RE: [RFC] V4L HDR Architecture Proposal

[Dylan Yip <dylany@xilinx.com>]

Hi Hans,

> -----Original Message-----
> From: linux-media-owner@vger.kernel.org <linux-media-
> owner@vger.kernel.org> On Behalf Of Hans Verkuil
> Sent: Tuesday, January 28, 2020 11:31 PM
> To: Dylan Yip <dylany@xilinx.com>; Laurent Pinchart
> <laurent.pinchart@ideasonboard.com>
> Cc: Vishal Sagar <vsagar@xilinx.com>; linux-media@vger.kernel.org;
> Varunkumar Allagadapa <VARUNKUM@xilinx.com>; Madhurkiran
> Harikrishnan <MADHURKI@xilinx.com>; Jianqiang Chen
> <jianqian@xilinx.com>; Hyun Kwon <hyunk@xilinx.com>; Cyril Chemparathy
> <cyrilc@xilinx.com>; Sandip Kothari <sandipk@xilinx.com>; Subhransu
> Sekhar Prusty <sprusty@xilinx.com>; Anil Kumar Chimbeti
> <anilchc@xilinx.com>
> Subject: Re: [RFC] V4L HDR Architecture Proposal
> 
> On 1/29/20 7:14 AM, Dylan Yip wrote:
> > Hi Laurent, Hans,
> >
> > Thanks for the insights!
> >
> >> -----Original Message-----
> >> From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> >> Sent: Friday, January 24, 2020 4:09 AM
> >> To: Hans Verkuil <hverkuil@xs4all.nl>
> >> Cc: Vishal Sagar <vsagar@xilinx.com>; Dylan Yip <dylany@xilinx.com>;
> >> linux- media@vger.kernel.org; Varunkumar Allagadapa
> >> <VARUNKUM@xilinx.com>; Madhurkiran Harikrishnan
> >> <MADHURKI@xilinx.com>; Jianqiang Chen <jianqian@xilinx.com>; Hyun
> >> Kwon <hyunk@xilinx.com>; Cyril Chemparathy <cyrilc@xilinx.com>;
> >> Sandip Kothari <sandipk@xilinx.com>; Subhransu Sekhar Prusty
> >> <sprusty@xilinx.com>; Anil Kumar Chimbeti <anilchc@xilinx.com>
> >> Subject: Re: [RFC] V4L HDR Architecture Proposal
> >>
> >> Hi Hans,
> >>
> >> On Fri, Jan 24, 2020 at 11:10:06AM +0100, Hans Verkuil wrote:
> >>> On 1/24/20 10:04 AM, Vishal Sagar wrote:
> >>>> On Thursday, January 23, 2020 6:36 PM, Hans Verkuil wrote:
> >>>>> On 1/22/20 9:13 PM, Dylan Yip wrote:
> >>>>>> Hi All,
> >>>>>>
> >>>>>> We are planning to add HDR10 and HDR10+ metadata support into
> the
> >>>>>> V4L
> >>>>> framework and were hoping for some feedback before we started
> >>>>> implementation.
> >>>>>
> >>>>> Nice!
> >>>>>
> >>>>>>
> >>>>>> For context, Xilinx HDMI RX IP currently uses a AXI LITE
> >>>>>> interface where HDR metadata is obtained from a hardware FIFO. To
> >>>>>> access these packets a CPU copy is required.
> >>>>>> We are in the process of migrating towards a AXI MM interface
> >>>>>> where the hardware will directly write HDR metadata into memory.
> >>>>>> Currently the HDMI RX driver (https://github.com/Xilinx/hdmi-
> >>>>>> modules/blob/master/hdmi/xilinx-hdmirx.c) is modeled as a v4l
> >>>>>> subdev. This is linked to a DMA IP which utilizes the DMA engine
> >>>>>> APIs and registers itself as a video node for video data.
> >>>>>>
> >>>>>> HDR10 will only consist of static metadata which will come once
> >>>>>> per
> >> stream.
> >>>>>> However, HDR10+ will have dynamic metadata which can potentially
> >>>>>> come once per frame and be up to ~4000 bytes. We would like V4L
> >>>>>> architecture to be flexible to support both.
> >>>>>
> >>>>> The key here is the difference between Extended InfoFrames that
> >>>>> can be long and the others, that have a maximum size. The latter
> >>>>> should be handled by controls, the first is more difficult.
> >>>>
> >>>> Are you suggesting to handle static HDR via read only v4l controls
> >>>> in a
> >> meta video node?
> >>>
> >>> Yes. It's very suitable for that purpose.
> >
> > So are you saying we should create a separate metadata node and add the
> v4l control there or would we add the v4l control to the existing video data
> node? If it is the former, what's the point of creating the metadata node
> since we won't qbuf/dqbuf to it?
> 
> I'm sorry, I misread your original question. Static HDR should be handled via
> read only v4l controls in the existing video node, not in a meta video node.
> 

Ah thanks for the clarification.

