[PATCH v3] spi: tegra114: initialize native chip selects inactive

[Abraham Zukor <abe@maticrobots.com>]

tegra_spi_probe() initializes SPI_COMMAND1 with only SPI_M_S, leaving
CS_POL_INACTIVE clear for every chip select. This drives every native
active-low chip select low until tegra_spi_setup() runs for that device.

SPI children are registered and probed one at a time. A synchronous
probe of an earlier child can therefore transfer while a later child's
chip select is still asserted. On a Tegra234 system with active-low
devices on CS0 and CS1, this caused both devices to be selected when the
CS1 device probed first. Its initialization then intermittently failed.
A logic analyzer showed CS0 remained low throughout the CS1 transfer and
went high only after the CS1 probe failed.

Walk the controller's device tree children before SPI_COMMAND1 is
written and set CS_POL_INACTIVE for each valid chip select without
spi-cs-high. That way the line is already deasserted when the first
child transfers and does not change level when setup runs.

Disabled children are walked too. The core only registers available
children, so tegra_spi_setup() never runs for a disabled one and probe
is the only chance to park its chip select at the level its hardware
needs.

Fixes: f333a331adfa ("spi/tegra114: add spi driver")
Assisted-by: Claude:Opus-5
Signed-off-by: Abraham Zukor <abe@maticrobots.com>
---
v3:
- Drop the cs-gpios check; programming an unused native chip select does
  not affect a device using a GPIO chip select.
- Split the compound condition into separate checks for readability.
- Retest on a Jetson Orin Nano with CS1 disabled. The old module left CS1
  asserted, while the patched module parked both chip selects inactive.

v2:
- Derive the inactive level per chip select from the device tree instead
  of setting the whole CS_POL_INACTIVE field, which is incorrect for an
  active-high or mixed-polarity bus.

drivers/spi/spi-tegra114.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/drivers/spi/spi-tegra114.c b/drivers/spi/spi-tegra114.c
index aa44ffd09e61..463bb7e61b6e 100644
--- a/drivers/spi/spi-tegra114.c
+++ b/drivers/spi/spi-tegra114.c
@@ -1395,6 +1395,32 @@ static int tegra_spi_probe(struct platform_device *pdev)
 	udelay(2);
 	reset_control_deassert(tspi->rst);
 	tspi->def_command1_reg  = SPI_M_S;
+
+	/*
+	 * SPI_COMMAND1 is written before any child is registered, so a chip
+	 * select with CS_POL_INACTIVE clear stays asserted until
+	 * tegra_spi_setup() runs for that device. Program the level the
+	 * device tree asks for up front instead.
+	 *
+	 * Disabled children are included deliberately. The core never
+	 * registers them, so this is the only chance to park their chip
+	 * select at the level their hardware needs.
+	 */
+	for_each_child_of_node_scoped(pdev->dev.of_node, np) {
+		u32 cs;
+
+		if (of_property_read_bool(np, "spi-cs-high"))
+			continue;
+
+		if (of_property_read_u32(np, "reg", &cs))
+			continue;
+
+		if (cs >= MAX_CHIP_SELECT)
+			continue;
+
+		tspi->def_command1_reg |= SPI_CS_POL_INACTIVE(cs);
+	}
+
 	tegra_spi_writel(tspi, tspi->def_command1_reg, SPI_COMMAND1);
 	tspi->spi_cs_timing1 = tegra_spi_readl(tspi, SPI_CS_TIMING1);
 	tspi->spi_cs_timing2 = tegra_spi_readl(tspi, SPI_CS_TIMING2);
-- 
2.54.0