linux-kernel - Re: [PATCH v2 26/27] dyndbg: 4 new trace-events: pr_debug, dev_dbg, drm

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Date:   Wed, 29 Jun 2022 16:30:52 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jim Cromie <jim.cromie@...il.com>
Cc:     jbaron@...mai.com, 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, gregkh@...uxfoundation.org,
        daniel.vetter@...ll.ch, seanpaul@...omium.org, robdclark@...il.com,
        mathieu.desnoyers@...icios.com, quic_saipraka@...cinc.com,
        will@...nel.org, catalin.marinas@....com,
        quic_psodagud@...cinc.com, maz@...nel.org, arnd@...db.de,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, mingo@...hat.com
Subject: Re: [PATCH v2 26/27] dyndbg: 4 new trace-events: pr_debug, dev_dbg,
 drm_{,dev}debug


Sorry for the late review. I finally got some time to look at this.

On Mon, 16 May 2022 16:56:39 -0600
Jim Cromie <jim.cromie@...il.com> wrote:


> diff --git a/include/trace/events/drm.h b/include/trace/events/drm.h
> new file mode 100644
> index 000000000000..6de80dd68620
> --- /dev/null
> +++ b/include/trace/events/drm.h
> @@ -0,0 +1,68 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#undef TRACE_SYSTEM
> +#define TRACE_SYSTEM drm
> +
> +#if !defined(_TRACE_DRM_H) || defined(TRACE_HEADER_MULTI_READ)
> +#define _TRACE_DRM_H
> +
> +#include <linux/tracepoint.h>
> +
> +/* drm_debug() was called, pass its args */
> +TRACE_EVENT(drm_debug,
> +	    TP_PROTO(int drm_debug_category, struct va_format *vaf),
> +
> +	    TP_ARGS(drm_debug_category, vaf),
> +
> +	    TP_STRUCT__entry(
> +		    __field(int, drm_debug_category)
> +		    __dynamic_array(char, msg, 256)
> +		    ),
> +
> +	    TP_fast_assign(
> +		    int len;
> +
> +		    __entry->drm_debug_category = drm_debug_category;
> +		    vsnprintf(__get_str(msg), 256, vaf->fmt, *vaf->va);
> +
> +		    len = strlen(__get_str(msg));
> +		    if (len > 0 && (__get_str(msg)[len - 1] == '\n'))
> +			    len -= 1;
> +		    __get_str(msg)[len] = 0;
> +		    ),
> +
> +	    TP_printk("%s", __get_str(msg))
> +);
> +
> +/* drm_devdbg() was called, pass its args, preserving order */
> +TRACE_EVENT(drm_devdbg,
> +	    TP_PROTO(const struct device *dev, int drm_debug_category, struct va_format *vaf),
> +
> +	    TP_ARGS(dev, drm_debug_category, vaf),
> +
> +	    TP_STRUCT__entry(
> +		    __field(const struct device*, dev)
> +		    __field(int, drm_debug_category)
> +		    __dynamic_array(char, msg, 256)

You do not want to hardcode the 256 here. That will cause 256 bytes to be
reserved on the buffer, and you will not get that back. Might as well make
it a static array, as you also add 4 bytes to for the offset and size.

I think you want (haven't tested it)

		__dynamic_array(char, msg, get_msg_size(vaf))

Where you have:

static unsigned int get_msg_size(struct va_format *vaf)
{
	va_list aq;
	unsigned int ret;

	va_copy(aq, vaf->va);
	ret = vsnprintf(NULL, 0, vaf->fmt, aq);
	va_end(aq);

	return min(ret + 1, 256);
}

What is in the last parameter of __dynamic_array() is used to calculate the
size needed to store the dynamic array.

Hmm, looking at other users of __dynamic_array(), this appears to be a
constant problem. I need to document this better.

-- Steve


> +		    ),
> +
> +	    TP_fast_assign(
> +		    int len;
> +
> +		    __entry->drm_debug_category = drm_debug_category;
> +		    __entry->dev = dev;
> +		    vsnprintf(__get_str(msg), 256, vaf->fmt, *vaf->va);
> +
> +		    len = strlen(__get_str(msg));
> +		    if (len > 0 && (__get_str(msg)[len - 1] == '\n'))
> +			    len -= 1;
> +		    __get_str(msg)[len] = 0;
> +		    ),
> +
> +	    TP_printk("cat:%d, %s %s", __entry->drm_debug_category,
> +		      dev_name(__entry->dev), __get_str(msg))
> +);
> +
> +#endif /* _TRACE_DRM_H */
> +