linux-kernel - Re: [PATCH] drm: use trace_printk rather than printk in drm

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Date:   Tue, 30 Jul 2019 23:06:22 -0700
From:   Joe Perches <joe@...ches.com>
To:     Fuqian Huang <huangfq.daxian@...il.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: use trace_printk rather than printk in drm_dbg.

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...)