mfd: cros ec: spi: Don't send first message too soon

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.

Cc: <stable@vger.kernel.org>
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

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();
 
 	err = cros_ec_register(ec_dev);
 	if (err) {