[PATCH] spi: tegra114: assert CS GPIO for every transfer in a message

[Carlos Jones]

From: Carlos Jones Jr <carlosjr.jones@xxxxxxxxxx>

The driver provides its own transfer_one_message() and therefore drives
the chip-select GPIO itself rather than through the SPI core. The GPIO is
asserted only in the is_first_of_msg path of
tegra_spi_setup_transfer_one(), but tegra_spi_transfer_end() deasserts it
whenever a transfer has cs_change set. For a multi-transfer message whose
earlier transfers request cs_change -- for example a register read that
sends the address in one transfer and clocks the data back in the next --
the CS GPIO is released after the first transfer and never re-asserted,
so the remaining transfers are clocked with chip-select inactive and the
peripheral never drives the bus.

This was observed with an ADIS16470 IMU (adis16475 driver), whose
register read is a two-transfer, cs_change-framed exchange: a logic
analyzer showed the data frame clocked with CS high, and the driver read
0 while the device drove 0x4056 on the wire.

Re-assert the CS GPIO for every transfer so that multi-transfer messages
keep chip-select active across the whole message. gpiod_set_value() is
idempotent and polarity-aware, so this is a no-op for the first transfer
and for messages that keep CS asserted throughout.

Fixes: 63c1440596ff ("spi: tegra114: add support for gpio based CS")
Signed-off-by: Carlos Jones Jr <carlosjr.jones@xxxxxxxxxx>
---
 drivers/spi/spi-tegra114.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/spi/spi-tegra114.c b/drivers/spi/spi-tegra114.c
index 1234567..89abcde 100644
--- a/drivers/spi/spi-tegra114.c
+++ b/drivers/spi/spi-tegra114.c
@@ -859,6 +859,13 @@ static u32 tegra_spi_setup_transfer_one(struct spi_device *spi,
 		command1 |= SPI_BIT_LENGTH(bits_per_word - 1);
 	}

+	/*
+	 * Re-assert the CS GPIO for every transfer: transfer_end() drops it
+	 * on cs_change, and the assert above only covers is_first_of_msg.
+	 */
+	if (spi_get_csgpiod(spi, 0))
+		gpiod_set_value(spi_get_csgpiod(spi, 0), 1);
+
 	return command1;
 }

--
2.43.0