nb/intel/haswell: Implement proper backlight PWM config

Further backport the backlight-PWM handling from Skylake. Beside
configuring the PWM frequency in Hz, we also use the PCH's logic
for the brightness setting via BLM_PCH_OVERRIDE_ENABLE. Linux
would toggle it anyway and that might confuse our ASL code.

We assume that the 183Hz value that was set before for Slippy
variants was overridden by Linux with the 200Hz VBT value, like
it was for the Broadwell Chromebooks. So we set 200Hz for them
in the devicetrees. The calculated value for the T440p of 220Hz
seems sane and also matches the VBT.

Change-Id: I17dfe1a3610d5e2918c617cf5d10896692fdccb3
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39769
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

2 files changed, 39 insertions, 10 deletions

diff --git a/src/northbridge/intel/haswell/chip.h b/src/northbridge/intel/haswell/chip.h
index cfc28845b2..d7ef27df16 100644
--- a/src/northbridge/intel/haswell/chip.h
+++ b/src/northbridge/intel/haswell/chip.h
@@ -36,8 +36,11 @@ struct northbridge_intel_haswell_config {
 	u16 gpu_panel_power_backlight_on_delay;  /* T5 time sequence */
 	u16 gpu_panel_power_backlight_off_delay; /* Tx time sequence */
 
-	u32 gpu_cpu_backlight;	/* CPU Backlight PWM value */
-	u32 gpu_pch_backlight;	/* PCH Backlight PWM value */
+	unsigned int gpu_pch_backlight_pwm_hz;
+	enum {
+		GPU_BACKLIGHT_POLARITY_HIGH = 0,
+		GPU_BACKLIGHT_POLARITY_LOW,
+	} gpu_pch_backlight_polarity;
 
 	bool gpu_ddi_e_connected;
 
diff --git a/src/northbridge/intel/haswell/gma.c b/src/northbridge/intel/haswell/gma.c
index eed6740bc3..65c5cf3e75 100644
--- a/src/northbridge/intel/haswell/gma.c
+++ b/src/northbridge/intel/haswell/gma.c
@@ -12,6 +12,7 @@
  * GNU General Public License for more details.
  */
 
+#include <commonlib/helpers.h>
 #include <arch/io.h>
 #include <device/mmio.h>
 #include <device/pci_ops.h>
@@ -347,14 +348,39 @@ static void gma_setup_panel(struct device *dev)
 		gtt_write(PCH_PP_DIVISOR, reg32);
 	}
 
-	/* Enable Backlight if needed */
-	if (conf->gpu_cpu_backlight) {
-		gtt_write(BLC_PWM_CPU_CTL2, BLC_PWM2_ENABLE);
-		gtt_write(BLC_PWM_CPU_CTL, conf->gpu_cpu_backlight);
-	}
-	if (conf->gpu_pch_backlight) {
-		gtt_write(BLC_PWM_PCH_CTL1, BLM_PCH_PWM_ENABLE);
-		gtt_write(BLC_PWM_PCH_CTL2, conf->gpu_pch_backlight);
+	/* Enforce the PCH PWM function, as so does Linux.
+	   The CPU PWM controls are disabled after reset.  */
+	if (conf->gpu_pch_backlight_pwm_hz) {
+		/* Reference clock is either 24MHz or 135MHz. We can choose
+		   either a 16 or a 128 step increment. Use 16 if we would
+		   have less than 100 steps otherwise. */
+		const unsigned int refclock = CONFIG(INTEL_LYNXPOINT_LP) ? 24*MHz : 135*MHz;
+		const unsigned int hz_limit = refclock / 128 / 100;
+		unsigned int pwm_increment, pwm_period;
+		u32 south_chicken2;
+
+		south_chicken2 = gtt_read(SOUTH_CHICKEN2);
+		if (conf->gpu_pch_backlight_pwm_hz > hz_limit) {
+			pwm_increment = 16;
+			south_chicken2 |= LPT_PWM_GRANULARITY;
+		} else {
+			pwm_increment = 128;
+			south_chicken2 &= ~LPT_PWM_GRANULARITY;
+		}
+		gtt_write(SOUTH_CHICKEN2, south_chicken2);
+
+		pwm_period = refclock / pwm_increment / conf->gpu_pch_backlight_pwm_hz;
+		printk(BIOS_INFO,
+			"GMA: Setting backlight PWM frequency to %uMHz / %u / %u = %uHz\n",
+			refclock / MHz, pwm_increment, pwm_period,
+			DIV_ROUND_CLOSEST(refclock, pwm_increment * pwm_period));
+
+		/* Start with a 50% duty cycle. */
+		gtt_write(BLC_PWM_PCH_CTL2, pwm_period << 16 | pwm_period / 2);
+
+		gtt_write(BLC_PWM_PCH_CTL1,
+			(conf->gpu_pch_backlight_polarity == GPU_BACKLIGHT_POLARITY_LOW) << 29 |
+			BLM_PCH_OVERRIDE_ENABLE | BLM_PCH_PWM_ENABLE);
 	}
 
 	/* Get display,pipeline,and DDI registers into a basic sane state */