drm/i915: Add support for forcing the link bpp on a connector

Add support for forcing the link bpp on a connector via a connector
debugfs entry. During reducing link bpps due to a link BW limit, keep
bpps close to their forced value.

Add the debugfs entry to all relevant connectors: all DP connectors and
on an FDI link CRT/SDVO/LVDS/HDMI connectors.

v2:
- Move adding the debugfs entries to this patch.
- Rename i915_force_link_bpp to intel_force_link_bpp. (Jani)
- Select the relevant connectors via platform checks. (Jani)
- Use for_each_new_intel_connector_in_state(). (Jani)
- Fix 64 bit division vs. 32 bit build when converting str to q4. (lkp)
- Avoid division and addition overflow when converting str to q4.

v3:
- Add TODO: to make the non-DSC min bpp value connector specific. (Ankit)

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250509180340.554867-12-imre.deak@intel.com

drivers/gpu/drm/i915/display/intel_display_debugfs.c

@@ -39,6 +39,7 @@
 #include "intel_hdcp.h"
 #include "intel_hdmi.h"
 #include "intel_hotplug.h"
+#include "intel_link_bw.h"
 #include "intel_panel.h"
 #include "intel_pps.h"
 #include "intel_psr.h"
@@ -1325,6 +1326,7 @@ void intel_connector_debugfs_add(struct intel_connector *connector)
 	intel_psr_connector_debugfs_add(connector);
 	intel_alpm_lobf_debugfs_add(connector);
 	intel_dp_link_training_debugfs_add(connector);
+	intel_link_bw_connector_debugfs_add(connector);
 
 	if (DISPLAY_VER(display) >= 11 &&
 	    ((connector_type == DRM_MODE_CONNECTOR_DisplayPort && !connector->mst.dp) ||

drivers/gpu/drm/i915/display/intel_display_device.h

@@ -172,6 +172,7 @@ struct intel_display_platforms {
 #define HAS_GMBUS_BURST_READ(__display)	(DISPLAY_VER(__display) >= 10 || (__display)->platform.kabylake)
 #define HAS_GMBUS_IRQ(__display)	(DISPLAY_VER(__display) >= 4)
 #define HAS_GMCH(__display)		(DISPLAY_INFO(__display)->has_gmch)
+#define HAS_FDI(__display)		(IS_DISPLAY_VER((__display), 5, 8) && !HAS_GMCH(__display))
 #define HAS_HOTPLUG(__display)		(DISPLAY_INFO(__display)->has_hotplug)
 #define HAS_HW_SAGV_WM(__display)	(DISPLAY_VER(__display) >= 13 && !(__display)->platform.dgfx)
 #define HAS_IPC(__display)		(DISPLAY_INFO(__display)->has_ipc)

drivers/gpu/drm/i915/display/intel_display_types.h

@@ -550,6 +550,10 @@ struct intel_connector {
 		struct intel_dp *dp;
 	} mst;
 
+	struct {
+		int force_bpp_x16;
+	} link;
+
 	/* Work struct to schedule a uevent on link train failure */
 	struct work_struct modeset_retry_work;
 

drivers/gpu/drm/i915/display/intel_link_bw.c

@@ -3,6 +3,11 @@
  * Copyright © 2023 Intel Corporation
  */
 
+#include <linux/ctype.h>
+#include <linux/debugfs.h>
+#include <linux/int_log.h>
+#include <linux/math.h>
+
 #include <drm/drm_fixed.h>
 #include <drm/drm_print.h>
 
@@ -10,11 +15,33 @@
 #include "intel_crtc.h"
 #include "intel_display_core.h"
 #include "intel_display_types.h"
+#include "intel_dp.h"
 #include "intel_dp_mst.h"
 #include "intel_dp_tunnel.h"
 #include "intel_fdi.h"
 #include "intel_link_bw.h"
 
+static int get_forced_link_bpp_x16(struct intel_atomic_state *state,
+				   const struct intel_crtc *crtc)
+{
+	struct intel_digital_connector_state *conn_state;
+	struct intel_connector *connector;
+	int force_bpp_x16 = INT_MAX;
+	int i;
+
+	for_each_new_intel_connector_in_state(state, connector, conn_state, i) {
+		if (conn_state->base.crtc != &crtc->base)
+			continue;
+
+		if (!connector->link.force_bpp_x16)
+			continue;
+
+		force_bpp_x16 = min(force_bpp_x16, connector->link.force_bpp_x16);
+	}
+
+	return force_bpp_x16 < INT_MAX ? force_bpp_x16 : 0;
+}
+
 /**
  * intel_link_bw_init_limits - initialize BW limits
  * @state: Atomic state
@@ -31,9 +58,10 @@ void intel_link_bw_init_limits(struct intel_atomic_state *state,
 	limits->force_fec_pipes = 0;
 	limits->bpp_limit_reached_pipes = 0;
 	for_each_pipe(display, pipe) {
+		struct intel_crtc *crtc = intel_crtc_for_pipe(display, pipe);
 		const struct intel_crtc_state *crtc_state =
-			intel_atomic_get_new_crtc_state(state,
-							intel_crtc_for_pipe(display, pipe));
+			intel_atomic_get_new_crtc_state(state, crtc);
+		int forced_bpp_x16 = get_forced_link_bpp_x16(state, crtc);
 
