linux-kernel - Re: [PATCH v4 3/3] platform/chrome: cros_ec_spi: Request the SPI thread be realtime

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Date:   Fri, 24 May 2019 12:26:07 +0200
From:   Enric Balletbo i Serra <enric.balletbo@...labora.com>
To:     Douglas Anderson <dianders@...omium.org>,
        Mark Brown <broonie@...nel.org>,
        Benson Leung <bleung@...omium.org>
Cc:     linux-rockchip@...ts.infradead.org, drinkcat@...omium.org,
        Guenter Roeck <groeck@...omium.org>, briannorris@...omium.org,
        mka@...omium.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/3] platform/chrome: cros_ec_spi: Request the SPI
 thread be realtime

Hi,

On 15/5/19 18:48, Douglas Anderson wrote:
> All currently known ECs in the wild are very sensitive to timing.
> Specifically the ECs are known to drop a transfer if more than 8 ms
> passes from the assertion of the chip select until the transfer
> finishes.
> 
> Let's use the new feature introduced in the patch (spi: Allow SPI
> devices to request the pumping thread be realtime") to request the SPI
> pumping thread be realtime.  This means that if we get shunted off to
> the SPI thread for whatever reason we won't get downgraded to low
> priority.
> 
> NOTES:
> - We still need to keep ourselves as high priority since the SPI core
>   doesn't guarantee that all transfers end up on the pumping thread
>   (in fact, it tries pretty hard to do them in the calling context).
> - If future Chrome OS ECs ever fix themselves to be less sensitive
>   then we could consider adding a property (or compatible string) to
>   not set this property.  For now we need it across the board.
> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> Reviewed-by: Guenter Roeck <groeck@...omium.org>
> ---

queued for 5.3

Thanks,
 Enric

> 
> Changes in v4: None
> Changes in v3:
> - Updated description and variable name since we no longer force.
> 
> Changes in v2:
> - Renamed variable to "force_rt_transfers".
> 
>  drivers/platform/chrome/cros_ec_spi.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/platform/chrome/cros_ec_spi.c b/drivers/platform/chrome/cros_ec_spi.c
> index 1e38a885c539..daf3119191c8 100644
> --- a/drivers/platform/chrome/cros_ec_spi.c
> +++ b/drivers/platform/chrome/cros_ec_spi.c
> @@ -740,6 +740,7 @@ static int cros_ec_spi_probe(struct spi_device *spi)
>  
>  	spi->bits_per_word = 8;
>  	spi->mode = SPI_MODE_0;
> +	spi->rt = true;
>  	err = spi_setup(spi);
>  	if (err < 0)
>  		return err;
>