[PATCH 05/32] spi: dw: Introduce DW_SPI_CAP_POLL_NODELAY

[Damien Le Moal <damien.lemoal@xxxxxxx>]

On slow systems, i.e. systems with a slow CPU resulting in slow context
switches, calling spi_delay_exec() when executing polled transfers
using dw_spi_poll_transfer() can lead to RX FIFO overflows. Allow
platforms to opt out of delayed polling by introducing the
DW_SPI_CAP_POLL_NODELAY DW SPI capability flag to disable
the execution of spi_delay_exec() in dw_spi_poll_transfer().

Signed-off-by: Damien Le Moal <damien.lemoal@xxxxxxx>
---
 drivers/spi/spi-dw-core.c | 12 ++++++++----
 drivers/spi/spi-dw.h      |  1 +
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/spi/spi-dw-core.c b/drivers/spi/spi-dw-core.c
index c2ef1d8d46d5..16a6fd569145 100644
--- a/drivers/spi/spi-dw-core.c
+++ b/drivers/spi/spi-dw-core.c
@@ -385,14 +385,18 @@ static int dw_spi_poll_transfer(struct dw_spi *dws,
 	u16 nbits;
 	int ret;
 
-	delay.unit = SPI_DELAY_UNIT_SCK;
-	nbits = dws->n_bytes * BITS_PER_BYTE;
+	if (!(dws->caps & DW_SPI_CAP_POLL_NODELAY)) {
+		delay.unit = SPI_DELAY_UNIT_SCK;
+		nbits = dws->n_bytes * BITS_PER_BYTE;
+	}
 
 	do {
 		dw_writer(dws);
 
-		delay.value = nbits * (dws->rx_len - dws->tx_len);
-		spi_delay_exec(&delay, transfer);
+		if (!(dws->caps & DW_SPI_CAP_POLL_NODELAY)) {
+			delay.value = nbits * (dws->rx_len - dws->tx_len);
+			spi_delay_exec(&delay, transfer);
+		}
 
 		dw_reader(dws);
 
diff --git a/drivers/spi/spi-dw.h b/drivers/spi/spi-dw.h
index 48a11a51a407..25f6372b993a 100644
--- a/drivers/spi/spi-dw.h
+++ b/drivers/spi/spi-dw.h
@@ -130,6 +130,7 @@ enum dw_ssi_type {
 #define DW_SPI_CAP_KEEMBAY_MST		BIT(1)
 #define DW_SPI_CAP_DWC_SSI		BIT(2)
 #define DW_SPI_CAP_DFS_32		BIT(3)
+#define DW_SPI_CAP_POLL_NODELAY		BIT(4)
 
 /* Slave spi_transfer/spi_mem_op related */
 struct dw_spi_cfg {
-- 
2.28.0