[PATCH i-g-t v6 1/3] lib/i915/i915_dp: Add UHBR helpers and const-correct set_link_params

[Sowmiya S <sowmiya.s@intel.com>]

Add i915_dp_is_uhbr_rate() and i915_dp_get_next_lower_rate(), prep for
the UHBR-to-HBR fallback subtest later in this series; no caller yet.

i915_dp_get_next_lower_rate() parses intel_dp_allowed_link_configs
(source & sink & current limits) rather than i915_dp_force_link_rate,
since the latter is source-only and lets the kernel silently clamp a
rate the sink never advertised.

Also take link_rate/lane_count as const char * in
i915_dp_set_link_params() so callers can pass string literals.

v4: - name the UHBR10 threshold and make i915_dp_is_uhbr_rate() a
      static inline in the header to avoid a cross-TU call (Kunal)
    - parse intel_dp_allowed_link_configs instead of the source-only
      i915_dp_force_link_rate list in i915_dp_get_next_lower_rate()
      (Kunal)
    - avoid the implicit long to int narrowing on the return value
      (Kunal)

Signed-off-by: Sowmiya S <sowmiya.s@intel.com>
Reviewed-by: Kunal Joshi <kunal1.joshi@intel.com>
---
 lib/i915/i915_dp.c | 58 +++++++++++++++++++++++++++++++++++++++++++++-
 lib/i915/i915_dp.h | 24 ++++++++++++++++++-
 2 files changed, 80 insertions(+), 2 deletions(-)

diff --git a/lib/i915/i915_dp.c b/lib/i915/i915_dp.c
index e54058580..3d570f7cc 100644
--- a/lib/i915/i915_dp.c
+++ b/lib/i915/i915_dp.c
@@ -334,7 +334,7 @@ void i915_dp_reset_link_params(int drm_fd, igt_output_t *output)
  * to set link rate and lane count to auto on exit
  */
 void i915_dp_set_link_params(int drm_fd, igt_output_t *output,
-			     char *link_rate, char *lane_count)
+			     const char *link_rate, const char *lane_count)
 {
 	bool valid;
 	drmModeConnector *temp;
@@ -390,3 +390,59 @@ int i915_dp_get_max_supported_rate(int drm_fd, const igt_output_t *output)
 
 	return max_rate;
 }
+
+/**
+ * i915_dp_get_next_lower_rate:
+ * @drm_fd: A drm file descriptor
+ * @output: Target output
+ * @rate: reference link rate in 10 kbit/s units
+ *
+ * Parse the intel_dp_allowed_link_configs debugfs file and return the highest
+ * allowed link rate strictly below @rate.
+ *
+ * This file lists the configurations the driver would actually pick from, i.e.
+ * the intersection of the source rates, the rates the sink advertises and the
+ * current link limits, as "<lanes>x<rate>" entries. The i915_dp_force_link_rate
+ * list is only the source rates: writing a rate from it that the sink never
+ * advertised still succeeds and the kernel silently clamps the effective rate
+ * down, so the caller would report a rate the link was never trained at.
+ *
+ * Returns: highest allowed rate below @rate in 10 kbit/s units, or 0 if none.
+ */
+int i915_dp_get_next_lower_rate(int drm_fd, igt_output_t *output, int rate)
+{
+	char buf[4096];
+	const char *p;
+	int res, next = 0;
+
+	res = igt_debugfs_read_connector_file(drm_fd, igt_output_name(output),
+					      "intel_dp_allowed_link_configs",
+					      buf, sizeof(buf));
+	igt_assert_f(res == 0,
+		     "Unable to read %s/intel_dp_allowed_link_configs\n",
+		     igt_output_name(output));
+
+	/*
+	 * Entries are "<lanes>x<rate>". Key off the 'x' separator rather than
+	 * tokenising the whole file, so the header and any decoration around
+	 * the list are skipped without having to model them.
+	 */
+	for (p = buf; (p = strchr(p, 'x')); p++) {
+		char *endptr;
+		long r;
+
+		/* Must be preceded by the lane count to be a config entry. */
+		if (p == buf || !isdigit((unsigned char)p[-1]))
+			continue;
+
+		errno = 0;
+		r = strtol(p + 1, &endptr, 10);
+		if (errno || endptr == p + 1)
+			continue;
+
+		if (r < rate && r > next)
+			next = (int)r;
+	}
+
+	return next;
+}
diff --git a/lib/i915/i915_dp.h b/lib/i915/i915_dp.h
index b13629147..5d17713e2 100644
--- a/lib/i915/i915_dp.h
+++ b/lib/i915/i915_dp.h
@@ -3,8 +3,29 @@
 #ifndef _I915_DP_H_
 #define _I915_DP_H_
 
+#include <stdbool.h>
+
 #include "igt_kms.h"
 
+/* DP link rates are in 10 kbit/s units; UHBR10 is 10 Gbps. */
+#define I915_DP_UHBR10_LINK_RATE	1000000
+
+/**
+ * i915_dp_is_uhbr_rate:
+ * @link_rate: DP link rate in 10 kbit/s units, as reported by the
+ *	       i915_dp_*_link_rate debugfs files
+ *
+ * UHBR (Ultra High Bit Rate) link rates use 128b/132b channel encoding,
+ * everything below uses legacy 8b/10b. Mirrors the kernel's
+ * drm_dp_is_uhbr_rate().
+ *
+ * Returns: true if @link_rate is a UHBR rate, false otherwise.
+ */
+static inline bool i915_dp_is_uhbr_rate(int link_rate)
+{
+	return link_rate >= I915_DP_UHBR10_LINK_RATE;
+}
+
 int i915_dp_get_current_link_rate(int drm_fd, igt_output_t *output);
 int i915_dp_get_current_lane_count(int drm_fd, igt_output_t *output);
 int i915_dp_get_max_link_rate(int drm_fd, igt_output_t *output);
@@ -17,7 +38,8 @@ int i915_dp_get_pending_lt_failures(int drm_fd, igt_output_t *output);
 int i915_dp_get_pending_retrain(int drm_fd, igt_output_t *output);
 void i915_dp_reset_link_params(int drm_fd, igt_output_t *output);
 void i915_dp_set_link_params(int drm_fd, igt_output_t *output,
-			     char *link_rate, char *lane_count);
+			     const char *link_rate, const char *lane_count);
 int i915_dp_get_max_supported_rate(int drm_fd, const igt_output_t *output);
+int i915_dp_get_next_lower_rate(int drm_fd, igt_output_t *output, int rate);
 
 #endif
-- 
2.43.0