linux-kernel - Re: [PATCH] spi: Limit the spi device max speed to controller's max speed

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Message-ID: <20201209194636.32f4ioxxdggezklr@mobilestation>
Date:   Wed, 9 Dec 2020 22:46:36 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Tudor Ambarus <tudor.ambarus@...rochip.com>
Cc:     broonie@...nel.org, linux-spi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] spi: Limit the spi device max speed to controller's max
 speed

Hello Tudor,

On Wed, Dec 09, 2020 at 07:35:14PM +0200, Tudor Ambarus wrote:
> Make sure the max_speed_hz of spi_device does not override
> the max_speed_hz of controller.

I have doubts that's right thing to do. It seems better to let
the controller driver to handle the speed clamping itself, while
to leave the SPI client device max_speed_hz field describing the
device speed capability. Moreover the SPI-transfers passed to the
controller will have a SPI-bus speed fixed in accordance with the
controller and client device capabilities anyway.
See the __spi_validate() method for details:
https://elixir.bootlin.com/linux/v5.10-rc7/source/drivers/spi/spi.c#L3570

-Sergey

> 
> Signed-off-by: Tudor Ambarus <tudor.ambarus@...rochip.com>
> ---
>  drivers/spi/spi.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> index cd3c395b4e90..51d7c004fbab 100644
> --- a/drivers/spi/spi.c
> +++ b/drivers/spi/spi.c
> @@ -3378,7 +3378,8 @@ int spi_setup(struct spi_device *spi)
>  	if (status)
>  		return status;
>  
> -	if (!spi->max_speed_hz)
> +	if (!spi->max_speed_hz ||
> +	    spi->max_speed_hz > spi->controller->max_speed_hz)
>  		spi->max_speed_hz = spi->controller->max_speed_hz;
>  
>  	mutex_lock(&spi->controller->io_mutex);
> -- 
> 2.25.1
>