[PATCH v3 07/16] spi: dw: Use DMA max burst to set the request thresholds

[Serge Semin]

Each channel of DMA controller may have a limited length of burst
transaction (number of IO operations performed at ones in a single
DMA client request). This parameter can be used to setup the most
optimal DMA Tx/Rx data level values. In order to avoid the Tx buffer
overrun we can set the DMA Tx level to be of FIFO depth minus the
maximum burst transactions length. To prevent the Rx buffer underflow
the DMA Rx level should be set to the maximum burst transactions length.
This commit setups the DMA channels and the DW SPI DMA Tx/Rx levels
in accordance with these rules.

Signed-off-by: Serge Semin <Sergey.Semin@xxxxxxxxxxxxxxxxxxxx>
Cc: Alexey Malahov <Alexey.Malahov@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
Cc: Paul Burton <paulburton@xxxxxxxxxx>
Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Cc: Rob Herring <robh+dt@xxxxxxxxxx>
Cc: linux-mips@xxxxxxxxxxxxxxx
Cc: devicetree@xxxxxxxxxxxxxxx

---

Changelog v3:
- Use min() method to calculate the optimal burst values.
---
 drivers/spi/spi-dw-mid.c | 37 +++++++++++++++++++++++++++++++++----
 drivers/spi/spi-dw.h     |  2 ++
 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/drivers/spi/spi-dw-mid.c b/drivers/spi/spi-dw-mid.c
index be02fedd87cb..0e95d8bc85c5 100644
--- a/drivers/spi/spi-dw-mid.c
+++ b/drivers/spi/spi-dw-mid.c
@@ -34,6 +34,31 @@ static bool mid_spi_dma_chan_filter(struct dma_chan *chan, void *param)
 	return true;
 }
 
+static void mid_spi_maxburst_init(struct dw_spi *dws)
+{
+	struct dma_slave_caps caps;
+	u32 max_burst, def_burst;
+	int ret;
+
+	def_burst = dws->fifo_len / 2;
+
+	ret = dma_get_slave_caps(dws->rxchan, &caps);
+	if (!ret && caps.max_burst)
+		max_burst = caps.max_burst;
+	else
+		max_burst = RX_BURST_LEVEL;
+
+	dws->rxburst = min(max_burst, def_burst);
+
+	ret = dma_get_slave_caps(dws->txchan, &caps);
+	if (!ret && caps.max_burst)
+		max_burst = caps.max_burst;
+	else
+		max_burst = TX_BURST_LEVEL;
+
+	dws->txburst = min(max_burst, def_burst);
+}
+
 static int mid_spi_dma_init_mfld(struct device *dev, struct dw_spi *dws)
 {
 	struct dw_dma_slave slave = {
@@ -69,6 +94,8 @@ static int mid_spi_dma_init_mfld(struct device *dev, struct dw_spi *dws)
 	dws->master->dma_rx = dws->rxchan;
 	dws->master->dma_tx = dws->txchan;
 
+	mid_spi_maxburst_init(dws);
+
 	return 0;
 
 free_rxchan:
@@ -94,6 +121,8 @@ static int mid_spi_dma_init_generic(struct device *dev, struct dw_spi *dws)
 	dws->master->dma_rx = dws->rxchan;
 	dws->master->dma_tx = dws->txchan;
 
+	mid_spi_maxburst_init(dws);
+
 	return 0;
 }
 
@@ -216,7 +245,7 @@ static struct dma_async_tx_descriptor *dw_spi_dma_prepare_tx(struct dw_spi *dws,
 	memset(&txconf, 0, sizeof(txconf));
 	txconf.direction = DMA_MEM_TO_DEV;
 	txconf.dst_addr = dws->dma_addr;
-	txconf.dst_maxburst = TX_BURST_LEVEL;
+	txconf.dst_maxburst = dws->txburst;
 	txconf.src_addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;
 	txconf.dst_addr_width = convert_dma_width(dws->n_bytes);
 	txconf.device_fc = false;
@@ -290,7 +319,7 @@ static struct dma_async_tx_descriptor *dw_spi_dma_prepare_rx(struct dw_spi *dws,
 	memset(&rxconf, 0, sizeof(rxconf));
 	rxconf.direction = DMA_DEV_TO_MEM;
 	rxconf.src_addr = dws->dma_addr;
-	rxconf.src_maxburst = RX_BURST_LEVEL;
+	rxconf.src_maxburst = dws->rxburst;
 	rxconf.dst_addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;
 	rxconf.src_addr_width = convert_dma_width(dws->n_bytes);
 	rxconf.device_fc = false;
@@ -315,8 +344,8 @@ static int mid_spi_dma_setup(struct dw_spi *dws, struct spi_transfer *xfer)
 {
 	u16 imr = 0, dma_ctrl = 0;
 
-	dw_writel(dws, DW_SPI_DMARDLR, RX_BURST_LEVEL - 1);
-	dw_writel(dws, DW_SPI_DMATDLR, TX_BURST_LEVEL);
+	dw_writel(dws, DW_SPI_DMARDLR, dws->rxburst - 1);
+	dw_writel(dws, DW_SPI_DMATDLR, dws->fifo_len - dws->txburst);
 
 	if (xfer->tx_buf) {
 		dma_ctrl |= SPI_DMA_TDMAE;
diff --git a/drivers/spi/spi-dw.h b/drivers/spi/spi-dw.h
index 4902f937c3d7..d0c8b7d3a5d2 100644
--- a/drivers/spi/spi-dw.h
+++ b/drivers/spi/spi-dw.h
@@ -141,7 +141,9 @@ struct dw_spi {
 
 	/* DMA info */
 	struct dma_chan		*txchan;
+	u32			txburst;
 	struct dma_chan		*rxchan;
+	u32			rxburst;
 	unsigned long		dma_chan_busy;
 	dma_addr_t		dma_addr; /* phy address of the Data register */
 	const struct dw_spi_dma_ops *dma_ops;
-- 
2.25.1