> Regards,
> 
> 	Hans
> 
> >
> > Best,
> > Dylan Yip
> >
> >>>
> >>>>> Can you tell a bit more about how the hardware operates? Are all
> >>>>> InfoFrames obtained through the hw fifo, or are some stored in
> >>>>> registers and some go through the fifo?
> >>>>
> >>>> In the current implementation of the HDMI Rx IP, all InfoFrames are
> >>>> read
> >> from a register byte by byte which has FIFO at the back.
> >>>> The register is accessible by an AXI Lite interface.
> >>>> The FIFO can store maximum 8 packets. Each packet is 36 bytes in
> >>>> size (31
> >> bytes data and 5 bytes ECC calculated by IP).
> >>>> InfoFrames are one type of packets.
> >>>
> >>> Does one packet correspond to one InfoFrame? Or are they all
> >>> concatenated and hacked up into packets for the FIFO?
> >>>
> >>> This probably won't work well for large Extended InfoFrames of 4kB
> >>> or
> >>> more: the driver would have to be able to read from the FIFO very
> >>> quickly in order to prevent data from being lost, right? Hence the
> >> development of the AXIMM interface referred to below.
> >>>
> >>>> There are other types like General Control Packet, Audio Clock
> >>>> Regeneration Packet, etc. referred in Table 5-8 packet types in
> >>>> HDMI specification v1.4b)
> >>>>
> >>>> In future we plan on adding an AXIMM interface in the IP to handle
> >>>> Dynamic HDR. The tentative behavior will be as below - The driver
> >>>> will
> >> provide a buffer pointer to the IP via a register. The IP will dump
> >> the infoframes's extracted data into this buffer.
> >>>> With Frame sync, IP will return the length of the buffer in the
> >>>> provided
> >> buffer.
> >>>>
> >>>>> Does the hardware set maximum sizes for specific InfoFrames or the
> >>>>> total size of all InfoFrames combined? Or can it be any size?
> >>>>>
> >>>> Hope the above info about FIFO depth for current HDMI Rx IP answers
> >> this.
> >>>
> >>> Right, so the driver will provide the maximum size for all
> >>> InfoFrames that can occur between two video frames.
> >>>
> >>> And the driver will parse the received InfoFrames.
> >>>
> >>> I am strongly leaning towards using a control for the HDR10+
> >>> InfoFrame as well: it fits well with the Request API where controls
> >>> can be cleanly associated with a specific video frame, and the
> >>> amount of data isn't
> >> that large.
> >>
> >> This however leads me to a simple question: why do we have a metadata
> >> API in the first place if everything should go through controls ?

I have the same concern as Laurent here. Why are we supporting HDR metadata through controls but using the metadata API for other types of metadata? Wouldn't it be cleaner to follow the existing metadata API since HDR is a type of metadata?

This is why we were originally thinking that a 2 node approach (1 for video 1 for metadata) would have been cleaner. 

> >>
> >>> That said, some work in the control framework is probably needed to
> >>> streamline things a
> >>> bit: 
> >>>
> >>> 1) it should be possible to increase the size of compound controls
> >>> later if
> >> new fields are
> >>>    added. This is on the TODO list already since it is desired
> >>> functionality for
> >> codecs.
> >>>
> >>> 2) tentative, needs research first: add some sort of mechanism to
> >>> mmap
> >> the control
> >>>    payload to avoid mem copies. That would make controls much more
> >> useful for large metadata.
> >>
> >> Let's not forget that we would then also need to mmap the control per
> >> request, which will become challenging if we want to be able to
> >> pre-map everything like we do for buffers instead of mapping and
> >> unmapping for every request.

Same concern here. If we want to pre-map everything like buffers, wouldn't we essentially be replicating the behavior of buffers. Then the only difference would be that we are doing g_ctrl instead of qbuf/dqbuf right?

Best,
Dylan Yip

> >>
> >>> I'm not sure when I will have time to work on that, though.
> >>>
> >>>>> Does it accept any InfoFrame or only specific InfoFrame types? Or
> >>>>> is this programmable?
> >>>>
> >>>> HDMI Rx IP accepts all types of InfoFrames.
> >>>>
> >>>>>> We have 2 different proposals that we believe will work:
> >>>>>>
> >>>>>> A. 2 video node approach (1 for video, 1 for metadata) - This
> >>>>>> will align with current v4l metadata structure (i.e. uvc) but
> >>>>>> will require our HDMI RX driver to register a subdev and device node
> >>>>>> 	a. Our HDMI RX driver will register a v4l subdev (for video
> >>>>>> data) and a metadata node
> >>>>>> 		i. Is this acceptable?
> >>>>>> 	b. Applications will qbuf/dqbuf to both video and metadata
> nodes
> >>>>>> for each frame
> >>>>>>
> >>>>>> B. 1 video node approach - This will avoid mixing v4l subdev and
> >>>>>> v4l device node functionality inside HDMI RX driver but it strays
> >>>>>> from current v4l metadata architecture and also changes v4l
> >>>>>> subdev
> >> functionality
> >>>>>> 	a. We would add a "read" function to v4l subdev's
> >>>>>> 		i. This will also require us to add some "capabilities"
> field
> >>>>>> to subdev or be able to query for the "read" function
> >>>>>> 	b. HDMI Rx driver will register a v4l subdev with "read"
> >>>>>> function/capability
> >>>>>> 	c. Application can directly pass a buffer in the "read" function
> >>>>>> to HDMI RX subdev to obtain HDR metadata
> >>>>>> 		i. We will need to pass subdev name from application
> or be
> >> able
> >>>>>> to query all subdevs for this "read" capability, is this acceptable?
> >>>>>>
> >>>>>> Please let me know your opinions on which approach is best or
> >>>>>> propose another approach if these 2 are unfit. Thanks
> >>
> >> --
> >> Regards,
> >>
> >> Laurent Pinchart