linux-kernel - Re: [RFC] spi: expand bits_per_word

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Message-ID: <71b66cbb-ab2f-44e3-926f-9ae4bcb3aadc@baylibre.com>
Date: Wed, 11 Jun 2025 09:16:06 -0500
From: David Lechner <dlechner@...libre.com>
To: Da Xue <da@...re.computer>, Lars-Peter Clausen <lars@...afoo.de>,
 Michael Hennerich <Michael.Hennerich@...log.com>,
 Jonathan Cameron <jic23@...nel.org>, Nuno Sá
 <nuno.sa@...log.com>, Andy Shevchenko <andy@...nel.org>,
 Mark Brown <broonie@...nel.org>, Rui Miguel Silva <rmfrfs@...il.com>,
 Viresh Kumar <vireshk@...nel.org>, Johan Hovold <johan@...nel.org>,
 Alex Elder <elder@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-spi@...r.kernel.org, greybus-dev@...ts.linaro.org,
 linux-staging@...ts.linux.dev
Subject: Re: [RFC] spi: expand bits_per_word_mask to 64 bits

On 6/10/25 7:05 PM, Da Xue wrote:
> Most current controller IP support 64-bit words.
> Update the mask to u64 from u32.
> 
> Signed-off-by: Da Xue <da@...re.computer>
> ---
>  drivers/iio/adc/ad7949.c         | 2 +-
>  drivers/spi/spi-dln2.c           | 2 +-
>  drivers/spi/spi-ingenic.c        | 2 +-
>  drivers/spi/spi-sh-msiof.c       | 2 +-
>  drivers/spi/spi.c                | 4 ++--
>  drivers/staging/greybus/spilib.c | 2 +-
>  include/linux/spi/altera.h       | 2 +-
>  include/linux/spi/spi.h          | 6 +++---
>  8 files changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
> index edd0c3a35ab7..469789ffa4a3 100644
> --- a/drivers/iio/adc/ad7949.c
> +++ b/drivers/iio/adc/ad7949.c
> @@ -308,7 +308,7 @@ static void ad7949_disable_reg(void *reg)
>  
>  static int ad7949_spi_probe(struct spi_device *spi)
>  {
> -	u32 spi_ctrl_mask = spi->controller->bits_per_word_mask;
> +	u64 spi_ctrl_mask = spi->controller->bits_per_word_mask;

I think this driver is incorrectly accessing bits_per_word_mask
directly and should be using spi_is_bpw_supported() instead.

This driver checks for SPI_BPW_MASK(8) at one point but doesn't
take into account that if bits_per_word_mask == 0, then 8 is
implied. spi_is_bpw_supported(), on the other hand, takes this
into account.

>  	struct device *dev = &spi->dev;
>  	const struct ad7949_adc_spec *spec;
>  	struct ad7949_adc_chip *ad7949_adc;

...

> diff --git a/drivers/staging/greybus/spilib.c b/drivers/staging/greybus/spilib.c
> index 24e9c909fa02..087eed1879b1 100644
> --- a/drivers/staging/greybus/spilib.c
> +++ b/drivers/staging/greybus/spilib.c
> @@ -27,7 +27,7 @@ struct gb_spilib {
>  	unsigned int		op_timeout;
>  	u16			mode;
>  	u16			flags;
> -	u32			bits_per_word_mask;
> +	u64			bits_per_word_mask;

This is assigned by:

	spi->bits_per_word_mask = le32_to_cpu(response.bits_per_word_mask);

in gb_spi_get_master_config(), so changing to u64 doesn't have any
effect and should likely be omitted to avoid confusion.

(The response struct is defined by a communication protocol and can't be
changed, otherwise it would break the communications.)

>  	u8			num_chipselect;
>  	u32			min_speed_hz;
>  	u32			max_speed_hz;