Re: [PATCH v8 48/55] [media] media_device: add a topology version field

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Mauro,

On Friday 04 September 2015 14:08:27 Mauro Carvalho Chehab wrote:
> Em Mon, 31 Aug 2015 15:35:38 +0200 Hans Verkuil escreveu:
> > On 08/31/2015 02:52 PM, Mauro Carvalho Chehab wrote:
> >> Em Mon, 31 Aug 2015 14:29:28 +0200 Hans Verkuil escreveu:
> >>> On 08/30/2015 05:06 AM, Mauro Carvalho Chehab wrote:
> >>>> Every time a graph object is added or removed, the version
> >>>> of the topology changes. That's a requirement for the new
> >>>> MEDIA_IOC_G_TOPOLOGY, in order to allow userspace to know
> >>>> that the topology has changed after a previous call to it.
> >>>> 
> >>>> Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
> >>> 
> >>> I think this should be postponed until we actually have dynamic
> >>> reconfigurable graphs.
> >> 
> >> So far, we're using the term "dynamic" to mean partial graph object
> >> removal.
> >> 
> >> But even today, MC does support "dynamic" in the sense of graph object
> >> additions.
> >> 
> >> You should notice that having a topology_version is something that IMHO,
> >> it is needed since the beginning, even without dynamic reconfigurable
> >> graphs, because the graph may grow in runtime.
> >> 
> >> That will happen, for example, if usb-snd-audio is blacklisted at
> >> /etc/modprobe*, and someone connects an au0828.
> >> 
> >> New entities/links will be created (after Shuah patches) if one would
> >> modprobe latter snd-usb-audio.
> > 
> > latter -> later :-)
> > 
> > You are right, this would trigger a topology change. I hadn't thought
> > about that.

First of all it won't be very useful without a topology change notification, 
so until we have them it doesn't matter too much. Then, the code is full of 
race conditions when it comes to dynamic updates, and I'm afraid Shua's 
patches can't go in before we fix them.

> >>> I would also like to reserve version 0: if 0 is returned, then the graph
> >>> is static.
> >> 
> >> Why? Implementing this would be really hard, as that would mean that
> >> G_TOPOLOGY would need to be blocked until all drivers and subdevices get
> >> probed.
> >> 
> >> In order to implement that, some logic would be needed at the drivers to
> >> identify if everything was set and unlock G_TOPOLOGY.
> > 
> > That wouldn't be needed if the media device node was created last. Which I
> > think is a good idea anyway.
> 
> Creating the media device node last won't work. It should be the first thing
> to be created, as all objects should be added to media_device linked lists.

I disagree with that. The media_device needs to be initialized first, but can 
be registered with userspace last. We don't want to generate a topology update 
event for every new entity, link or pad during the device probe sequence. 
Drivers should be in control and tell when they're done with initialization.

> Also, the numberspace should be local to a given media_device, as the graph
> traversal algorithm relies on having the number of entities <= 64,
> currently, in order to be able to detect loops. We should increase that
> number, but removing an "as low as possible" entity number limit is not
> trivial.
> 
> > > What would be the gain for that? I fail to see any.
> > 
> > It would tell userspace that it doesn't have to cope with dynamically
> > changing graphs.
> > 
> > Even though with the au0828 example you can expect to see cases like that,
> > I can pretty much guarantee that no generic v4l2 applications will ever
> > support dynamic changes.
> 
> Well, my test app supports it and it is generic ;) My plan is to use it as a
> basis for the library to be used on userspace for generic apps, extending it
> to be used by other tools like xawtv, qv4l2 and the dvbv5 apps.
> 
> I don't think it is hard to handle it on a generic app,

I'll quote you later on that :-)

> and this should be done anyway if we want dynamic support.
>
> The logic seems actually be simple:
> 
> at G_TOPOLOGY, if the topology changes, reload the objects;

And update everything in userspace. That's a very hard task if you don't 
design your applications extremely carefully.

> at SETUP_LINK_V2, the topology will be sent. if the driver detects that
> topology changed, it returns an error.
> 
> The caller will then need to reload the topology and re-apply the
> transaction to select the links, if the entities affected still exists. In
> other words, if the user's intent were to change the pipeline to receive the
> data at /dev/video2, e. g. something like:
> 	./yavta/yavta -f UYVY -s 720x312 -n 1 --capture=1 -F /dev/video2
> 
> What userspace would do is to reload everything, check if /dev/video2 still
> exists and then redo the function that it is equivalent to the above
> command, failing otherwise. That doesn't sound hard to implement.
> 
> > Those that will support it will be custom-made.
> > 
> > Being able to see that graphs can change dynamically would allow such apps
> > to either refuse to use the device, or warn the user.
> 
> The way I see is that applications that will assume that the graph
> is static will be the custom-made ones.

Or (some of) the generic ones.

> As they know the hardware, they can just either ignore the topology_version
> or wait for it to stabilize when the hardware is still being probed.
> 
> In any case, if we end by needing to have an explicit way for the
> Kernelspace to tell userspace that a graph is static, that could be done via
> an extra flag at MEDIA_INFO.

Why ? I don't see anything wrong with reversing version 0 for that purpose, as 
Hans proposed.

> Enabling this flag could be as easy as waiting for all graph elements to be
> created (where the topology is still dynamic), and raise such flag after
> finishing the probe sequence.

-- 
Regards,

Laurent Pinchart