[PATCH v1 3/5] spi: fsl-cpm: Use 16 bit mode for large transfers with even size

[Christophe Leroy]

On CPM, the RISC core is a lot more efficiant when doing transfers
in 16-bits chunks than in 8-bits chunks, but unfortunately the
words need to be byte swapped as seen in a previous commit.

So, for large tranfers with an even size, allocate a temporary tx
buffer and byte-swap data before and after transfer.

This change allows setting higher speed for transfer. For instance
on an MPC 8xx (CPM1 comms RISC processor), the documentation tells
that transfer in byte mode at 1 kbit/s uses 0.200% of CPM load
at 25 MHz while a word transfer at the same speed uses 0.032%
of CPM load. This means the speed can be 6 times higher in
word mode for the same CPM load.

For the time being, only do it on CPM1 as there must be a
trade-off between the CPM load reduction and the CPU load required
to byte swap the data.

Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
---
 drivers/spi/spi-fsl-cpm.c | 23 +++++++++++++++++++++++
 drivers/spi/spi-fsl-spi.c |  3 +++
 2 files changed, 26 insertions(+)

diff --git a/drivers/spi/spi-fsl-cpm.c b/drivers/spi/spi-fsl-cpm.c
index 17a44d4f5021..38452089e8f3 100644
--- a/drivers/spi/spi-fsl-cpm.c
+++ b/drivers/spi/spi-fsl-cpm.c
@@ -21,6 +21,7 @@
 #include <linux/spi/spi.h>
 #include <linux/types.h>
 #include <linux/platform_device.h>
+#include <linux/byteorder/generic.h>
 
 #include "spi-fsl-cpm.h"
 #include "spi-fsl-lib.h"
@@ -120,6 +121,21 @@ int fsl_spi_cpm_bufs(struct mpc8xxx_spi *mspi,
 		mspi->rx_dma = mspi->dma_dummy_rx;
 		mspi->map_rx_dma = 0;
 	}
+	if (t->bits_per_word == 16 && t->tx_buf) {
+		const u16 *src = t->tx_buf;
+		u16 *dst;
+		int i;
+
+		dst = kmalloc(t->len, GFP_KERNEL);
+		if (!dst)
+			return -ENOMEM;
+
+		for (i = 0; i < t->len >> 1; i++)
+			dst[i] = cpu_to_le16p(src + i);
+
+		mspi->tx = dst;
+		mspi->map_tx_dma = 1;
+	}
 
 	if (mspi->map_tx_dma) {
 		void *nonconst_tx = (void *)mspi->tx; /* shut up gcc */
@@ -173,6 +189,13 @@ void fsl_spi_cpm_bufs_complete(struct mpc8xxx_spi *mspi)
 	if (mspi->map_rx_dma)
 		dma_unmap_single(dev, mspi->rx_dma, t->len, DMA_FROM_DEVICE);
 	mspi->xfer_in_progress = NULL;
+
+	if (t->bits_per_word == 16 && t->rx_buf) {
+		int i;
+
+		for (i = 0; i < t->len; i += 2)
+			le16_to_cpus(t->rx_buf + i);
+	}
 }
 EXPORT_SYMBOL_GPL(fsl_spi_cpm_bufs_complete);
 
diff --git a/drivers/spi/spi-fsl-spi.c b/drivers/spi/spi-fsl-spi.c
index 7e0aca620b72..b14f430a699d 100644
--- a/drivers/spi/spi-fsl-spi.c
+++ b/drivers/spi/spi-fsl-spi.c
@@ -351,6 +351,9 @@ static int fsl_spi_prepare_message(struct spi_controller *ctlr,
 				return -EINVAL;
 			if (t->bits_per_word == 16 || t->bits_per_word == 32)
 				t->bits_per_word = 8; /* pretend its 8 bits */
+			if (t->bits_per_word == 8 && t->len >= 256 &&
+			    (mpc8xxx_spi->flags & SPI_CPM1))
+				t->bits_per_word = 16;
 		}
 	}
 	return fsl_spi_setup_transfer(m->spi, first);
-- 
2.39.2