
> > > And the code would look like
> > > 
> > > foo |= u32_encode_bits(SDCA_CONTROL_DEST_MASK1,
> > > FIELD_GET(SDCA_CONTROL_ORIG_MASK1, fun));
> > > foo |= u32_encode_bits(SDCA_CONTROL_DEST_MASK2,
> > > FIELD_GET(SDCA_CONTROL_ORIG_MASK2, fun));
> > > 
> > > The original suggestion was:
> > > 
> > > FIELD_PREP(GENMASK(20, 19), FIELD_GET(GENMASK(5, 4), (ctl)))	|	
> > > FIELD_PREP(GENMASK(6, 3), FIELD_GET(GENMASK(3, 0), (ctl)))	|	
> > > 
> > > I prefer the original... Adding these defines doesn't really add value
> > > because
> > > a) the values will not be reused anywhere else.
> > > b) we need 12 of those defines
> > > b) we need a prefix for those defines which makes the code heavier
> > 

-- 
~Vinod