[PATCH 2/2] v1 ARM: sun4i: spi: Allow Tx transfers larger than FIFO size

[Alexandru Gagniuc <mr.nuke.me@xxxxxxxxx>]

Enable and use the Tx FIFO 3/4 interrupt to replenish the FIFO on
large SPI bursts. This requires more care in when the interrupt is
left enabled, as this interrupt will continually trigger when the FIFO
is less than 1/4 full, even though we acknowledge it.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@xxxxxxxxx>
---
 drivers/spi/spi-sun4i.c | 44 ++++++++++++++++++++++++++++++++++++++------
 1 file changed, 38 insertions(+), 6 deletions(-)

diff --git a/drivers/spi/spi-sun4i.c b/drivers/spi/spi-sun4i.c
index 09d4b54..174736c 100644
--- a/drivers/spi/spi-sun4i.c
+++ b/drivers/spi/spi-sun4i.c
@@ -48,6 +48,7 @@
 
 #define SUN4I_INT_CTL_REG		0x0c
 #define SUN4I_INT_CTL_RF_F34			BIT(4)
+#define SUN4I_INT_CTL_TF_E34			BIT(12)
 #define SUN4I_INT_CTL_TC			BIT(16)
 
 #define SUN4I_INT_STA_REG		0x10
@@ -100,6 +101,21 @@ static inline void sun4i_spi_write(struct sun4i_spi 
*sspi, u32 reg, u32 value)
 	writel(value, sspi->base_addr + reg);
 }
 
+static inline int sun4i_spi_get_tx_fifo_count(struct sun4i_spi *sspi)
+{
+	u32 reg;
+	reg = sun4i_spi_read(sspi, SUN4I_FIFO_STA_REG);
+	reg >>= SUN4I_FIFO_STA_TF_CNT_BITS;
+	return reg & SUN4I_FIFO_STA_TF_CNT_MASK;
+}
+
+static inline void sun4i_spi_disable_interrupt(struct sun4i_spi *sspi, u32 
intm)
+{
+	u32 reg = sun4i_spi_read(sspi, SUN4I_INT_CTL_REG);
+	reg &= ~intm;
+	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, reg);
+}
+
 static inline void sun4i_spi_drain_fifo(struct sun4i_spi *sspi, int len)
 {
 	u32 reg, cnt;
@@ -181,10 +197,6 @@ static int sun4i_spi_transfer_one(struct spi_master 
*master,
 	if (tfr->len > SUN4I_MAX_XFER_SIZE)
 		return -EINVAL;
 
-	/* We only support read transfers larger than the FIFO */
-	if ((tfr->len > SUN4I_FIFO_DEPTH) && tfr->tx_buf)
-		return -EINVAL;
-
 	reinit_completion(&sspi->done);
 	sspi->tx_buf = tfr->tx_buf;
 	sspi->rx_buf = tfr->rx_buf;
@@ -280,8 +292,11 @@ static int sun4i_spi_transfer_one(struct spi_master 
*master,
 	sun4i_spi_fill_fifo(sspi, SUN4I_FIFO_DEPTH);
 
 	/* Enable the interrupts */
-	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, SUN4I_INT_CTL_TC |
-						 SUN4I_INT_CTL_RF_F34);
+	reg = SUN4I_INT_CTL_TC | SUN4I_INT_CTL_RF_F34;
+	/* Only enable Tx FIFO interrupt if we really need it */
+	if (tx_len > SUN4I_FIFO_DEPTH)
+		reg |= SUN4I_INT_CTL_TF_E34;
+	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, reg);
 
 	/* Start the transfer */
 	reg = sun4i_spi_read(sspi, SUN4I_CTL_REG);
@@ -306,6 +321,7 @@ static irqreturn_t sun4i_spi_handler(int irq, void 
*dev_id)
 {
 	struct sun4i_spi *sspi = dev_id;
 	u32 status = sun4i_spi_read(sspi, SUN4I_INT_STA_REG);
+	u32 cnt;
 
 	/* Transfer complete */
 	if (status & SUN4I_INT_CTL_TC) {
@@ -323,6 +339,22 @@ static irqreturn_t sun4i_spi_handler(int irq, void 
*dev_id)
 		return IRQ_HANDLED;
 	}
 
+	/* Transmit FIFO 3/4 empty */
+	if (status & SUN4I_INT_CTL_TF_E34) {
+		/* See how much data can fit */
+		cnt = SUN4I_FIFO_DEPTH - sun4i_spi_get_tx_fifo_count(sspi);
+		sun4i_spi_fill_fifo(sspi, cnt);
+
+		if(!sspi->len)
+			/* nothing left to transmit */
+			sun4i_spi_disable_interrupt(sspi, 
SUN4I_INT_CTL_TF_E34);
+
+		/* Only clear the interrupt _after_ re-seeding the FIFO */
+		sun4i_spi_write(sspi, SUN4I_INT_STA_REG, 
SUN4I_INT_CTL_TF_E34);
+
+		return IRQ_HANDLED;
+	}
+
 	return IRQ_NONE;
 }
 
-- 
1.8.5.3

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