[alsa-devel] [PATCH v2] [media] media-device: use kref for media_device instance

[Mauro Carvalho Chehab]

Em Mon, 21 Mar 2016 13:10:33 +0200
Laurent Pinchart <laurent.pinchart at ideasonboard.com> escreveu:

> Hi Mauro,
> 
> Thank you for the patch.
> 
> On Friday 18 Mar 2016 21:42:16 Mauro Carvalho Chehab wrote:
> > Now that the media_device can be used by multiple drivers,
> > via devres, we need to be sure that it will be dropped only
> > when all drivers stop using it.  
> 
> I've discussed this with Shuah previously and I'm surprised to see that the 
> problem hasn't been fixed : using devres for this purpose is just plain wrong. 

I didn't follow your discussions with Shuah. I'm pretty sure I didn't
see any such reply to the /22 patch series. 

For sure there are other approaches, although I wouldn't say that this
approach is plain wrong. It was actually suggested by Greg KH at the
USB summit, back in 2011:
	https://lkml.org/lkml/2011/8/21/61

It works fine in the cases like the ones Shuah is currently addressing: 
USB devices that have multiple interfaces handled by independent drivers.

Going further, right now, as far as I'm aware of, there are only two use
cases for a driver-independent media_device struct in the media subsystem
(on the upstream Kernel):

- USB devices with USB Audio Class: au0828 and em28xx drivers,
  plus snd-usb-audio;

- bt878/bt879 PCI devices, where the DVB driver is independent
  from the V4L2 one (affects bt87x and bttv drivers).

The devres approach fits well for both use cases.

Ok, there are a plenty of OOT SoC drivers that might benefit of some
other solution, but we should care about them only if/when they got
upstreamed.

> The empty media_device_release_devres() function should have given you a hint.
> 
> What we need instead is a list of media devices indexed by struct device (for 
> this use case) or by struct device_node (for DT use cases). It will both 
> simplify the code and get rid of the devres abuse.

Yeah, Shuah's approach should be changed to a different one, in order to
work for DT use cases. It would be good to have a real DT use case for us
to validate the solution, before we start implementing something in the
wild.

Still, it would very likely need a kref there, in order to destroy the
media controller struct only after all drivers stop using it.

> Shuah, if I recall correctly you worked on implementing such a solution after 
> our last discussion on the topic. Could you please update us on the status ?

I saw a Shuah's email proposing to discuss this at the media summit.

> In the mean time, let's hold off on this patch, and merge a proper solution 
> instead.

Well, we should send a fix for the current issues for Kernel 4.6.

As the number of drivers that would be using this internal API is small
(right now, only 2 drivers), replacing devres by some other strategy
in the future should be easy.

Regards,
Mauro