clk: bcm: rpi: Run some clocks at the minimum rate allowed

The core clock and M2MC clocks are shared between some devices (Unicam
controllers and the HVS, and the HDMI controllers, respectively) that
will have various, varying, requirements depending on their current work
load.

Since those loads can require a fairly high clock rate in extreme
conditions (up to ~600MHz), we can end up running those clocks at their
maximum frequency even though we no longer require such a high rate.

Fortunately, those devices don't require an exact rate but a minimum
rate, and all the drivers are using clk_set_min_rate. Thus, we can just
rely on the fact that the clk_request minimum (which is the aggregated
minimum of all the clock users) is what we want at all times.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220225143534.405820-11-maxime@cerno.tech
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/bcm/clk-raspberrypi.c

@@ -77,6 +77,7 @@ struct raspberrypi_clk_variant {
 	bool		export;
 	char		*clkdev;
 	unsigned long	min_rate;
+	bool		minimize;
 };
 
 static struct raspberrypi_clk_variant
@@ -87,6 +88,18 @@ raspberrypi_clk_variants[RPI_FIRMWARE_NUM_CLK_ID] = {
 	},
 	[RPI_FIRMWARE_CORE_CLK_ID] = {
 		.export = true,
+
+		/*
+		 * The clock is shared between the HVS and the CSI
+		 * controllers, on the BCM2711 and will change depending
+		 * on the pixels composited on the HVS and the capture
+		 * resolution on Unicam.
+		 *
+		 * Since the rate can get quite large, and we need to
+		 * coordinate between both driver instances, let's
+		 * always use the minimum the drivers will let us.
+		 */
+		.minimize = true,
 	},
 	[RPI_FIRMWARE_M2MC_CLK_ID] = {
 		.export = true,
@@ -102,6 +115,16 @@ raspberrypi_clk_variants[RPI_FIRMWARE_NUM_CLK_ID] = {
 		 * in this situation.
 		 */
 		.min_rate = 120000000,
+
+		/*
+		 * The clock is shared between the two HDMI controllers
+		 * on the BCM2711 and will change depending on the
+		 * resolution output on each. Since the rate can get
+		 * quite large, and we need to coordinate between both
+		 * driver instances, let's always use the minimum the
+		 * drivers will let us.
+		 */
+		.minimize = true,
 	},
 	[RPI_FIRMWARE_V3D_CLK_ID] = {
 		.export = true,
@@ -206,12 +229,26 @@ static int raspberrypi_fw_set_rate(struct clk_hw *hw, unsigned long rate,
 static int raspberrypi_fw_dumb_determine_rate(struct clk_hw *hw,
 					      struct clk_rate_request *req)
 {
+	struct raspberrypi_clk_data *data =
+		container_of(hw, struct raspberrypi_clk_data, hw);
+	struct raspberrypi_clk_variant *variant = data->variant;
+
 	/*
 	 * The firmware will do the rounding but that isn't part of
 	 * the interface with the firmware, so we just do our best
 	 * here.
 	 */
+
 	req->rate = clamp(req->rate, req->min_rate, req->max_rate);
+
+	/*
+	 * We want to aggressively reduce the clock rate here, so let's
+	 * just ignore the requested rate and return the bare minimum
+	 * rate we can get away with.
+	 */
+	if (variant->minimize && req->min_rate > 0)
+		req->rate = req->min_rate;
+
 	return 0;
 }