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

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Date:   Thu, 10 Sep 2020 11:52:23 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
Cc:     alsa-devel@...a-project.org, tiwai@...e.de, broonie@...nel.org,
        gregkh@...uxfoundation.org,
        Bard liao <yung-chuan.liao@...ux.intel.com>,
        Rander Wang <rander.wang@...ux.intel.com>,
        Guennadi Liakhovetski <guennadi.liakhovetski@...ux.intel.com>,
        Kai Vehmanen <kai.vehmanen@...ux.intel.com>,
        Sanyog Kale <sanyog.r.kale@...el.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] soundwire: SDCA: add helper macro to access
 controls

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

> 
> 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

> 
> 
> > > > 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:
> 
> #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)
> 
> 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