Re: [PATCH v7 16/44] [media] media: convert links from array to list

From: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
To: Shuah Khan <shuahkhan@gmail.com>
Cc: "Linux Media Mailing List" <linux-media@vger.kernel.org>,
	"Mauro Carvalho Chehab" <mchehab@infradead.org>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Dan Carpenter" <dan.carpenter@oracle.com>,
	"Antti Palosaari" <crope@iki.fi>,
	"Tina Ruchandani" <ruchandani.tina@gmail.com>,
	"Akihiro Tsukada" <tskd08@gmail.com>,
	"Rafael Lourenço de Lima Chehab" <chehabrafael@gmail.com>,
	"Hans Verkuil" <hans.verkuil@cisco.com>,
	"Shuah Khan" <shuahkh@osg.samsung.com>,
	"Prabhakar Lad" <prabhakar.csengg@gmail.com>,
	"Sakari Ailus" <sakari.ailus@linux.intel.com>,
	"Laurent Pinchart" <laurent.pinchart@ideasonboard.com>,
	"Boris BREZILLON" <boris.brezillon@free-electrons.com>
Subject: Re: [PATCH v7 16/44] [media] media: convert links from array to list
Date: Wed, 26 Aug 2015 12:28:03 -0300	[thread overview]
Message-ID: <20150826122803.1714f295@recife.lan> (raw)
In-Reply-To: <CAKocOOMDGbNn_a+y-PmfKv7TgDjdfVoe8Z9QTre8QeQPOV0ZhA@mail.gmail.com>

Em Tue, 25 Aug 2015 16:48:56 -0600
Shuah Khan <shuahkhan@gmail.com> escreveu:

> On Sun, Aug 23, 2015 at 2:17 PM, Mauro Carvalho Chehab
> <mchehab@osg.samsung.com> wrote:
> > The entire logic that represent graph links were developed on a
> > time where there were no needs to dynamic remove links. So,
> > although links are created/removed one by one via some
> > functions, they're stored as an array inside the entity struct.
> >
> > As the array may grow, there's a logic inside the code that
> > checks if the amount of space is not enough to store
> > the needed links. If it isn't the core uses krealloc()
> > to change the size of the link, with is bad, as it
> > leaves the memory fragmented.
> >
> > So, convert links into a list.
> >
> > Also, currently,  both source and sink entities need the link
> > at the graph traversal logic inside media_entity. So there's
> > a logic duplicating all links. That makes it to spend
> > twice the memory needed. This is not a big deal for today's
> > usage, where the number of links are not big.
> >
> > Yet, if during the MC workshop discussions, it was said that
> > IIO graphs could have up to 4,000 entities. So, we may
> > want to remove the duplication on some future. The problem
> > is that it would require a separate linked list to store
> > the backlinks inside the entity, or to use a more complex
> > algorithm to do graph backlink traversal, with is something
> > that the current graph traversal inside the core can't cope
> > with. So, let's postpone a such change if/when it is actually
> > needed.
> >
> > Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
> >
> 
> I think it makes sense to add remove  support in phase2 after the
> patches 1-15 in this
> series that add the graph object and the API changes that go with it
> through some
> use-cases before adding support for remove.

Sorry, I'm totally lost with the above comment.

This patch series is a "pre-phase 1"[1]. It is the minimal amount of
stuff for us to start playing with the new API and testing/debugging
the graph creation.

We can only work on userspace cases after having the foundation
provided by this patchset (plus the 3 additional patches I sent on
Sunday), e. g. after implementing the MEDIA_IOC_G_TOPOLOGY ioctl.

[1] For a "phase 1", we also need the 3 additional patches I sent on
Sunday, plus a patch that will create the indirect interface->entities
links at the V4L2 side.

> That way we have confidence in the patch set 1-15 and then we can
> build on top with
> solid foundation.

Regards,
Mauro