linux-kernel - [RFC PATCH 4/4] spi: spi-fsl-spi: automatically adapt bits-per-word in cpu mode

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Date:   Wed, 27 Mar 2019 14:30:52 +0000
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Mark Brown <broonie@...nel.org>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Fabio Estevam <festevam@...il.com>,
        Rasmus Villemoes <Rasmus.Villemoes@...vas.se>
Subject: [RFC PATCH 4/4] spi: spi-fsl-spi: automatically adapt bits-per-word
 in cpu mode

Taking one interrupt for every byte is rather slow. Since the
controller is perfectly capable of transmitting 32 bits at a time,
change t->bits_per-word to 32 when the length is divisible by 4 and
large enough that the reduced number of interrupts easily compensates
for the one or two extra fsl_spi_setup_transfer() calls this causes.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
---
 drivers/spi/spi-fsl-spi.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/spi/spi-fsl-spi.c b/drivers/spi/spi-fsl-spi.c
index e2b341943796..b36ac6aa3b1f 100644
--- a/drivers/spi/spi-fsl-spi.c
+++ b/drivers/spi/spi-fsl-spi.c
@@ -363,12 +363,28 @@ static int fsl_spi_bufs(struct spi_device *spi, struct spi_transfer *t,
 static int fsl_spi_do_one_msg(struct spi_master *master,
 			      struct spi_message *m)
 {
+	struct mpc8xxx_spi *mpc8xxx_spi = spi_master_get_devdata(master);
 	struct spi_device *spi = m->spi;
 	struct spi_transfer *t, *first;
 	unsigned int cs_change;
 	const int nsecs = 50;
 	int status, last_bpw;
 
+	/*
+	 * In CPU mode, optimize large byte transfers to use larger
+	 * bits_per_word values to reduce number of interrupts taken.
+	 */
+	if (!(mpc8xxx_spi->flags & SPI_CPM_MODE)) {
+		list_for_each_entry(t, &m->transfers, transfer_list) {
+			if (t->len < 256 || t->bits_per_word != 8)
+				continue;
+			if ((t->len & 3) == 0)
+				t->bits_per_word = 32;
+			else if ((t->len & 1) == 0)
+				t->bits_per_word = 16;
+		}
+	}
+
 	/* Don't allow changes if CS is active */
 	cs_change = 1;
 	list_for_each_entry(t, &m->transfers, transfer_list) {
-- 
2.20.1