[PATCH v3 2/2] spi: rockchip: skip the unused FIFO direction on a one-wire device

[Cole Munz]

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 <munzz...@proton.me>
Tested-by: Alexey Charkov <alch...@flipper.net>
---
Changes in v3: reworded the toread comment to name what the RX drain
was doing for the transmit path (pacing the loop against the wire),
per Quentin. Comment only; compile check rerun, exit 0.

 drivers/spi/rk_spi.c | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

diff --git a/drivers/spi/rk_spi.c b/drivers/spi/rk_spi.c
index 2c3d70ba7159..82009e61202f 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 +486,14 @@ 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. Draining it was also what
+                * paced this loop against the wire, so the transmit-only
+                * path relies on the wait_till_not_busy() below for
+                * completion instead.
+                */
+               toread = (priv->mode & SPI_NO_RX) ? 0 : todo;
                /* Only write if we have something to write */
                towrite = out ? todo : 0;
                while (toread || towrite) {
@@ -513,7 +534,7 @@ 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