spi: bcm-qspi: choose sysclk setting based on requested speed

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>
Link: https://lore.kernel.org/r/20211124193353.32311-2-kdasu.kdev@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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)
@@ -621,10 +633,18 @@ 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 */
+			/* 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) {
 			/* data_reg_size 1 (64bit) */
@@ -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);