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

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Date:   Fri, 10 May 2019 17:32:17 -0700
From:   Guenter Roeck <groeck@...gle.com>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Mark Brown <broonie@...nel.org>,
        Benson Leung <bleung@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        Nicolas Boichat <drinkcat@...omium.org>,
        Guenter Roeck <groeck@...omium.org>,
        Brian Norris <briannorris@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] platform/chrome: cros_ec_spi: Set ourselves as timing sensitive

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

> 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@...omium.org>

Reviewed-by: Guenter Roeck <groeck@...omium.org>

> ---
>
>  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
>