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

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Date: Fri, 26 Jan 2024 08:49:22 +0000
From: Tudor Ambarus <tudor.ambarus@...aro.org>
To: Sam Protsenko <semen.protsenko@...aro.org>,
 Mark Brown <broonie@...nel.org>
Cc: 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 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
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.