clk: bcm: rpi: Set a default minimum rate

The M2MC clock provides the state machine clock for both HDMI
controllers.

However, if no HDMI monitor is plugged in at boot, its clock rate will
be left at 0 by the firmware and will make any register access end up in
a CPU stall, even though the clock was enabled.

We had some code in the HDMI controller to deal with this before, but it
makes more sense to have it in the clock driver. Move it there.

Signed-off-by: Maxime Ripard <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Stephen Boyd <[e-mail]>

drivers/clk/bcm/clk-raspberrypi.c

···
 struct raspberrypi_clk_variant {
 	bool		export;
 	char		*clkdev;
+	unsigned long	min_rate;
 };
 
 static struct raspberrypi_clk_variant
···
 	},
 	[RPI_FIRMWARE_M2MC_CLK_ID] = {
 		.export = true,
+
+		/*
+		 * If we boot without any cable connected to any of the
+		 * HDMI connector, the firmware will skip the HSM
+		 * initialization and leave it with a rate of 0,
+		 * resulting in a bus lockup when we're accessing the
+		 * registers even if it's enabled.
+		 *
+		 * Let's put a sensible default so that we don't end up
+		 * in this situation.
+		 */
+		.min_rate = 120000000,
 	},
 	[RPI_FIRMWARE_V3D_CLK_ID] = {
 		.export = true,
···
 		if (ret) {
 			dev_err(rpi->dev, "Failed to initialize clkdev\n");
 			return ERR_PTR(ret);
+		}
+	}
+
+	if (variant->min_rate) {
+		unsigned long rate;
+
+		clk_hw_set_rate_range(&data->hw, variant->min_rate, max_rate);
+
+		rate = raspberrypi_fw_get_rate(&data->hw, 0);
+		if (rate < variant->min_rate) {
+			ret = raspberrypi_fw_set_rate(&data->hw, variant->min_rate, 0);
+			if (ret)
+				return ERR_PTR(ret);
 		}
 	}