[PATCH 2/2] drivers/gpu/drm/drm_print: fix drm_printer dynamic debug bypass

[Jim Cromie]

drm_debug_enabled() is the canonical bit-test for drm.debug.

Commit 6ce6fae84536 ("drm_print: optimize drm_debug_enabled for
jump-label") renamed the original bit-test to drm_debug_enabled_raw()
and introduced an internal bypass for dyndbg. When [1]=y, it defined
__drm_debug_enabled() to evaluate to 'true', allowing dyndbg's
static-key to handle the filtering at the callsite. It also provided
drm_debug_enabled() for cases where an explicit bit-mask check is
still required.

Later, commit 9fd6f61a297e ("drm/print: add drm_dbg_printer() for drm
device specific printer") added __drm_printfn_dbg(), but mistakenly
used the internal bypass __drm_debug_enabled() instead of the
canonical drm_debug_enabled(). This went unnoticed because at the
time, [1]=y was marked BROKEN.

Because __drm_printfn_dbg() is a shared helper where the callsite is
not directly guarded by dyndbg's static-key, this caused it to hit
the 'true' bypass and always print, ignoring the drm.debug bit-mask.

This results in a flood of messages in environments with slow serial
consoles, as seen in DRM-CI on i915 CML devices. When IGT causes a
mismatch in intel_pipe_config_compare(), the resulting UART storm
causes a hard timeout after 20 minutes.

To fix this, change __drm_printfn_dbg() to use
drm_debug_enabled_instrumented() instead. This ensures the bit-test
is performed at runtime even when dyndbg is enabled. It also adds a
pr_debug() to help track the frequency of this bit-test.

Additionally, update __drm_dev_dbg() to use the canonical
drm_debug_enabled() instead of the internal __drm_debug_enabled().
While __drm_dev_dbg() is wrapped by a dyndbg factory and thus safe,
this change ensures consistency and clarifies the intended usage.

[1] CONFIG_DRM_USE_DYNAMIC_DEBUG

Fixes: 9fd6f61a297e ("drm/print: add drm_dbg_printer() for drm device specific printer")
Signed-off-by: Jim Cromie <jim.cromie@xxxxxxxxx>
---
 drivers/gpu/drm/drm_print.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_print.c b/drivers/gpu/drm/drm_print.c
index ded9461df5f2..9b622345e2eb 100644
--- a/drivers/gpu/drm/drm_print.c
+++ b/drivers/gpu/drm/drm_print.c
@@ -218,7 +218,7 @@ void __drm_printfn_dbg(struct drm_printer *p, struct va_format *vaf)
 	const struct device *dev = drm ? drm->dev : NULL;
 	enum drm_debug_category category = p->category;
 
-	if (!__drm_debug_enabled(category))
+	if (!drm_debug_enabled_instrumented(category))
 		return;
 
 	__drm_dev_vprintk(dev, KERN_DEBUG, p->origin, p->prefix, vaf);
@@ -335,7 +335,7 @@ void __drm_dev_dbg(struct _ddebug *desc, const struct device *dev,
 	struct va_format vaf;
 	va_list args;
 
-	if (!__drm_debug_enabled(category))
+	if (!drm_debug_enabled(category))
 		return;
 
 	/* we know we are printing for either syslog, tracefs, or both */
-- 
2.53.0