linux-kernel - [PATCH 1/2] drm: mark CONFIG_DRM_USE_DYNAMIC

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Date:   Mon,  5 Dec 2022 09:10:04 -0700
From:   Jim Cromie <jim.cromie@...il.com>
To:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        amd-gfx@...ts.freedesktop.org, intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org
Cc:     jani.nikula@...el.com, ville.syrjala@...ux.intel.com,
        daniel.vetter@...ll.ch, seanpaul@...omium.org, robdclark@...il.com,
        Jim Cromie <jim.cromie@...il.com>
Subject: [PATCH 1/2] drm: mark CONFIG_DRM_USE_DYNAMIC_DEBUG as BROKEN for now

CONFIG_DRM_USE_DYNAMIC_DEBUG=y has a regression, due to a chicken-egg
initialization problem:

1- modprobe i915
   i915 needs drm.ko, which is loaded 1st

2- "modprobe drm drm.debug=0x1ff" (virtual/implied)
   drm.debug is set post-initialization, from boot-args etc

3- `modprobe i915` finishes

W/O drm.debug-on-dyndbg that just works, because drm_dbg* does
drm_debug_enabled() to check __drm_debug & DRM_UT_<CAT> before
printing.

But the whole point of CONFIG_DRM_USE_DYNAMIC_DEBUG is to avoid that
runtime test, by enabling (at end/after module_init) a static-key at
selected callsites in the just-loaded module.

And since drm.ko is loaded before all dependent modules, no other
modules are "just-loaded", and their drm.debug callsites are not
present yet, just those in drm.ko itself.

CC: <ville.syrjala@...ux.intel.com>
Signed-off-by: Jim Cromie <jim.cromie@...il.com>
---
.v2 - default=N, cuz CI tests BROKEN stuff too. @ville.syrjala
---
 drivers/gpu/drm/Kconfig | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index 34f5a092c99e..5adc8d5b6a40 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -53,7 +53,8 @@ config DRM_DEBUG_MM
 
 config DRM_USE_DYNAMIC_DEBUG
 	bool "use dynamic debug to implement drm.debug"
-	default y
+	default n
+	depends on BROKEN	# chicken-egg initial enable problem
 	depends on DRM
 	depends on DYNAMIC_DEBUG || DYNAMIC_DEBUG_CORE
 	depends on JUMP_LABEL
-- 
2.38.1