spi: img-spfi: check for max speed supported by the spfi block

Maximum speed supported by spfi is limited to 1/4 of the spfi clock.

But in some SoCs the maximum speed supported by the spfi block can
be limited to less than 1/4 of the spfi clock. In such cases we have
to define the limit in the device tree so that the driver can pick
it up.
Signed-off-by: Sifan Naeem <sifan.naeem@imgtec.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-img-spfi.txt

@@ -21,6 +21,7 @@ Required properties:
 Optional properties:
 - img,supports-quad-mode: Should be set if the interface supports quad mode
   SPI transfers.
+- spfi-max-frequency: Maximum speed supported by the spfi block.
 
 Example:
 

drivers/spi/spi-img-spfi.c

@@ -546,6 +546,7 @@ static int img_spfi_probe(struct platform_device *pdev)
 	struct img_spfi *spfi;
 	struct resource *res;
 	int ret;
+	u32 max_speed_hz;
 
 	master = spi_alloc_master(&pdev->dev, sizeof(*spfi));
 	if (!master)
@@ -610,6 +611,19 @@ static int img_spfi_probe(struct platform_device *pdev)
 	master->max_speed_hz = clk_get_rate(spfi->spfi_clk) / 4;
 	master->min_speed_hz = clk_get_rate(spfi->spfi_clk) / 512;
 
+	/*
+	 * Maximum speed supported by spfi is limited to the lower value
+	 * between 1/4 of the SPFI clock or to "spfi-max-frequency"
+	 * defined in the device tree.
+	 * If no value is defined in the device tree assume the maximum
+	 * speed supported to be 1/4 of the SPFI clock.
+	 */
+	if (!of_property_read_u32(spfi->dev->of_node, "spfi-max-frequency",
+				  &max_speed_hz)) {
+		if (master->max_speed_hz > max_speed_hz)
+			master->max_speed_hz = max_speed_hz;
+	}
+
 	master->setup = img_spfi_setup;
 	master->cleanup = img_spfi_cleanup;
 	master->transfer_one = img_spfi_transfer_one;