spi: imx: mx51-ecspi: Fix low-speed CONFIGREG delay calculation

The spi_imx->spi_bus_clk may be uninitialized and thus also zero in
mx51_ecspi_prepare_message(), which would lead to division by zero
in kernel. Since bitbang .setup_transfer callback which initializes
the spi_imx->spi_bus_clk is called after bitbang prepare_message
callback, iterate over all the transfers in spi_message, find the
one with lowest bus frequency, and use that bus frequency for the
delay calculation.

Note that it is not possible to move this CONFIGREG delay back into
the .setup_transfer callback, because that is invoked too late, after
the GPIO chipselects were already configured.

Fixes: 135cbd37 ("spi: imx: mx51-ecspi: Reinstate low-speed CONFIGREG delay")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210726100102.5188-1-marex@denx.deSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-imx.c

@@ -505,7 +505,9 @@ static int mx51_ecspi_prepare_message(struct spi_imx_data *spi_imx,
 				      struct spi_message *msg)
 {
 	struct spi_device *spi = msg->spi;
+	struct spi_transfer *xfer;
 	u32 ctrl = MX51_ECSPI_CTRL_ENABLE;
+	u32 min_speed_hz = ~0U;
 	u32 testreg, delay;
 	u32 cfg = readl(spi_imx->base + MX51_ECSPI_CONFIG);
 
@@ -577,8 +579,20 @@ static int mx51_ecspi_prepare_message(struct spi_imx_data *spi_imx,
 	 * be asserted before the SCLK polarity changes, which would disrupt
 	 * the SPI communication as the device on the other end would consider
 	 * the change of SCLK polarity as a clock tick already.
+	 *
+	 * Because spi_imx->spi_bus_clk is only set in bitbang prepare_message
+	 * callback, iterate over all the transfers in spi_message, find the
+	 * one with lowest bus frequency, and use that bus frequency for the
+	 * delay calculation. In case all transfers have speed_hz == 0, then
+	 * min_speed_hz is ~0 and the resulting delay is zero.
 	 */
-	delay = (2 * 1000000) / spi_imx->spi_bus_clk;
+	list_for_each_entry(xfer, &msg->transfers, transfer_list) {
+		if (!xfer->speed_hz)
+			continue;
+		min_speed_hz = min(xfer->speed_hz, min_speed_hz);
+	}
+
+	delay = (2 * 1000000) / min_speed_hz;
 	if (likely(delay < 10))	/* SCLK is faster than 100 kHz */
 		udelay(delay);
 	else			/* SCLK is _very_ slow */