Add OPP and SoC core voltage scaling support to the display controller
driver. This is required for enabling system-wide DVFS on pre-Tegra186
SoCs.

Tested-by: Peter Geis <redacted>
Tested-by: Nicolas Chauvet <redacted>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
---
 drivers/gpu/drm/tegra/dc.c | 66 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 65 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/tegra/dc.c b/drivers/gpu/drm/tegra/dc.c
index b6676f1fe358..105ad786e432 100644
--- a/drivers/gpu/drm/tegra/dc.c
+++ b/drivers/gpu/drm/tegra/dc.c

@@ -11,9 +11,12 @@
 #include <linux/interconnect.h>
 #include <linux/module.h>
 #include <linux/of_device.h>
+#include <linux/pm_domain.h>
+#include <linux/pm_opp.h>
 #include <linux/pm_runtime.h>
 #include <linux/reset.h>

+#include <soc/tegra/common.h>
 #include <soc/tegra/pmc.h>

 #include <drm/drm_atomic.h>

@@ -1699,6 +1702,48 @@ int tegra_dc_state_setup_clock(struct tegra_dc *dc,
        return 0;
 }

+static void tegra_dc_update_voltage_state(struct tegra_dc *dc,
+                                         struct tegra_dc_state *state)
+{
+       unsigned long rate, pstate;
+       struct dev_pm_opp *opp;
+       int err;
+
+       /* calculate actual pixel clock rate which depends on internal divider */
+       rate = DIV_ROUND_UP(clk_get_rate(dc->clk) * 2, state->div + 2);
+
+       /* find suitable OPP for the rate */
+       opp = dev_pm_opp_find_freq_ceil(dc->dev, &rate);
+
+       if (opp == ERR_PTR(-ERANGE))
+               opp = dev_pm_opp_find_freq_floor(dc->dev, &rate);
+
+       /* -ENOENT means that this device-tree doesn't have OPP table */
+       if (opp == ERR_PTR(-ENOENT))
+               return;
+
+       if (IS_ERR(opp)) {
+               dev_err(dc->dev, "failed to find OPP for %luHz: %pe\n",
+                       rate, opp);
+               return;
+       }
+
+       pstate = dev_pm_opp_get_voltage(opp);
+       dev_pm_opp_put(opp);
+
+       /*
+        * The minimum core voltage depends on the pixel clock rate (which
+        * depends on internal clock divider of the CRTC) and not on the
+        * rate of the display controller clock. This is why we're not using
+        * dev_pm_opp_set_rate() API and instead controlling the power domain
+        * directly.
+        */
+       err = dev_pm_genpd_set_performance_state(dc->dev, pstate);