[PATCH v2] drm: use trace_printk rather than printk in drm_dbg.

[Fuqian Huang]

In drivers/gpu/drm/amd/amdgpu/amdgpu_ih.c,
amdgpu_ih_process calls DRM_DEBUG which calls drm_dbg and
finally calls printk.
As amdgpu_ih_process is called from an interrupt handler,
and interrupt handler should be short as possible.

As printk may lead to bogging down the system or can even
create a live lock. printk should not be used in IRQ context.
Instead, trace_printk is recommended in IRQ context.
Link: https://lwn.net/Articles/365835

Reviewed-by: Joe Perches <joe at perches.com> 
Signed-off-by: Fuqian Huang <huangfq.daxian at gmail.com>
---
Changes in v2:
  - Only make the interrupt uses the trace_printk to avoid
    all 4000+ drm_dbg/DRM_DEBUG uses emitting a trace_printk.

 drivers/gpu/drm/drm_print.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_print.c b/drivers/gpu/drm/drm_print.c
index a17c8a14dba4..747835d16fa6 100644
--- a/drivers/gpu/drm/drm_print.c
+++ b/drivers/gpu/drm/drm_print.c
@@ -236,9 +236,13 @@ void drm_dbg(unsigned int category, const char *format, ...)
 	vaf.fmt = format;
 	vaf.va = &args;
 
-	printk(KERN_DEBUG "[" DRM_NAME ":%ps] %pV",
-	       __builtin_return_address(0), &vaf);
-
+	if (in_interrupt()) {
+		trace_printk(KERN_DEBUG "[" DRM_NAME ":%ps] %pV",
+		       __builtin_return_address(0), &vaf);
+	} else {
+		printk(KERN_DEBUG "[" DRM_NAME ":%ps] %pV",
+		       __builtin_return_address(0), &vaf);
+	}
 	va_end(args);
 }
 EXPORT_SYMBOL(drm_dbg);
-- 
2.11.0