From: Thierry Reding <treding@xxxxxxxxxx>
This helper chooses an appropriate configuration, according to the
bitrate requirements of the video mode and the capabilities of the
DisplayPort sink.
Signed-off-by: Thierry Reding <treding@xxxxxxxxxx>
---
drivers/gpu/drm/drm_dp_helper.c | 55 +++++++++++++++++++++++++++++++++
include/drm/drm_dp_helper.h | 4 +++
2 files changed, 59 insertions(+)
diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 85956f3a24e3..01d34f33c658 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -550,6 +550,61 @@ int drm_dp_link_configure(struct drm_dp_aux *aux, struct drm_dp_link *link)
}
EXPORT_SYMBOL(drm_dp_link_configure);
+/**
+ * drm_dp_link_choose() - choose the lowest possible configuration for a mode
+ * @link: DRM DP link object
+ * @mode: DRM display mode
+ * @info: DRM display information
+ *
+ * According to the eDP specification, a source should select a configuration
+ * with the lowest number of lanes and the lowest possible link rate that can
+ * match the bitrate requirements of a video mode. However it must ensure not
+ * to exceed the capabilities of the sink.
+ *
+ * Returns: 0 on success or a negative error code on failure.
+ */
+int drm_dp_link_choose(struct drm_dp_link *link,
+ const struct drm_display_mode *mode,
+ const struct drm_display_info *info)
+{
+ /* available link symbol clock rates */
+ static const unsigned int rates[3] = { 162000, 270000, 540000 };
+ /* available number of lanes */
+ static const unsigned int lanes[3] = { 1, 2, 4 };
+ unsigned long requirement, capacity;
+ unsigned int rate = link->max_rate;
+ unsigned int i, j;
+
+ /* bandwidth requirement */
+ requirement = mode->clock * info->bpc * 3;
+
+ for (i = 0; i < ARRAY_SIZE(lanes) && lanes[i] <= link->max_lanes; i++) {
+ for (j = 0; j < ARRAY_SIZE(rates) && rates[j] <= rate; j++) {
+ /*
+ * Capacity for this combination of lanes and rate,
+ * factoring in the ANSI 8B/10B encoding.
+ *
+ * Link rates in the DRM DP helpers are really link
+ * symbol frequencies, so a tenth of the actual rate
+ * of the link.
+ */
+ capacity = lanes[i] * (rates[j] * 10) * 8 / 10;
+
+ if (capacity >= requirement) {
+ DRM_DEBUG_KMS("using %u lanes at %u kHz (%lu/%lu kbps)\n",
+ lanes[i], rates[j], requirement,
+ capacity);
+ link->lanes = lanes[i];
+ link->rate = rates[j];
+ return 0;
+ }
+ }
+ }
+
+ return -ERANGE;
+}
+EXPORT_SYMBOL(drm_dp_link_choose);
+
/**
* drm_dp_downstream_max_clock() - extract branch device max
* pixel rate for legacy VGA
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index cd728b8a4dee..0f256458a57d 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -28,6 +28,7 @@
#include <linux/time64.h>
#include <linux/types.h>
+#include <drm/drm_crtc.h>
#include <drm/drm_print.h>
/*
@@ -1470,6 +1471,9 @@ int drm_dp_link_probe(struct drm_dp_aux *aux, struct drm_dp_link *link);
int drm_dp_link_power_up(struct drm_dp_aux *aux, struct drm_dp_link *link);
int drm_dp_link_power_down(struct drm_dp_aux *aux, struct drm_dp_link *link);
int drm_dp_link_configure(struct drm_dp_aux *aux, struct drm_dp_link *link);
+int drm_dp_link_choose(struct drm_dp_link *link,
+ const struct drm_display_mode *mode,
+ const struct drm_display_info *info);
int drm_dp_downstream_max_clock(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
const u8 port_cap[4]);
int drm_dp_downstream_max_bpc(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
--
2.22.0
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