linux-kernel - Re: [PATCH v2] spi: dw: Fix wrong FIFO level setting for long xfers

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Date:   Fri, 13 Jan 2023 21:17:32 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Serge Semin <fancer.lancer@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        Sergey Nazarov <Sergey.Nazarov@...kalelectronics.ru>,
        linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] spi: dw: Fix wrong FIFO level setting for long xfers

On Fri, Jan 13, 2023 at 09:59:42PM +0300, Serge Semin wrote:
> Due to using the u16 type in the min_t() macros the SPI transfer length
> will be cast to word before participating in the conditional statement
> implied by the macro. Thus if the transfer length is greater than 64KB the
> Tx/Rx FIFO threshold level value will be determined by the leftover of the
> truncated after the type-case length. In the worst case it will cause the
> dramatical performance drop due to the "Tx FIFO Empty" or "Rx FIFO Full"
> interrupts triggered on each xfer word sent/received to/from the bus.
> 
> The problem can be easily fixed by specifying the unsigned int type in the
> min_t() macros thus preventing the possible data loss.

LGTM,

Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

thanks!

> Fixes: ea11370fffdf ("spi: dw: get TX level without an additional variable")
> Reported-by: Sergey Nazarov <Sergey.Nazarov@...kalelectronics.ru>
> Signed-off-by: Serge Semin <Sergey.Semin@...kalelectronics.ru>
> 
> ---
> 
> Changelog v2:
> - Use min_t(unisgned int, ...) macros instead of just min(). (@Andy)
> ---
>  drivers/spi/spi-dw-core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/spi/spi-dw-core.c b/drivers/spi/spi-dw-core.c
> index 99edddf9958b..c3bfb6c84cab 100644
> --- a/drivers/spi/spi-dw-core.c
> +++ b/drivers/spi/spi-dw-core.c
> @@ -366,7 +366,7 @@ static void dw_spi_irq_setup(struct dw_spi *dws)
>  	 * will be adjusted at the final stage of the IRQ-based SPI transfer
>  	 * execution so not to lose the leftover of the incoming data.
>  	 */
> -	level = min_t(u16, dws->fifo_len / 2, dws->tx_len);
> +	level = min_t(unsigned int, dws->fifo_len / 2, dws->tx_len);
>  	dw_writel(dws, DW_SPI_TXFTLR, level);
>  	dw_writel(dws, DW_SPI_RXFTLR, level - 1);
>  
> -- 
> 2.39.0
> 
> 

-- 
With Best Regards,
Andy Shevchenko