 		if (state->base.duplicated && crtc_state) {
 			limits->max_bpp_x16[pipe] = crtc_state->max_link_bpp_x16;
@@ -42,15 +70,19 @@ void intel_link_bw_init_limits(struct intel_atomic_state *state,
 		} else {
 			limits->max_bpp_x16[pipe] = INT_MAX;
 		}
+
+		if (forced_bpp_x16)
+			limits->max_bpp_x16[pipe] = min(limits->max_bpp_x16[pipe], forced_bpp_x16);
 	}
 }
 
 /**
- * intel_link_bw_reduce_bpp - reduce maximum link bpp for a selected pipe
+ * __intel_link_bw_reduce_bpp - reduce maximum link bpp for a selected pipe
  * @state: atomic state
  * @limits: link BW limits
  * @pipe_mask: mask of pipes to select from
  * @reason: explanation of why bpp reduction is needed
+ * @reduce_forced_bpp: allow reducing bpps below their forced link bpp
  *
  * Select the pipe from @pipe_mask with the biggest link bpp value and set the
  * maximum of link bpp in @limits below this value. Modeset the selected pipe,
@@ -64,10 +96,11 @@ void intel_link_bw_init_limits(struct intel_atomic_state *state,
  *   - %-ENOSPC if no pipe can further reduce its link bpp
  *   - Other negative error, if modesetting the selected pipe failed
  */
-int intel_link_bw_reduce_bpp(struct intel_atomic_state *state,
-			     struct intel_link_bw_limits *limits,
-			     u8 pipe_mask,
-			     const char *reason)
+static int __intel_link_bw_reduce_bpp(struct intel_atomic_state *state,
+				      struct intel_link_bw_limits *limits,
+				      u8 pipe_mask,
+				      const char *reason,
+				      bool reduce_forced_bpp)
 {
 	struct intel_display *display = to_intel_display(state);
 	enum pipe max_bpp_pipe = INVALID_PIPE;
@@ -97,6 +130,10 @@ int intel_link_bw_reduce_bpp(struct intel_atomic_state *state,
 			 */
 			link_bpp_x16 = fxp_q4_from_int(crtc_state->pipe_bpp);
 
+		if (!reduce_forced_bpp &&
+		    link_bpp_x16 <= get_forced_link_bpp_x16(state, crtc))
+			continue;
+
 		if (link_bpp_x16 > max_bpp_x16) {
 			max_bpp_x16 = link_bpp_x16;
 			max_bpp_pipe = crtc->pipe;
@@ -112,6 +149,21 @@ int intel_link_bw_reduce_bpp(struct intel_atomic_state *state,
 						 BIT(max_bpp_pipe));
 }
 
+int intel_link_bw_reduce_bpp(struct intel_atomic_state *state,
+			     struct intel_link_bw_limits *limits,
+			     u8 pipe_mask,
+			     const char *reason)
+{
+	int ret;
+
+	/* Try to keep any forced link BPP. */
+	ret = __intel_link_bw_reduce_bpp(state, limits, pipe_mask, reason, false);
+	if (ret == -ENOSPC)
+		ret = __intel_link_bw_reduce_bpp(state, limits, pipe_mask, reason, true);
+
+	return ret;
+}
+
 /**
  * intel_link_bw_set_bpp_limit_for_pipe - set link bpp limit for a pipe to its minimum
  * @state: atomic state
@@ -245,3 +297,176 @@ int intel_link_bw_atomic_check(struct intel_atomic_state *state,
 
