clk: bcm2835: Mark the CM SDRAM clock's parent as critical

While the SDRAM is being driven by its dedicated PLL most of the time,
there is a little loop running in the firmware that periodically turns
on the CM SDRAM clock (using its pre-initialized parent) and switches
SDRAM to using the CM clock to do PVT recalibration.

This avoids system hangs if we choose SDRAM's parent for some other
clock, then disable that clock.

Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/clk/bcm/clk-bcm2835.c

@@ -36,6 +36,7 @@
 
 #include <linux/clk-provider.h>
 #include <linux/clkdev.h>
+#include <linux/clk.h>
 #include <linux/clk/bcm2835.h>
 #include <linux/debugfs.h>
 #include <linux/module.h>
@@ -1827,6 +1828,25 @@ static const struct bcm2835_clk_desc clk_desc_array[] = {
 		.ctl_reg = CM_PERIICTL),
 };
 
+/*
+ * Permanently take a reference on the parent of the SDRAM clock.
+ *
+ * While the SDRAM is being driven by its dedicated PLL most of the
+ * time, there is a little loop running in the firmware that
+ * periodically switches the SDRAM to using our CM clock to do PVT
+ * recalibration, with the assumption that the previously configured
+ * SDRAM parent is still enabled and running.
+ */
+static int bcm2835_mark_sdc_parent_critical(struct clk *sdc)
+{
+	struct clk *parent = clk_get_parent(sdc);
+
+	if (IS_ERR(parent))
+		return PTR_ERR(parent);
+
+	return clk_prepare_enable(parent);
+}
+
 static int bcm2835_clk_probe(struct platform_device *pdev)
 {
 	struct device *dev = &pdev->dev;
@@ -1836,6 +1856,7 @@ static int bcm2835_clk_probe(struct platform_device *pdev)
 	const struct bcm2835_clk_desc *desc;
 	const size_t asize = ARRAY_SIZE(clk_desc_array);
 	size_t i;
+	int ret;
 
 	cprman = devm_kzalloc(dev,
 			      sizeof(*cprman) + asize * sizeof(*clks),
@@ -1866,6 +1887,10 @@ static int bcm2835_clk_probe(struct platform_device *pdev)
 			clks[i] = desc->clk_register(cprman, desc->data);
 	}
 
+	ret = bcm2835_mark_sdc_parent_critical(clks[BCM2835_CLOCK_SDRAM]);
+	if (ret)
+		return ret;
+
 	return of_clk_add_provider(dev->of_node, of_clk_src_onecell_get,
 				   &cprman->onecell);
 }