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

[PATCH] 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.
Link: https://lwn.net/Articles/365835

Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
---
 drivers/gpu/drm/drm_print.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_print.c b/drivers/gpu/drm/drm_print.c
index a17c8a14dba4..90cded140146 100644
--- a/drivers/gpu/drm/drm_print.c
+++ b/drivers/gpu/drm/drm_print.c
@@ -236,7 +236,7 @@ void drm_dbg(unsigned int category, const char *format, ...)
 	vaf.fmt = format;
 	vaf.va = &args;
 
-	printk(KERN_DEBUG "[" DRM_NAME ":%ps] %pV",
+	trace_printk(KERN_DEBUG "[" DRM_NAME ":%ps] %pV",
 	       __builtin_return_address(0), &vaf);
 
 	va_end(args);
-- 
2.11.0

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

[Joe Perches]

On Wed, 2019-07-31 at 10:45 +0800, Fuqian Huang wrote:
> 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.
> Link: https://lwn.net/Articles/365835
[]
> diff --git a/drivers/gpu/drm/drm_print.c b/drivers/gpu/drm/drm_print.c
[]
> @@ -236,7 +236,7 @@ void drm_dbg(unsigned int category, const char *format, ...)
>  	vaf.fmt = format;
>  	vaf.va = &args;
>  
> -	printk(KERN_DEBUG "[" DRM_NAME ":%ps] %pV",
> +	trace_printk(KERN_DEBUG "[" DRM_NAME ":%ps] %pV",
>  	       __builtin_return_address(0), &vaf);
>  
>  	va_end(args);

This makes all 4000+ drm_dbg/DRM_DEBUG uses emit
a trace_printk.

I suggest instead you make only the interrupt uses
use a different function and not drm_dbg.

Or maybe add an in_interrupt() check like

	if (in_interrupt())
		printk(KERN_DEBUG etc...)
	else
		trace_printk(etc...)

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

[Fuqian Huang]

Joe Perches <joe@perches.com> 於 2019年7月31日週三 下午2:06寫道：
>
> On Wed, 2019-07-31 at 10:45 +0800, Fuqian Huang wrote:
> > 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.
> > Link: https://lwn.net/Articles/365835
> []
> > diff --git a/drivers/gpu/drm/drm_print.c b/drivers/gpu/drm/drm_print.c
> []
> > @@ -236,7 +236,7 @@ void drm_dbg(unsigned int category, const char *format, ...)
> >       vaf.fmt = format;
> >       vaf.va = &args;
> >
> > -     printk(KERN_DEBUG "[" DRM_NAME ":%ps] %pV",
> > +     trace_printk(KERN_DEBUG "[" DRM_NAME ":%ps] %pV",
> >              __builtin_return_address(0), &vaf);
> >
> >       va_end(args);
>
> This makes all 4000+ drm_dbg/DRM_DEBUG uses emit
> a trace_printk.
>
> I suggest instead you make only the interrupt uses
> use a different function and not drm_dbg.
>
> Or maybe add an in_interrupt() check like
>
>         if (in_interrupt())
>                 printk(KERN_DEBUG etc...)
>         else
>                 trace_printk(etc...)

I will send a v2 patch to fix this.