[alsa-devel] [PATCH v2] [media] media-device: use kref for media_device instance
Mauro Carvalho Chehab
mchehab at osg.samsung.com
Mon Mar 21 12:58:05 CET 2016
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
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