drm/tegra: dc - Compute shift clock divider in output drivers

The shift clock divider is highly dependent on the type of output, so
push computation of it down into the output drivers. The old code used
to work merely by accident.

Signed-off-by: Thierry Reding <treding@nvidia.com>

1 files changed, 29 insertions, 2 deletions

diff --git a/drivers/gpu/drm/tegra/rgb.c b/drivers/gpu/drm/tegra/rgb.c
index 0266fb40479..d6af9be48f4 100644
--- a/drivers/gpu/drm/tegra/rgb.c
+++ b/drivers/gpu/drm/tegra/rgb.c
@@ -159,11 +159,38 @@ static int tegra_output_rgb_disable(struct tegra_output *output)
 }
 
 static int tegra_output_rgb_setup_clock(struct tegra_output *output,
-					struct clk *clk, unsigned long pclk)
+					struct clk *clk, unsigned long pclk,
+					unsigned int *div)
 {
 	struct tegra_rgb *rgb = to_rgb(output);
+	int err;
+
+	err = clk_set_parent(clk, rgb->clk_parent);
+	if (err < 0) {
+		dev_err(output->dev, "failed to set parent: %d\n", err);
+		return err;
+	}
 
-	return clk_set_parent(clk, rgb->clk_parent);
+	/*
+	 * We may not want to change the frequency of the parent clock, since
+	 * it may be a parent for other peripherals. This is due to the fact
+	 * that on Tegra20 there's only a single clock dedicated to display
+	 * (pll_d_out0), whereas later generations have a second one that can
+	 * be used to independently drive a second output (pll_d2_out0).
+	 *
+	 * As a way to support multiple outputs on Tegra20 as well, pll_p is
+	 * typically used as the parent clock for the display controllers.
+	 * But this comes at a cost: pll_p is the parent of several other
+	 * peripherals, so its frequency shouldn't change out of the blue.
+	 *
+	 * The best we can do at this point is to use the shift clock divider
+	 * and hope that the desired frequency can be matched (or at least
+	 * matched sufficiently close that the panel will still work).
+	 */
+
+	*div = ((clk_get_rate(clk) * 2) / pclk) - 2;
+
+	return 0;
 }
 
 static int tegra_output_rgb_check_mode(struct tegra_output *output,