[libcamera-devel] [PATCH 1/9] libcamera: v4l2_device: Add method to lookup device path

[Jacopo Mondi]

Hi Niklas,

On Sat, Jul 18, 2020 at 03:23:16PM +0200, Niklas Söderlund wrote:
> Add a method to lookup a V4L2 devices path in sysfs. The device path
> describes the bus and device backing the V4L2 device and is guaranteed
> to be unique in the system and is persistent between reboots of the
> system.
>
> Signed-off-by: Niklas Söderlund <niklas.soderlund at ragnatech.se>
> ---
>  include/libcamera/internal/v4l2_device.h |  1 +
>  src/libcamera/v4l2_device.cpp            | 27 ++++++++++++++++++++++++
>  2 files changed, 28 insertions(+)
>
> diff --git a/include/libcamera/internal/v4l2_device.h b/include/libcamera/internal/v4l2_device.h
> index bf643f2ec966bb33..3b605aab343b3b94 100644
> --- a/include/libcamera/internal/v4l2_device.h
> +++ b/include/libcamera/internal/v4l2_device.h
> @@ -30,6 +30,7 @@ public:
>  	int setControls(ControlList *ctrls);
>
>  	const std::string &deviceNode() const { return deviceNode_; }
> +	std::string devicePath() const;
>
>  protected:
>  	V4L2Device(const std::string &deviceNode);
> diff --git a/src/libcamera/v4l2_device.cpp b/src/libcamera/v4l2_device.cpp
> index 56ea1ddda2c1425f..2f95e45261463f34 100644
> --- a/src/libcamera/v4l2_device.cpp
> +++ b/src/libcamera/v4l2_device.cpp
> @@ -9,6 +9,7 @@
>
>  #include <fcntl.h>
>  #include <iomanip>
> +#include <limits.h>
>  #include <string.h>
>  #include <sys/ioctl.h>
>  #include <sys/syscall.h>
> @@ -350,6 +351,32 @@ int V4L2Device::setControls(ControlList *ctrls)
>  	return ret;
>  }
>
> +/**
> + * \brief Retrieve the device path

unique device path in sysfs

> + *
> + * The device path describes the bus and device backing the V4L2 device and is
> + * guaranteed to be unique in the system and is persistent between reboots of
> + * the system.

drop the last 'the system' as it is repeated.

> + *
> + * The path is located by utilising that every V4L2 device have an entry under
> + * /sys/class/video4linux/ that contains a symlink the device backing it.

I'm not sure if it is required to state this, if that's part of the V4L2
specification. I would however change 'utilising' with 'assuming', but
maybe it's me not knowing how that word should be used.

> + *
> + * \todo When switching to C++17 use std::filesystem:: in the implementation.
> + *
> + * \return The device path

The unique device path in sysfs

Do we want to use 'predictable' as systemd does with network interface
names ?

> + */
> +std::string V4L2Device::devicePath() const
> +{
> +	std::string vdev = basename(deviceNode_.c_str());
> +	std::string sysfs = "/sys/class/video4linux/" + vdev + "/device";
> +
> +	char path[PATH_MAX];
> +	if (!realpath(sysfs.c_str(), path))
> +		LOG(V4L2, Fatal) << "Can not resolve path for " << deviceNode_;

I would cache the error and report why we failed (ie path non existing
or permission issues).

> +
> +	return path;
> +}
> +
>  /**
>   * \brief Perform an IOCTL system call on the device node
>   * \param[in] request The IOCTL request code
> --
> 2.27.0
>
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