[PATCH v4 2/2] spi: rockchip: skip the unused FIFO direction on a one-wire device

The controller has a transfer-mode field that can run transmit-only or
receive-only instead of both, which leaves the unused FIFO out of the
transfer entirely. The driver never used it for that: claim_bus always
programmed TMOD_TR, and the only other mode came from an opportunistic
switch to TMOD_RO for read-only transfers.

A device described with spi-{tx,rx}-bus-width = <0> has no wire in that
direction at all, so now that the width reaches plat->mode as
SPI_NO_TX/SPI_NO_RX, pick the transfer mode from it. A write-only
display stops clocking receive bytes nobody reads.

The transmit-only case needs one more change. The 8-bit loop paces
itself on the receive FIFO and sets toread unconditionally, so with no
receive path it would wait on a FIFO that stays empty forever. Leave
toread at zero there and let the existing wait_till_not_busy() at the
end of the chunk handle completion, which is the same thing that
already covers a transmit component today.

The restore at the end of a read-only transfer went back to a hardcoded
TMOD_TR, which would undo the device's own mode. Restore what the mode
asks for instead.

Signed-off-by: Cole Munz <[email protected]>
Tested-by: Alexey Charkov <[email protected]>
---
Changes in v4: comment rewords only, per the thread with Quentin. No
code change; compile check rerun, exit 0.

 drivers/spi/rk_spi.c | 33 ++++++++++++++++++++++++++-------
 1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/drivers/spi/rk_spi.c b/drivers/spi/rk_spi.c
index 2c3d70ba7159..54befd54b099 100644
--- a/drivers/spi/rk_spi.c
+++ b/drivers/spi/rk_spi.c
@@ -283,6 +283,20 @@ static int rockchip_spi_probe(struct udevice *bus)
 	return 0;
 }
 
+/*
+ * A device that declares spi-{tx,rx}-bus-width = <0> has no wire in that
+ * direction, so the controller can drop the matching FIFO entirely instead
+ * of clocking bytes nobody reads.
+ */
+static u32 rkspi_base_tmod(struct rockchip_spi_priv *priv)
+{
+	if (priv->mode & SPI_NO_RX)
+		return TMOD_TO;
+	if (priv->mode & SPI_NO_TX)
+		return TMOD_RO;
+	return TMOD_TR;
+}
+
 static int rockchip_spi_claim_bus(struct udevice *dev)
 {
 	struct udevice *bus = dev->parent;
@@ -329,8 +343,8 @@ static int rockchip_spi_claim_bus(struct udevice *dev)
 	/* Frame Format */
 	ctrlr0 |= FRF_SPI << FRF_SHIFT;
 
-	/* Tx and Rx mode */
-	ctrlr0 |= TMOD_TR << TMOD_SHIFT;
+	/* Configure RX/TX mode */
+	ctrlr0 |= rkspi_base_tmod(priv) << TMOD_SHIFT;
 
 	writel(ctrlr0, &regs->ctrlr0);
 
@@ -472,7 +486,11 @@ static int rockchip_spi_xfer(struct udevice *dev, unsigned int bitlen,
 		writel(todo - 1, &regs->ctrlr1);
 		rkspi_enable_chip(regs, true);
 
-		toread = todo;
+		/*
+		 * When the RX wire is not routed, the RX FIFO never fills,
+		 * so waiting on it would hang.
+		 */
+		toread = (priv->mode & SPI_NO_RX) ? 0 : todo;
 		/* Only write if we have something to write */
 		towrite = out ? todo : 0;
 		while (toread || towrite) {
@@ -492,9 +510,10 @@ static int rockchip_spi_xfer(struct udevice *dev, unsigned int bitlen,
 		}
 
 		/*
-		 * In case that there's a transmit-component, we need to wait
-		 * until the control goes idle before we can disable the SPI
-		 * control logic (as this will implicitly flush the FIFOs).
+		 * With a transmit component the TX FIFO can still hold data
+		 * that has not been shifted onto the wire. Wait until the
+		 * controller goes idle before disabling it, as disabling
+		 * clears the FIFOs.
 		 */
 		if (out) {
 			ret = rkspi_wait_till_not_busy(regs);
@@ -513,7 +532,7 @@ static int rockchip_spi_xfer(struct udevice *dev, unsigned int bitlen,
 	if (!out)
 		clrsetbits_le32(&regs->ctrlr0,
 				TMOD_MASK << TMOD_SHIFT,
-				TMOD_TR << TMOD_SHIFT);
+				rkspi_base_tmod(priv) << TMOD_SHIFT);
 
 	return ret;
 }
-- 
2.55.0