linux-kernel - Re: [PATCH v2 09/28] spi: s3c64xx: use bitfield access macros

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Message-ID: <CAPLW+4=pM=gY1bypGNhKmcftLFHWBMUZ7=JitMj_3TaxLF672A@mail.gmail.com>
Date: Fri, 26 Jan 2024 13:55:15 -0600
From: Sam Protsenko <semen.protsenko@...aro.org>
To: Tudor Ambarus <tudor.ambarus@...aro.org>
Cc: Mark Brown <broonie@...nel.org>, andi.shyti@...nel.org, arnd@...db.de, 
	robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org, 
	alim.akhtar@...sung.com, linux-spi@...r.kernel.org, 
	linux-samsung-soc@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-arch@...r.kernel.org, andre.draszik@...aro.org, 
	peter.griffin@...aro.org, kernel-team@...roid.com, willmcvicker@...gle.com
Subject: Re: [PATCH v2 09/28] spi: s3c64xx: use bitfield access macros

On Fri, Jan 26, 2024 at 2:49 AM Tudor Ambarus <tudor.ambarus@...aroorg> wrote:
>
>
>
> On 1/25/24 19:50, Sam Protsenko wrote:
> > On Thu, Jan 25, 2024 at 8:50 AM Tudor Ambarus <tudor.ambarus@...aro.org> wrote:
> >>
> >> Use the bitfield access macros in order to clean and to make the driver
> >> easier to read.
> >>
> >> Signed-off-by: Tudor Ambarus <tudor.ambarus@...aro.org>
> >> ---
> >>  drivers/spi/spi-s3c64xx.c | 196 +++++++++++++++++++-------------------
> >>  1 file changed, 99 insertions(+), 97 deletions(-)
> >>
> >> diff --git a/drivers/spi/spi-s3c64xx.c b/drivers/spi/spi-s3c64xx.c
> >> index 1e44b24f6401..d046810da51f 100644
> >> --- a/drivers/spi/spi-s3c64xx.c
> >> +++ b/drivers/spi/spi-s3c64xx.c
> >> @@ -4,6 +4,7 @@
>
> cut
>
> >> +#define S3C64XX_SPI_PSR_MASK                   GENMASK(15, 0)
> >
> > But it was 0xff (7:0) originally, and here you extend it up to 15:0.
>
> this is a bug from my side, I'll fix it, thanks!
>
> cut
>
> >>         default:
> >> -               val |= S3C64XX_SPI_MODE_BUS_TSZ_BYTE;
> >> -               val |= S3C64XX_SPI_MODE_CH_TSZ_BYTE;
> >> +               val |= FIELD_PREP(S3C64XX_SPI_MODE_BUS_TSZ_MASK,
> >> +                                 S3C64XX_SPI_MODE_BUS_TSZ_BYTE) |
> >> +                      FIELD_PREP(S3C64XX_SPI_MODE_CH_TSZ_MASK,
> >> +                                 S3C64XX_SPI_MODE_CH_TSZ_BYTE);
> >
> > I don't know. Maybe it's me, but using this FIELD_PREP() macro seems
> > to only making the code harder to read. At least in cases like this. I
> > would vote against its usage, to keep the code compact and easy to
> > read.
>
> I saw Andi complained about this too, maybe Mark can chime in.
>
> To me this is not a matter of taste, it's how it should be done. In this

Sure. But if you think it has to be done, I suggest it's done taking
next things into the account:
  1. It shouldn't make code harder to read
  2. Preferably stick to canonical ways of doing things
  3. IMHO patches like this *must* be tested thoroughly on different
boards with different test-cases, to make sure there are no
regressions. Because the benefits of cleanups are not that great, as I
see it, but we are risking to break some hardware/software combination
unintentionally while doing those cleanups. It's a good idea to
describe how it was tested in commit message or PATCH #0. Just my
$.02.

For (1) and (2): I noticed a lot of drivers are carrying additional
helper functions for read/write operations, to keep things tidy and
functional at the same time. Another mechanism that comes into mind is
regmap, though I'm not sure if it's needed for such low-level entities
as bus drivers. Also I think Andi has a point about FIELD_PREP and how
that can be handled.

> particular case you have more lines when using FIELD_PREP because the
> mask starts from bit 0. If the mask ever changes for new IPs then you'd
> have to hack the code, whereas if using FIELD_PREP you just have to
> update the mask field, something like:
>
>         FIELD_PREP(drv_prv_data->whatever_reg.field_mask,
>                    S3C64XX_SPI_MODE_CH_TSZ_BYTE);
>
> Thus it makes the code generic and more friendly for new IP additions.
> And I have to admit I like it better too. I know from the start that
> we're dealing with register fields and not some internal driver mask.