clk: clk-axi-clkgen: make sure to enable the AXI bus clock

In order to access the registers of the HW, we need to make sure that
the AXI bus clock is enabled. Hence let's increase the number of clocks
by one.

In order to keep backward compatibility and make sure old DTs still work
we check if clock-names is available or not. If it is, then we can
disambiguate between really having the AXI clock or a parent clock and
so we can enable the bus clock. If not, we fallback to what was done
before and don't explicitly enable the AXI bus clock.

Note that if clock-names is given, the axi clock must be the last one in
the phandle array (also enforced in the DT bindings) so that we can reuse
as much code as possible.

Fixes: 0e646c52cf0e ("clk: Add axi-clkgen driver")
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20241029-axi-clkgen-fix-axiclk-v2-2-bc5e0733ad76@analog.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/clk-axi-clkgen.c

@@ -7,6 +7,7 @@
  */
 
 #include <linux/platform_device.h>
+#include <linux/clk.h>
 #include <linux/clk-provider.h>
 #include <linux/slab.h>
 #include <linux/io.h>
@@ -512,6 +513,7 @@ static int axi_clkgen_probe(struct platform_device *pdev)
 	struct clk_init_data init;
 	const char *parent_names[2];
 	const char *clk_name;
+	struct clk *axi_clk;
 	unsigned int i;
 	int ret;
 
@@ -528,8 +530,24 @@ static int axi_clkgen_probe(struct platform_device *pdev)
 		return PTR_ERR(axi_clkgen->base);
 
 	init.num_parents = of_clk_get_parent_count(pdev->dev.of_node);
-	if (init.num_parents < 1 || init.num_parents > 2)
-		return -EINVAL;
+
+	axi_clk = devm_clk_get_enabled(&pdev->dev, "s_axi_aclk");
+	if (!IS_ERR(axi_clk)) {
+		if (init.num_parents < 2 || init.num_parents > 3)
+			return -EINVAL;
+
+		init.num_parents -= 1;
+	} else {
+		/*
+		 * Legacy... So that old DTs which do not have clock-names still
+		 * work. In this case we don't explicitly enable the AXI bus
+		 * clock.
+		 */
+		if (PTR_ERR(axi_clk) != -ENOENT)
+			return PTR_ERR(axi_clk);
+		if (init.num_parents < 1 || init.num_parents > 2)
+			return -EINVAL;
+	}
 
 	for (i = 0; i < init.num_parents; i++) {
 		parent_names[i] = of_clk_get_parent_name(pdev->dev.of_node, i);