drm/dp_helper: Add DP aux channel tracing

This is something we've needed for a very long time now, as it makes
debugging issues with faulty MST hubs along with debugging issues
regarding us interfacing with hubs correctly vastly easier to debug.
Currently this can actually be done if you trace the i2c devices for DP
using ftrace but that's significantly less useful for a couple of
reasons:

- Tracing the i2c devices through ftrace means all of the traces are
  going to contain a lot of "garbage" output that we're sending over the
  i2c line. Most of this garbage comes from retrying transactions, DRM's
  helper library adding extra transactions to work around bad hubs, etc.
- Having a user set up ftrace so that they can provide debugging
  information is a lot more difficult then being able to say "just boot
  with drm.debug=0x100"
- We can potentially expand upon this tracing in the future to print
  debugging information in regards to other DP transactions like MST
  sideband transactions

This is inspired by a patch Rob Clark sent to do this a long time back.
Neither of us could find the patch however, so we both assumed it would
probably just be easier to rewrite it anyway.

Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180716154432.13433-1-lyude@redhat.com

drivers/gpu/drm/drm_dp_helper.c

@@ -185,6 +185,20 @@ EXPORT_SYMBOL(drm_dp_bw_code_to_link_rate);
 
 #define AUX_RETRY_INTERVAL 500 /* us */
 
+static inline void
+drm_dp_dump_access(const struct drm_dp_aux *aux,
+		   u8 request, uint offset, void *buffer, int ret)
+{
+	const char *arrow = request == DP_AUX_NATIVE_READ ? "->" : "<-";
+
+	if (ret > 0)
+		drm_dbg(DRM_UT_DP, "%s: 0x%05x AUX %s (ret=%3d) %*ph\n",
+			aux->name, offset, arrow, ret, min(ret, 20), buffer);
+	else
+		drm_dbg(DRM_UT_DP, "%s: 0x%05x AUX %s (ret=%3d)\n",
+			aux->name, offset, arrow, ret);
+}
+
 /**
  * DOC: dp helpers
  *
@@ -288,10 +302,14 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
 	ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, DP_DPCD_REV, buffer,
 				 1);
 	if (ret != 1)
-		return ret;
+		goto out;
 
-	return drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, offset, buffer,
+	ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, offset, buffer,
 				 size);
+
+out:
+	drm_dp_dump_access(aux, DP_AUX_NATIVE_READ, offset, buffer, ret);
+	return ret;
 }
 EXPORT_SYMBOL(drm_dp_dpcd_read);
 
@@ -312,8 +330,12 @@ EXPORT_SYMBOL(drm_dp_dpcd_read);
 ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,
 			  void *buffer, size_t size)
 {
-	return drm_dp_dpcd_access(aux, DP_AUX_NATIVE_WRITE, offset, buffer,
+	int ret;
+
+	ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_WRITE, offset, buffer,
 				 size);
+	drm_dp_dump_access(aux, DP_AUX_NATIVE_WRITE, offset, buffer, ret);
+	return ret;
 }
 EXPORT_SYMBOL(drm_dp_dpcd_write);
 

drivers/gpu/drm/drm_drv.c

@@ -60,7 +60,8 @@ MODULE_PARM_DESC(debug, "Enable debug output, where each bit enables a debug cat
 "\t\tBit 3 (0x08)  will enable PRIME messages (prime code)\n"
 "\t\tBit 4 (0x10)  will enable ATOMIC messages (atomic code)\n"
 "\t\tBit 5 (0x20)  will enable VBL messages (vblank code)\n"
-"\t\tBit 7 (0x80) will enable LEASE messages (leasing code)");
+"\t\tBit 7 (0x80)  will enable LEASE messages (leasing code)\n"
+"\t\tBit 8 (0x100) will enable DP messages (displayport code)");
 module_param_named(debug, drm_debug, int, 0600);
 
 static DEFINE_SPINLOCK(drm_minor_lock);

include/drm/drm_print.h

@@ -195,6 +195,7 @@ static inline struct drm_printer drm_debug_printer(const char *prefix)
 #define DRM_UT_VBL		0x20
 #define DRM_UT_STATE		0x40
 #define DRM_UT_LEASE		0x80
+#define DRM_UT_DP		0x100
 
 __printf(3, 4)
 void drm_dev_printk(const struct device *dev, const char *level,
@@ -307,6 +308,11 @@ void drm_err(const char *format, ...);
 #define DRM_DEBUG_LEASE(fmt, ...)					\
 	drm_dbg(DRM_UT_LEASE, fmt, ##__VA_ARGS__)
 
+#define	DRM_DEV_DEBUG_DP(dev, fmt, ...)					\
+	drm_dev_dbg(dev, DRM_UT_DP, fmt, ## __VA_ARGS__)
+#define DRM_DEBUG_DP(dev, fmt, ...)					\
+	drm_dbg(DRM_UT_DP, fmt, ## __VA_ARGS__)
+
 #define _DRM_DEV_DEFINE_DEBUG_RATELIMITED(dev, category, fmt, ...)	\
 ({									\
 	static DEFINE_RATELIMIT_STATE(_rs,				\