Re: [PATCH 0/5] media: vimc: use configfs in order to configure devices topologies

[Dafna Hirschfeld <dafna.hirschfeld@collabora.com>]

Hi,

On Fri, 2019-09-20 at 15:17 +0200, Hans Verkuil wrote:
> Hi Dafna,
> 
> On 9/19/19 10:32 PM, Dafna Hirschfeld wrote:
> > This patchset introduces the usage of configfs in order to create vimc devices
> > with a configured topology. A patch introducing configfs usage was already sent by Helen Koike:
> > https://patchwork.linuxtv.org/patch/53397/ . The current patch is based on her patch but
> > suggests a new API for using configfs.
> > It uses symlinks to represent a link between two entities, an approach already used in the kernel
> > by usb gadgets composed with configfs to associate usb gadget's functions to its configurations.
> > For example, a topology of sensor->capture will be created with the following commands:
> > 
> > CONFIGFS_ROOT=/sys/kernel/config
> > 
> > mkdir ${CONFIGFS_ROOT}/vimc/mdev/
> > mkdir ${CONFIGFS_ROOT}/vimc/mdev/vimc-sensor:sen
> > mkdir ${CONFIGFS_ROOT}/vimc/mdev/vimc-capture:cap
> > tree ${CONFIGFS_ROOT}
> > /configfs/
> > `-- vimc
> >     `-- mdev
> >         |-- hotplug
> >         |-- vimc-capture:cap
> >         |   `-- pad:sink:0
> >         `-- vimc-sensor:sen
> >             `-- pad:source:0
> > 
> > mkdir ${CONFIGFS_ROOT}/vimc/mdev/vimc-sensor:sen/pad:source:0/to-cap
> > ln -s ${CONFIGFS_ROOT}/vimc/mdev/vimc-capture:cap/pad:sink:0 ${CONFIGFS_ROOT}/vimc/mdev/vimc-sensor:sen/pad:source:0/to-cap
> > tree ${CONFIGFS_ROOT}
> > /configfs/
> > `-- vimc
> >     `-- mdev
> >         |-- hotplug
> >         |-- vimc-capture:cap
> >         |   `-- pad:sink:0
> >         `-- vimc-sensor:sen
> >             `-- pad:source:0
> >                 `-- to-cap
> >                     |-- enabled
> >                     |-- immutable
> >                     `-- pad:sink:0 -> ../../../../../vimc/mdev/vimc-capture:cap/pad:sink:0
> > 
> > There are several reasons to prefer the symlink approach in order to represent links between entities.
> > The previous approach in which links are represented with directories of the form 'entity1:pad>-><entity2:pad'
> > requires userspace to parse the dirctories names in order to understand the topology, while in the symlink
> > approach userspace needs only to traverse the configfs tree.
> > Also, the usage of symlinks prevents userspace from creating links between entities that don't exist and also
> > an entity can't be removed if there is a symlink pointing to it or from it, while in the previous approach the
> 
> Why can't you remove an entity if there is a symlink pointing to it?
> 
> In the example above I can remove mdev/vimc-capture:cap just fine. Afterwards
> the pad:sink:0 symlink will point to a non-existing file, but that's valid
> symlink behavior.

Hmm, this should not be allowed, maybe you did something wrong?
I get:
# rmdir /sys/kernel/config/vimc/mdev/vimc-capture:cap
rmdir: failed to remove '/sys/kernel/config/vimc/mdev/vimc-capture:cap': Device or resource busy


> Or is vimc checking internally and prohibits the user from making invalid changes?

No, this is not internal to vimc, it is part of configfs, it is also written in the docs:

"A config_item cannot be removed while it links to any other item, nor
can it be removed while an item links to it.  Dangling symlinks are not
allowed in configfs."

Regards,
Dafna

> In any case, this cover letter is a bit confusing and needs to address this in more
> detail.
> 
> Regards,
> 
> 	Hans
> 
> > links were created by creating unrelated directories and care had to be taken to ensure consistency. This way
> > the topology configured from userspace is restricted to always be valid and represent the current topology of
> > the device. This results in less validation needed in kernel code when plugging the device and less possibility
> > for mistakes in the userspace side. Last, but not least, using symlinks is the natural way of associating things
> > in configfs.
> > 
> > This patch is meant to demonstrate the suggested configfs api and get comments and acceptance/disagreement from
> > the community. It passes few tests that configure basic topology and streams the capture entities.
> > Here is the tests script: https://gitlab.collabora.com/dafna/scripts/blob/master/configfs/sym-unit-tests-simple-topo.sh
> > Further versions will go through more extensive debugging.
> > 
> > The patchset is rebased on top of v5 of the patchset 'Collapse vimc into single monolithic driver' sent by Shuah Khan
> > https://lkml.org/lkml/2019/9/17/656
> > 
> > Patch 1, was sent by me before as a single patch and is needed for the configfs implementation.
> > 
> > Patch 2, documents how to use the new configfs api in order to create and set devices topologies.
> > 
> > Patch 3, only adds the new configfs api code but does not use it yet, so it still creates only the hardcoded device.
> > 
> > Patch 4, removes the hardcoded device topology and creates devices with topologies configured with the configfs.
> > 
> > Patch 5, implements indexing for the bus_info field since now there can be more than one vimc device.
> > 
> > Dafna Hirschfeld (5):
> >   media: vimc: upon streaming, check that the pipeline starts with a
> >     source entity
> >   docs: media: vimc: Documenting vimc topology configuration using
> >     configfs
> >   media: vimc: Add the implementation for the configfs api
> >   media: vimc: use configfs instead of having hardcoded configuration
> >   media: vimc: Add device index to the bus_info
> > 
> >  Documentation/media/v4l-drivers/vimc.dot    |  28 +-
> >  Documentation/media/v4l-drivers/vimc.rst    | 240 ++++++-
> >  drivers/media/platform/vimc/Kconfig         |   9 +-
> >  drivers/media/platform/vimc/Makefile        |   2 +-
> >  drivers/media/platform/vimc/vimc-capture.c  |  50 +-
> >  drivers/media/platform/vimc/vimc-common.h   |  86 ++-
> >  drivers/media/platform/vimc/vimc-configfs.c | 656 ++++++++++++++++++++
> >  drivers/media/platform/vimc/vimc-configfs.h |  41 ++
> >  drivers/media/platform/vimc/vimc-core.c     | 350 +++++------
> >  drivers/media/platform/vimc/vimc-debayer.c  |  35 +-
> >  drivers/media/platform/vimc/vimc-scaler.c   |  35 +-
> >  drivers/media/platform/vimc/vimc-sensor.c   |  33 +-
> >  drivers/media/platform/vimc/vimc-streamer.c |  39 +-
> >  13 files changed, 1289 insertions(+), 315 deletions(-)
> >  create mode 100644 drivers/media/platform/vimc/vimc-configfs.c
> >  create mode 100644 drivers/media/platform/vimc/vimc-configfs.h
> >