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

1 files changed, 8 insertions, 7 deletions

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 6eb935bb2f92..ea4941da9b27 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -54,13 +54,14 @@ MODULE_AUTHOR("Gareth Hughes, Leif Delgass, José Fonseca, Jon Smirl");
 MODULE_DESCRIPTION("DRM shared core routines");
 MODULE_LICENSE("GPL and additional rights");
 MODULE_PARM_DESC(debug, "Enable debug output, where each bit enables a debug category.\n"
-"\t\tBit 0 (0x01) will enable CORE messages (drm core code)\n"
-"\t\tBit 1 (0x02) will enable DRIVER messages (drm controller code)\n"
-"\t\tBit 2 (0x04) will enable KMS messages (modesetting code)\n"
-"\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 0 (0x01)  will enable CORE messages (drm core code)\n"
+"\t\tBit 1 (0x02)  will enable DRIVER messages (drm controller code)\n"
+"\t\tBit 2 (0x04)  will enable KMS messages (modesetting code)\n"
+"\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)\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);