Re: [PATCH 3/4] platform/chrome: cros_ec_spi: Set ourselves as timing sensitive

[Guenter Roeck]

From: Douglas Anderson <dianders@xxxxxxxxxxxx>
Date: Fri, May 10, 2019 at 3:35 PM
To: Mark Brown, Benson Leung, Enric Balletbo i Serra
Cc: <linux-rockchip@xxxxxxxxxxxxxxxxxxx>, <drinkcat@xxxxxxxxxxxx>,
Guenter Roeck, <briannorris@xxxxxxxxxxxx>, <mka@xxxxxxxxxxxx>, Douglas
Anderson, <linux-kernel@xxxxxxxxxxxxxxx>

> 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 specify that they are timing sensitive") to specify this
> and increase the success rate of our transfers.
>
> NOTE: 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.
>
> With this change we can revert the commit 37a186225a0c
> ("platform/chrome: cros_ec_spi: Transfer messages at high priority").
> ...and, in fact, transfers are _even more_ reliable than they were
> with that commit since the SPI framework will use a higher priority
> (realtime) and we no longer lose our priority when we get shunted over
> to the message pumping thread (because we now always get shunted and
> the thread is high priority).
>
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>

Reviewed-by: Guenter Roeck <groeck@xxxxxxxxxxxx>

> ---
>
>  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 8e9451720e73..757a115502ec 100644
> --- a/drivers/platform/chrome/cros_ec_spi.c
> +++ b/drivers/platform/chrome/cros_ec_spi.c
> @@ -703,6 +703,7 @@ static int cros_ec_spi_probe(struct spi_device *spi)
>
>         spi->bits_per_word = 8;
>         spi->mode = SPI_MODE_0;
> +       spi->timing_sensitive = true;
>         err = spi_setup(spi);
>         if (err < 0)
>                 return err;
> --
> 2.21.0.1020.gf2820cf01a-goog
>