Date:   Fri, 22 Apr 2022 10:58:39 -0400
From:   Nicolas Dufresne <nicolas.dufresne@...labora.com>
To:     Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Cc:     kernel@...labora.com, linux-kernel@...r.kernel.org,
        Sebastian Fricke <sebastian.fricke@...labora.com>,
        linux-media@...r.kernel.org
Subject: Re: [PATCH v3 09/24] media: v4l2: Trace calculated p/b0/b1 initial
 reflist

Le vendredi 22 avril 2022 à 09:26 +0200, Hans Verkuil a écrit :
> On 05/04/2022 22:44, Nicolas Dufresne wrote:
> > Add debug print statements to print the content of P & B reference
> > lists, to verify that the ordering of the generated reference lists is
> > correct. This is especially important for the field decoding mode,
> > where sorting is more complex.
> > 
> > Signed-off-by: Nicolas Dufresne <nicolas.dufresne@...labora.com>
> > Tested-by: Sebastian Fricke <sebastian.fricke@...labora.com>
> > Reviewed-by: Sebastian Fricke <sebastian.fricke@...labora.com>
> > ---
> >  drivers/media/v4l2-core/v4l2-h264.c | 86 +++++++++++++++++++++++++++++
> >  1 file changed, 86 insertions(+)
> > 
> > diff --git a/drivers/media/v4l2-core/v4l2-h264.c b/drivers/media/v4l2-core/v4l2-h264.c
> > index 38d8dbda0045..bcf9b7774560 100644
> > --- a/drivers/media/v4l2-core/v4l2-h264.c
> > +++ b/drivers/media/v4l2-core/v4l2-h264.c
> > @@ -241,6 +241,87 @@ static int v4l2_h264_b1_ref_list_cmp(const void *ptra, const void *ptrb,
> >  	return poca < pocb ? -1 : 1;
> >  }
> >  
> > +static char ref_type_to_char (u8 ref_type)
> 
> Spurious space before (.
> 
> Odd that checkpatch didn't catch that.
> 
> > +{
> > +	switch (ref_type) {
> > +	case V4L2_H264_FRAME_REF:
> > +		return 'f';
> > +	case V4L2_H264_TOP_FIELD_REF:
> > +		return 't';
> > +	case V4L2_H264_BOTTOM_FIELD_REF:
> > +		return 'b';
> > +	}
> > +
> > +	return '?';
> > +}
> > +
> > +static const char *format_ref_list_p(const struct v4l2_h264_reflist_builder *builder,
> > +				     struct v4l2_h264_reference *reflist,
> > +				     char *out_str, const int len)
> > +{
> > +	int n = 0, i;
> > +
> > +	n += snprintf(out_str + n, len - n, "|");
> > +
> > +	for (i = 0; i < builder->num_valid; i++) {
> > +		/* this is pic_num for frame and frame_num (wrapped) for field,
> > +		 * but for frame pic_num is equal to frame_num (wrapped).
> > +		 */
> > +		int frame_num = builder->refs[reflist[i].index].frame_num;
> > +		bool longterm = builder->refs[reflist[i].index].longterm;
> > +
> > +		n += scnprintf(out_str + n, len - n, "%i%c%c|",
> > +			       frame_num, longterm ? 'l' : 's',
> > +			       ref_type_to_char (reflist[i].fields));
> > +	}
> > +
> > +	return out_str;
> > +}
> > +
> > +static void print_ref_list_p(const struct v4l2_h264_reflist_builder *builder,
> > +			     struct v4l2_h264_reference *reflist)
> > +{
> > +	char buf[1024];
> > +
> > +	pr_debug("ref_pic_list_p (cur_poc %u%c) %s\n",
> > +		 builder->cur_pic_order_count,
> > +		 ref_type_to_char(builder->cur_pic_fields),
> > +		 format_ref_list_p(builder, reflist, buf, sizeof(buf)));
> > +}
> > +
> > +static const char *format_ref_list_b(const struct v4l2_h264_reflist_builder *builder,
> > +				     struct v4l2_h264_reference *reflist,
> > +				     char *out_str, const int len)
> > +{
> > +	int n = 0, i;
> > +
> > +	n += snprintf(out_str + n, len - n, "|");
> > +
> > +	for (i = 0; i < builder->num_valid; i++) {
> > +		int frame_num = builder->refs[reflist[i].index].frame_num;
> > +		u32 poc = v4l2_h264_get_poc(builder, reflist + i);
> > +		bool longterm = builder->refs[reflist[i].index].longterm;
> > +
> > +		n += scnprintf(out_str + n, len - n, "%i%c%c|",
> > +			       longterm ? frame_num : poc,
> > +			       longterm ? 'l' : 's',
> > +			       ref_type_to_char(reflist[i].fields));
> > +	}
> > +
> > +	return out_str;
> > +}
> > +
> > +static void print_ref_list_b(const struct v4l2_h264_reflist_builder *builder,
> > +			     struct v4l2_h264_reference *reflist, u8 list_num)
> > +{
> > +	char buf[1024];
> 
> I really don't like placing 1024 bytes on the stack. Can you find another way
> of doing this? Perhaps using pr_cont or writing each format_ref_list item
> on a separate line.

Thanks, I was strongly discourage of using pr_cont (which was my first
approach). Rationales are well covered on LKLM and in the pr_cont documentation,
so I won't say more then its not visually thread safe.

I would like to decline the second proposition, as having the lists spread out
on up to 32 lines will make the trace very hard to use. What I may suggest, as I
would really prefer keeping this trace useful, is to use an allocation instead.
The performance does not matter, and I explicitly call this function inside the
pr_debug call so it can be compiled out.

My last resort otherwise would be to use 32 %s formaters, and pass each of the
possible 32 entry (or empty string "") manually.

let me know what you believe is acceptable for you,
Nicolas

> 
> Regards,
> 
> 	Hans
> 
> > +
> > +	pr_debug("ref_pic_list_b%u (cur_poc %u%c) %s",
> > +		 list_num, builder->cur_pic_order_count,
> > +		 ref_type_to_char (builder->cur_pic_fields),
> > +		 format_ref_list_b(builder, reflist, buf, sizeof(buf)));
> > +}
> > +
> >  /**
> >   * v4l2_h264_build_p_ref_list() - Build the P reference list
> >   *
> > @@ -261,6 +342,8 @@ v4l2_h264_build_p_ref_list(const struct v4l2_h264_reflist_builder *builder,
> >  	       sizeof(builder->unordered_reflist[0]) * builder->num_valid);
> >  	sort_r(reflist, builder->num_valid, sizeof(*reflist),
> >  	       v4l2_h264_p_ref_list_cmp, NULL, builder);
> > +
> > +	print_ref_list_p(builder, reflist);
> >  }
> >  EXPORT_SYMBOL_GPL(v4l2_h264_build_p_ref_list);
> >  
> > @@ -296,6 +379,9 @@ v4l2_h264_build_b_ref_lists(const struct v4l2_h264_reflist_builder *builder,
> >  	if (builder->num_valid > 1 &&
> >  	    !memcmp(b1_reflist, b0_reflist, builder->num_valid))
> >  		swap(b1_reflist[0], b1_reflist[1]);
> > +
> > +	print_ref_list_b(builder, b0_reflist, 0);
> > +	print_ref_list_b(builder, b1_reflist, 1);
> >  }
> >  EXPORT_SYMBOL_GPL(v4l2_h264_build_b_ref_lists);
> >  
> 

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