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Message-ID: <20200911070649.GU77521@vkoul-mobl>
Date: Fri, 11 Sep 2020 12:36:49 +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 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
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