[PATCH 1/2] v1 ARM: sun4i: spi: Allow Rx transfers larger than FIFO size

[Alexandru Gagniuc]

SPI transfers were limited to one FIFO depth, which is 64 bytes.
This was an artificial limitation, however, as the hardware can handle
much larger bursts. To accommodate this, we enable the interrupt when
the Rx FIFO is 3/4 full, and drain the FIFO within the interrupt
handler. The 3/4 ratio was chosen arbitrarily, with the intention to
reduce the potential number of interrupts.

Since the SUN4I_CTL_TP bit is set, the hardware will pause
transmission whenever the FIFO is full, so there is no risk of losing
data if we can't service the interrupt in time.

This patch only handles long Rx transfers, and continues to return
-EINVAL on long Tx transfers. Tx transfers are handled in a subsequent
patch.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me at gmail.com>
---
 drivers/spi/spi-sun4i.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/drivers/spi/spi-sun4i.c b/drivers/spi/spi-sun4i.c
index 3f82705..09d4b54 100644
--- a/drivers/spi/spi-sun4i.c
+++ b/drivers/spi/spi-sun4i.c
@@ -47,6 +47,7 @@
 #define SUN4I_CTL_TP				BIT(18)
 
 #define SUN4I_INT_CTL_REG		0x0c
+#define SUN4I_INT_CTL_RF_F34			BIT(4)
 #define SUN4I_INT_CTL_TC			BIT(16)
 
 #define SUN4I_INT_STA_REG		0x10
@@ -68,6 +69,8 @@
 #define SUN4I_XMIT_CNT_REG		0x24
 #define SUN4I_XMIT_CNT(cnt)			((cnt) & 0xffffff)
 
+#define SUN4I_MAX_XFER_SIZE			0xffffff
+
 #define SUN4I_FIFO_STA_REG		0x28
 #define SUN4I_FIFO_STA_RF_CNT_MASK		0x7f
 #define SUN4I_FIFO_STA_RF_CNT_BITS		0
@@ -175,8 +178,11 @@ static int sun4i_spi_transfer_one(struct spi_master 
*master,
 	int ret = 0;
 	u32 reg;
 
-	/* We don't support transfer larger than the FIFO */
-	if (tfr->len > SUN4I_FIFO_DEPTH)
+	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);
@@ -274,7 +280,8 @@ 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_spi_write(sspi, SUN4I_INT_CTL_REG, SUN4I_INT_CTL_TC |
+						 SUN4I_INT_CTL_RF_F34);
 
 	/* Start the transfer */
 	reg = sun4i_spi_read(sspi, SUN4I_CTL_REG);
@@ -287,7 +294,7 @@ static int sun4i_spi_transfer_one(struct spi_master 
*master,
 		goto out;
 	}
 
-	sun4i_spi_drain_fifo(sspi, SUN4I_FIFO_DEPTH);
+	/* FIFO is drained during the interrupt handler */
 
 out:
 	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, 0);
@@ -303,10 +310,19 @@ static irqreturn_t sun4i_spi_handler(int irq, void 
*dev_id)
 	/* Transfer complete */
 	if (status & SUN4I_INT_CTL_TC) {
 		sun4i_spi_write(sspi, SUN4I_INT_STA_REG, SUN4I_INT_CTL_TC);
+		sun4i_spi_drain_fifo(sspi, SUN4I_FIFO_DEPTH);
 		complete(&sspi->done);
 		return IRQ_HANDLED;
 	}
 
+	/* Receive FIFO 3/4 full */
+	if (status & SUN4I_INT_CTL_RF_F34) {
+		sun4i_spi_drain_fifo(sspi, SUN4I_FIFO_DEPTH);
+		/* Only clear the interrupt _after_ draining the FIFO */
+		sun4i_spi_write(sspi, SUN4I_INT_STA_REG, 
SUN4I_INT_CTL_RF_F34);
+		return IRQ_HANDLED;
+	}
+
 	return IRQ_NONE;
 }
 
-- 
1.8.5.3