Re: [PATCH v6 2/6] media: v4l2-core: Add helpers to build the H264 P/B0/B1 reflists

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu, 05 Mar 2020 14:42:34 -0500
Nicolas Dufresne <nicolas@ndufresne.ca> wrote:

> Le lundi 02 mars 2020 à 15:44 +0100, Boris Brezillon a écrit :
> > On Mon, 2 Mar 2020 14:24:33 +0100
> > Mauro Carvalho Chehab <mchehab+huawei@kernel.org> wrote:
> >   
> > > Em Thu, 20 Feb 2020 13:30:12 -0300
> > > Ezequiel Garcia <ezequiel@collabora.com> escreveu:
> > >   
> > > > From: Boris Brezillon <boris.brezillon@collabora.com>
> > > > 
> > > > Building those list is a standard procedure described in section
> > > > '8.2.4 Decoding process for reference picture lists construction' of
> > > > the H264 specification.
> > > > 
> > > > We already have 2 drivers needing the same logic (hantro and rkvdec) and
> > > > I suspect we will soon have more.
> > > > 
> > > > Let's provide generic helpers to create those lists.
> > > > 
> > > > Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> > > > Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
> > > > ---
> > > >  drivers/media/v4l2-core/Kconfig     |   4 +
> > > >  drivers/media/v4l2-core/Makefile    |   1 +
> > > >  drivers/media/v4l2-core/v4l2-h264.c | 258 ++++++++++++++++++++++++++++
> > > >  include/media/v4l2-h264.h           |  85 +++++++++
> > > >  4 files changed, 348 insertions(+)
> > > >  create mode 100644 drivers/media/v4l2-core/v4l2-h264.c
> > > >  create mode 100644 include/media/v4l2-h264.h
> > > > 
> > > > diff --git a/drivers/media/v4l2-core/Kconfig b/drivers/media/v4l2-
> > > > core/Kconfig
> > > > index 39e3fb30ba0b..8a4ccfbca8cf 100644
> > > > --- a/drivers/media/v4l2-core/Kconfig
> > > > +++ b/drivers/media/v4l2-core/Kconfig
> > > > @@ -45,6 +45,10 @@ config VIDEO_PCI_SKELETON
> > > >  config VIDEO_TUNER
> > > >  	tristate
> > > >  
> > > > +# Used by drivers that need v4l2-h264.ko
> > > > +config V4L2_H264
> > > > +	tristate
> > > > +
> > > >  # Used by drivers that need v4l2-mem2mem.ko
> > > >  config V4L2_MEM2MEM_DEV
> > > >  	tristate
> > > > diff --git a/drivers/media/v4l2-core/Makefile b/drivers/media/v4l2-
> > > > core/Makefile
> > > > index 786bd1ec4d1b..c5c53e0941ad 100644
> > > > --- a/drivers/media/v4l2-core/Makefile
> > > > +++ b/drivers/media/v4l2-core/Makefile
> > > > @@ -21,6 +21,7 @@ obj-$(CONFIG_VIDEO_V4L2) += v4l2-dv-timings.o
> > > >  obj-$(CONFIG_VIDEO_TUNER) += tuner.o
> > > >  
> > > >  obj-$(CONFIG_V4L2_MEM2MEM_DEV) += v4l2-mem2mem.o
> > > > +obj-$(CONFIG_V4L2_H264) += v4l2-h264.o
> > > >  
> > > >  obj-$(CONFIG_V4L2_FLASH_LED_CLASS) += v4l2-flash-led-class.o
> > > >  
> > > > diff --git a/drivers/media/v4l2-core/v4l2-h264.c b/drivers/media/v4l2-
> > > > core/v4l2-h264.c
> > > > new file mode 100644
> > > > index 000000000000..4f68c27ec7fd
> > > > --- /dev/null
> > > > +++ b/drivers/media/v4l2-core/v4l2-h264.c
> > > > @@ -0,0 +1,258 @@
> > > > +// SPDX-License-Identifier: GPL-2.0
> > > > +/*
> > > > + * V4L2 H264 helpers.
> > > > + *
> > > > + * Copyright (C) 2019 Collabora, Ltd.
> > > > + *
> > > > + * Author: Boris Brezillon <boris.brezillon@collabora.com>
> > > > + */
> > > > +
> > > > +#include <linux/module.h>
> > > > +#include <linux/sort.h>
> > > > +
> > > > +#include <media/v4l2-h264.h>
> > > > +
> > > > +/**
> > > > + * v4l2_h264_init_reflist_builder() - Initialize a P/B0/B1 reference list
> > > > + *				      builder
> > > > + *
> > > > + * @b: the builder context to initialize
> > > > + * @dec_params: decode parameters control
> > > > + * @slice_params: first slice parameters control
> > > > + * @sps: SPS control
> > > > + * @dpb: DPB to use when creating the reference list
> > > > + */
> > > > +void
> > > > +v4l2_h264_init_reflist_builder(struct v4l2_h264_reflist_builder *b,
> > > > +		const struct v4l2_ctrl_h264_decode_params *dec_params,
> > > > +		const struct v4l2_ctrl_h264_slice_params *slice_params,
> > > > +		const struct v4l2_ctrl_h264_sps *sps,
> > > > +		const struct v4l2_h264_dpb_entry *dpb)    
> > > 
> > > The prototype here is not nice...
> > >   
> > > > +{
> > > > +	int cur_frame_num, max_frame_num;
> > > > +	unsigned int i;
> > > > +
> > > > +	max_frame_num = 1 << (sps->log2_max_frame_num_minus4 + 4);
> > > > +	cur_frame_num = slice_params->frame_num;
> > > > +
> > > > +	memset(b, 0, sizeof(*b));
> > > > +	if (!(slice_params->flags & V4L2_H264_SLICE_FLAG_FIELD_PIC))
> > > > +		b->cur_pic_order_count = min(dec_params->bottom_field_order_cnt,
> > > > +					     dec_params->top_field_order_cnt);
> > > > +	else if (slice_params->flags & V4L2_H264_SLICE_FLAG_BOTTOM_FIELD)
> > > > +		b->cur_pic_order_count = dec_params->bottom_field_order_cnt;
> > > > +	else
> > > > +		b->cur_pic_order_count = dec_params->top_field_order_cnt;
> > > > +
> > > > +	for (i = 0; i < 16; i++) {
> > > > +		u32 pic_order_count;
> > > > +
> > > > +		if (!(dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_ACTIVE))
> > > > +			continue;
> > > > +
> > > > +		b->refs[i].pic_num = dpb[i].pic_num;    
> > > 
> > > ... as you're expecting a fixed number of elements at DPB array, and using
> > > a magic number (16) inside the for loop.  
> > 
> > I used to have a '#define V4L2_H264_NUM_DPB_ENTRIES 16' but have been
> > told that this is an arbitrary limitation (the spec does not explicitly
> > limit the DPB size, even if all the HW we've seen seem to limit it to
> > 16). Maybe we can pass the DPB array size as an argument so it stays
> > HW-specific.  
> 
> it's formalized in A.31 h), to quote it:
> 
> max_dec_frame_buffering <= MaxDpbFrames, where MaxDpbFrames is equal to
> 
>   Min( MaxDpbMbs / ( PicWidthInMbs * FrameHeightInMbs ), 16 )
> 
> So a DPB larger then this is not an H.24 DPB.

My bad, I mixed "max number of slices per frame" and "max number of
entries in the DPB". The first one I couldn't find a clear answer to,
while the second one has been 16 from the start. Sorry for the
confusion.