[PATCH] spi: spi-imx: Fix spi_bus_clk if requested clock is higher than input clock

[Frieder Schrempf <frieder@fris.de>]

From: Frieder Schrempf <frieder.schrempf@kontron.de>

In case the requested bus clock is higher than the input clock, the correct
dividers (pre = 0, post = 0) are returned from mx51_ecspi_clkdiv(), but
*fres is left uninitialized and therefore contains an arbitrary value.

This causes trouble for the recently introduced PIO polling feature as the
value in spi_imx->spi_bus_clk is used there to calculate for which
transfers to enable PIO polling.

Less serious but also incorrect, it causes the calculation of the delay for
propagation of register changes added in

commit 6fd8b8503a0d ("spi: spi-imx: Fix out-of-order CS/SCLK operation at low speeds")

to be wrong, as not the actual bus clock but the potentially higher requested
bus clock is used.

Fix this by setting *fres even if no clock dividers are in use.

This issue was observed on Kontron BL i.MX8MM with an SPI peripheral clock set
to 50 MHz by default and a requested SPI bus clock of 80 MHz for the SPI NOR
flash.

With the fix applied the debug message from mx51_ecspi_clkdiv() now prints the
following:

spi_imx 30820000.spi: mx51_ecspi_clkdiv: fin: 50000000, fspi: 50000000,
post: 0, pre: 0

Fixes: 6fd8b8503a0d ("spi: spi-imx: Fix out-of-order CS/SCLK operation at low speeds")
Fixes: 07e759387788 ("spi: spi-imx: add PIO polling support")
Cc: Marc Kleine-Budde <mkl@pengutronix.de>
Cc: David Jander <david@protonic.nl>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Marek Vasut <marex@denx.de>
Cc: stable@vger.kernel.org
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
---
 drivers/spi/spi-imx.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/spi/spi-imx.c b/drivers/spi/spi-imx.c
index 30d82cc7300b..468ce0a2b282 100644
--- a/drivers/spi/spi-imx.c
+++ b/drivers/spi/spi-imx.c
@@ -444,8 +444,7 @@ static unsigned int mx51_ecspi_clkdiv(struct spi_imx_data *spi_imx,
 	unsigned int pre, post;
 	unsigned int fin = spi_imx->spi_clk;
 
-	if (unlikely(fspi > fin))
-		return 0;
+	fspi = min(fspi, fin);
 
 	post = fls(fin) - fls(fspi);
 	if (fin > fspi << post)
-- 
2.38.1