[PATCH 1/2] spi: bcm-qspi: choose sysclk setting based on requested speed

[Kamal Dasu <kdasu.kdev@gmail.com>]

Check requested speed for a given transfer before setting
27MHz or 108Mhz sysclk on SoCs that support both. This way
for baud rates below 212Khz we can use 27Mhz clock.

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
---
 drivers/spi/spi-bcm-qspi.c | 32 ++++++++++++++++++++++++++------
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/drivers/spi/spi-bcm-qspi.c b/drivers/spi/spi-bcm-qspi.c
index f3de3305d0f5..38e6e2cb62ca 100644
--- a/drivers/spi/spi-bcm-qspi.c
+++ b/drivers/spi/spi-bcm-qspi.c
@@ -287,6 +287,18 @@ static inline int bcm_qspi_spbr_min(struct bcm_qspi *qspi)
 		return 8;
 }
 
+static u32 bcm_qspi_calc_spbr(u32 clk_speed_hz,
+			      const struct bcm_qspi_parms *xp)
+{
+	u32 spbr = 0;
+
+	/* SPBR = System Clock/(2 * SCK Baud Rate) */
+	if (xp->speed_hz)
+		spbr = clk_speed_hz / (xp->speed_hz * 2);
+
+	return spbr;
+}
+
 /* Read qspi controller register*/
 static inline u32 bcm_qspi_read(struct bcm_qspi *qspi, enum base_type type,
 				unsigned int offset)
@@ -621,9 +633,17 @@ static void bcm_qspi_hw_set_parms(struct bcm_qspi *qspi,
 			spcr |= MSPI_SPCR3_HALFDUPLEX |  MSPI_SPCR3_HDOUTTYPE;
 
 		if (bcm_qspi_has_sysclk_108(qspi)) {
-			/* SYSCLK_108 */
-			spcr |= MSPI_SPCR3_SYSCLKSEL_108;
-			qspi->base_clk = MSPI_BASE_FREQ * 4;
+			/* check requested baud rate before moving to 108Mhz */
+			spbr = bcm_qspi_calc_spbr(MSPI_BASE_FREQ * 4, xp);
+			if (spbr > QSPI_SPBR_MAX) {
+				/* use SYSCLK_27Mhz for slower baud rates */
+				spcr &= ~MSPI_SPCR3_SYSCLKSEL_MASK;
+				qspi->base_clk = MSPI_BASE_FREQ;
+			} else {
+				/* SYSCLK_108Mhz */
+				spcr |= MSPI_SPCR3_SYSCLKSEL_108;
+				qspi->base_clk = MSPI_BASE_FREQ * 4;
+			}
 		}
 
 		if (xp->bits_per_word > 16) {
@@ -649,9 +669,9 @@ static void bcm_qspi_hw_set_parms(struct bcm_qspi *qspi,
 		bcm_qspi_write(qspi, MSPI, MSPI_SPCR3, spcr);
 	}
 
-	if (xp->speed_hz)
-		spbr = qspi->base_clk / (2 * xp->speed_hz);
-
+	/* SCK Baud Rate = System Clock/(2 * SPBR) */
+	qspi->max_speed_hz = qspi->base_clk / (bcm_qspi_spbr_min(qspi) * 2);
+	spbr = bcm_qspi_calc_spbr(qspi->base_clk, xp);
 	spbr = clamp_val(spbr, bcm_qspi_spbr_min(qspi), QSPI_SPBR_MAX);
 	bcm_qspi_write(qspi, MSPI, MSPI_SPCR0_LSB, spbr);
 
-- 
2.17.1