 	return -EAGAIN;
 }
+
+static int force_link_bpp_show(struct seq_file *m, void *data)
+{
+	struct intel_connector *connector = m->private;
+
+	seq_printf(m, FXP_Q4_FMT "\n", FXP_Q4_ARGS(connector->link.force_bpp_x16));
+
+	return 0;
+}
+
+static int str_to_fxp_q4_nonneg_int(const char *str, int *val_x16)
+{
+	unsigned int val;
+	int err;
+
+	err = kstrtouint(str, 10, &val);
+	if (err)
+		return err;
+
+	if (val > INT_MAX >> 4)
+		return -ERANGE;
+
+	*val_x16 = fxp_q4_from_int(val);
+
+	return 0;
+}
+
+/* modifies str */
+static int str_to_fxp_q4_nonneg(char *str, int *val_x16)
+{
+	const char *int_str;
+	char *frac_str;
+	int frac_digits;
+	int frac_val;
+	int err;
+
+	int_str = strim(str);
+	frac_str = strchr(int_str, '.');
+
+	if (frac_str)
+		*frac_str++ = '\0';
+
+	err = str_to_fxp_q4_nonneg_int(int_str, val_x16);
+	if (err)
+		return err;
+
+	if (!frac_str)
+		return 0;
+
+	/* prevent negative number/leading +- sign mark */
+	if (!isdigit(*frac_str))
+		return -EINVAL;
+
+	err = str_to_fxp_q4_nonneg_int(frac_str, &frac_val);
+	if (err)
+		return err;
+
+	frac_digits = strlen(frac_str);
+	if (frac_digits > intlog10(INT_MAX) >> 24 ||
+	    frac_val > INT_MAX - int_pow(10, frac_digits) / 2)
+		return -ERANGE;
+
+	frac_val = DIV_ROUND_CLOSEST(frac_val, (int)int_pow(10, frac_digits));
+
+	if (*val_x16 > INT_MAX - frac_val)
+		return -ERANGE;
+
+	*val_x16 += frac_val;
+
+	return 0;
+}
+
+static int user_str_to_fxp_q4_nonneg(const char __user *ubuf, size_t len, int *val_x16)
+{
+	char *kbuf;
+	int err;
+
+	kbuf = memdup_user_nul(ubuf, len);
+	if (IS_ERR(kbuf))
+		return PTR_ERR(kbuf);
+
+	err = str_to_fxp_q4_nonneg(kbuf, val_x16);
+
+	kfree(kbuf);
+
+	return err;
+}
+
+static bool connector_supports_dsc(struct intel_connector *connector)
+{
+	struct intel_display *display = to_intel_display(connector);
+
+	switch (connector->base.connector_type) {
+	case DRM_MODE_CONNECTOR_eDP:
+		return intel_dp_has_dsc(connector);
+	case DRM_MODE_CONNECTOR_DisplayPort:
+		if (connector->mst.dp)
+			return HAS_DSC_MST(display);
+
+		return HAS_DSC(display);
+	default:
+		return false;
+	}
+}
+
+static ssize_t
+force_link_bpp_write(struct file *file, const char __user *ubuf, size_t len, loff_t *offp)
+{
+	struct seq_file *m = file->private_data;
+	struct intel_connector *connector = m->private;
+	struct intel_display *display = to_intel_display(connector);
+	int min_bpp;
+	int bpp_x16;
+	int err;
+
+	err = user_str_to_fxp_q4_nonneg(ubuf, len, &bpp_x16);
+	if (err)
+		return err;
+
+	/* TODO: Make the non-DSC min_bpp value connector specific. */
+	if (connector_supports_dsc(connector))
+		min_bpp = intel_dp_dsc_min_src_compressed_bpp();
+	else
+		min_bpp = intel_display_min_pipe_bpp();
+
+	if (bpp_x16 &&
+	    (bpp_x16 < fxp_q4_from_int(min_bpp) ||
+	     bpp_x16 > fxp_q4_from_int(intel_display_max_pipe_bpp(display))))
+		return -EINVAL;
+
+	err = drm_modeset_lock_single_interruptible(&display->drm->mode_config.connection_mutex);
+	if (err)
+		return err;
+
+	connector->link.force_bpp_x16 = bpp_x16;
+
+	drm_modeset_unlock(&display->drm->mode_config.connection_mutex);
+
+	*offp += len;
+
+	return len;
+}
+DEFINE_SHOW_STORE_ATTRIBUTE(force_link_bpp);
+
+void intel_link_bw_connector_debugfs_add(struct intel_connector *connector)
+{
+	struct intel_display *display = to_intel_display(connector);
+	struct dentry *root = connector->base.debugfs_entry;
+
+	switch (connector->base.connector_type) {
+	case DRM_MODE_CONNECTOR_DisplayPort:
+	case DRM_MODE_CONNECTOR_eDP:
+		break;
+	case DRM_MODE_CONNECTOR_VGA:
+	case DRM_MODE_CONNECTOR_SVIDEO:
+	case DRM_MODE_CONNECTOR_LVDS:
+	case DRM_MODE_CONNECTOR_DVID:
+		if (HAS_FDI(display))
+			break;
+
+		return;
+	case DRM_MODE_CONNECTOR_HDMIA:
+		if (HAS_FDI(display) && !HAS_DDI(display))
+			break;
+
+		return;
+	default:
+		return;
+	}
+
+	debugfs_create_file("intel_force_link_bpp", 0644, root,
+			    connector, &force_link_bpp_fops);
+}

drivers/gpu/drm/i915/display/intel_link_bw.h

@@ -11,6 +11,7 @@
 #include "intel_display_limits.h"
 
 struct intel_atomic_state;
+struct intel_connector;
 struct intel_crtc_state;
 
 struct intel_link_bw_limits {
@@ -32,5 +33,6 @@ bool intel_link_bw_set_bpp_limit_for_pipe(struct intel_atomic_state *state,
 					  enum pipe pipe);
 int intel_link_bw_atomic_check(struct intel_atomic_state *state,
 			       struct intel_link_bw_limits *new_limits);
+void intel_link_bw_connector_debugfs_add(struct intel_connector *connector);
 
 #endif