spi: spi-nxp-fspi: add the support for sample data from DQS pad

JIRA: https://issues.redhat.com/browse/RHEL-104570

commit c07f270
Author: Haibo Chen <haibo.chen@nxp.com>
Date:   Wed Sep 17 15:27:09 2025 +0800

    spi: spi-nxp-fspi: add the support for sample data from DQS pad

    flexspi define four mode for sample clock source selection.
    Here is the list of modes:
    mode 0: Dummy Read strobe generated by FlexSPI Controller and loopback
            internally
    mode 1: Dummy Read strobe generated by FlexSPI Controller and loopback
            from DQS pad
    mode 2: Reserved
    mode 3: Flash provided Read strobe and input from DQS pad

    In default, flexspi use mode 0 after reset. And for DTR mode, flexspi
    only support 8D-8D-8D mode. For 8D-8D-8D mode, IC suggest to use mode 3,
    otherwise read always get incorrect data.

    For DTR mode, flexspi will automatically div 2 of the root clock
    and output to device. the formula is:
        device_clock = root_clock / (is_dtr ? 2 : 1)
    So correct the clock rate setting for DTR mode to get the max
    performance.

    Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
    Reviewed-by: Frank Li <Frank.Li@nxp.com>
    Link: https://patch.msgid.link/20250917-flexspi-ddr-v2-4-bb9fe2a01889@nxp.com
    Signed-off-by: Mark Brown <broonie@kernel.org>

Signed-off-by: Mattijs Korpershoek <mkorpershoek@redhat.com>

Author: makohoek

drivers/spi/spi-nxp-fspi.c

@@ -399,7 +399,8 @@ struct nxp_fspi {
 	struct mutex lock;
 	struct pm_qos_request pm_qos_req;
 	int selected;
-#define FSPI_NEED_INIT		(1 << 0)
+#define FSPI_NEED_INIT		BIT(0)
+#define FSPI_DTR_MODE		BIT(1)
 	int flags;
 };
 
@@ -655,6 +656,40 @@ static void nxp_fspi_clk_disable_unprep(struct nxp_fspi *f)
 	return;
 }
 
+/*
+ * Sample Clock source selection for Flash Reading
+ * Four modes defined by fspi:
+ * mode 0: Dummy Read strobe generated by FlexSPI Controller
+ *         and loopback internally
+ * mode 1: Dummy Read strobe generated by FlexSPI Controller
+ *         and loopback from DQS pad
+ * mode 2: Reserved
+ * mode 3: Flash provided Read strobe and input from DQS pad
+ *
+ * fspi default use mode 0 after reset
+ */
+static void nxp_fspi_select_rx_sample_clk_source(struct nxp_fspi *f,
+						 bool op_is_dtr)
+{
+	u32 reg;
+
+	/*
+	 * For 8D-8D-8D mode, need to use mode 3 (Flash provided Read
+	 * strobe and input from DQS pad), otherwise read operaton may
+	 * meet issue.
+	 * This mode require flash device connect the DQS pad on board.
+	 * For other modes, still use mode 0, keep align with before.
+	 * spi_nor_suspend will disable 8D-8D-8D mode, also need to
+	 * change the mode back to mode 0.
+	 */
+	reg = fspi_readl(f, f->iobase + FSPI_MCR0);
+	if (op_is_dtr)
+		reg |= FSPI_MCR0_RXCLKSRC(3);
+	else	/*select mode 0 */
+		reg &= ~FSPI_MCR0_RXCLKSRC(3);
+	fspi_writel(f, reg, f->iobase + FSPI_MCR0);
+}
+
 static void nxp_fspi_dll_calibration(struct nxp_fspi *f)
 {
 	int ret;
@@ -736,15 +771,18 @@ static void nxp_fspi_dll_override(struct nxp_fspi *f)
 static void nxp_fspi_select_mem(struct nxp_fspi *f, struct spi_device *spi,
 				const struct spi_mem_op *op)
 {
+	/* flexspi only support one DTR mode: 8D-8D-8D */
+	bool op_is_dtr = op->cmd.dtr && op->addr.dtr && op->dummy.dtr && op->data.dtr;
 	unsigned long rate = op->max_freq;
 	int ret;
 	uint64_t size_kb;
 
 	/*
 	 * Return, if previously selected target device is same as current
-	 * requested target device.
+	 * requested target device. Also the DTR or STR mode do not change.
 	 */
-	if (f->selected == spi_get_chipselect(spi, 0))
+	if ((f->selected == spi_get_chipselect(spi, 0)) &&
+	    (!!(f->flags & FSPI_DTR_MODE) == op_is_dtr))
 		return;
 
 	/* Reset FLSHxxCR0 registers */
@@ -761,6 +799,18 @@ static void nxp_fspi_select_mem(struct nxp_fspi *f, struct spi_device *spi,
 
 	dev_dbg(f->dev, "Target device [CS:%x] selected\n", spi_get_chipselect(spi, 0));
 
+	nxp_fspi_select_rx_sample_clk_source(f, op_is_dtr);
+
+	if (op_is_dtr) {
+		f->flags |= FSPI_DTR_MODE;
+		/* For DTR mode, flexspi will default div 2 and output to device.
+		 * so here to config the root clock to 2 * device rate.
+		 */
+		rate = rate * 2;
+	} else {
+		f->flags &= ~FSPI_DTR_MODE;
+	}
+
 	nxp_fspi_clk_disable_unprep(f);
 
 	ret = clk_set_rate(f->clk, rate);