linux-kernel - Re: [PATCH 1/2] mfd: cros ec: spi: Don't send first message too soon

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Date:   Mon, 13 Nov 2017 13:19:23 -0800
From:   Brian Norris <briannorris@...omium.org>
To:     Jon Hunter <jonathanh@...dia.com>
Cc:     Lee Jones <lee.jones@...aro.org>, linux-kernel@...r.kernel.org,
        linux-tegra@...r.kernel.org,
        Shawn Nematbakhsh <shawnn@...omium.org>,
        Benson Leung <bleung@...omium.org>,
        Doug Anderson <dianders@...omium.org>
Subject: Re: [PATCH 1/2] mfd: cros ec: spi: Don't send first message too soon

+ others

On Mon, Nov 13, 2017 at 09:05:39PM +0000, Jon Hunter wrote:
> On the Tegra124 Nyan-Big chromebook the very first SPI message sent to
> the EC is failing.
> 
> The Tegra SPI driver configures the SPI chip-selects to be active-high
> by default (and always has for many years). The EC SPI requires an
> active-low chip-select and so the Tegra chip-select is reconfigured to
> be active-low when the EC SPI driver calls spi_setup(). The problem is
> that if the first SPI message to the EC is sent too soon after
> reconfiguring the SPI chip-select, it fails.
> 
> The EC SPI driver prevents back-to-back SPI messages being sent too
> soon by keeping track of the time the last transfer was sent via the
> variable 'last_transfer_ns'. To prevent the very first transfer being
> sent too soon, initialise the 'last_transfer_ns' variable after calling
> spi_setup() and before sending the first SPI message.
> 
> Signed-off-by: Jon Hunter <jonathanh@...dia.com>
> ---
> Looks like this issue has been around for several Linux releases now
> and it just depends on timing if this issue is seen or not and so there
> is no specific commit this fixes. However, would be good to include for
> v4.15.

I wonder if that doesn't mean we should have a stable tag still?

Cc: <stable@...r.kernel.org>

>  drivers/mfd/cros_ec_spi.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/mfd/cros_ec_spi.c b/drivers/mfd/cros_ec_spi.c
> index c9714072e224..a14196e95e9b 100644
> --- a/drivers/mfd/cros_ec_spi.c
> +++ b/drivers/mfd/cros_ec_spi.c
> @@ -667,6 +667,7 @@ static int cros_ec_spi_probe(struct spi_device *spi)
>  			   sizeof(struct ec_response_get_protocol_info);
>  	ec_dev->dout_size = sizeof(struct ec_host_request);
>  
> +	ec_spi->last_transfer_ns = ktime_get_ns();

Seems pretty reasonable to me:

Reviewed-by: Brian Norris <briannorris@...omium.org>

>  
>  	err = cros_ec_register(ec_dev);
>  	if (err) {
> -- 
> 2.7.4
>