[libcamera-devel] [PATCH v6 3/9] libcamera: v4l2_device: Add method to lookup device path

[Laurent Pinchart]

Hi Niklas,

Thank you for the patch.

On Mon, Aug 03, 2020 at 11:17:27PM +0200, Niklas Söderlund wrote:
> Add a method to lookup a V4L2 devices path in sysfs.
> 
> Signed-off-by: Niklas Söderlund <niklas.soderlund at ragnatech.se>
> ---
> * Changes since v5
> - Expand documentation
> - Use DeviceEnumerator interface to lookup device path
> - Include stdlib.h
> - Use realpath(..., nullptr); ...; free();
> 
> * Changes since v3
> - s/the device path/the device path in sysfs/
> ---
>  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..ef601d8e7e04b32a 100644
> --- a/src/libcamera/v4l2_device.cpp
> +++ b/src/libcamera/v4l2_device.cpp
> @@ -9,11 +9,14 @@
>  
>  #include <fcntl.h>
>  #include <iomanip>
> +#include <limits.h>
> +#include <stdlib.h>
>  #include <string.h>
>  #include <sys/ioctl.h>
>  #include <sys/syscall.h>
>  #include <unistd.h>
>  
> +#include "libcamera/internal/device_enumerator.h"
>  #include "libcamera/internal/log.h"
>  #include "libcamera/internal/utils.h"
>  #include "libcamera/internal/v4l2_controls.h"
> @@ -350,6 +353,30 @@ int V4L2Device::setControls(ControlList *ctrls)
>  	return ret;
>  }
>  
> +/**
> + * \brief Retrieve the device path in sysfs
> + *
> + * This function returns the sysfs path to the physical device backing the V4L2
> + * device. The path is guaranteed to be an absolute path, without any symbolic
> + * link.
> + *
> + * \return The device path in sysfs
> + */
> +std::string V4L2Device::devicePath() const
> +{
> +	std::unique_ptr<DeviceEnumerator> de = DeviceEnumerator::create();

Creating a new device enumerator just for this is a bit expensive :-( If
you agree with my suggestion in 1/9, I think we can ditch the device
enumerator. Otherwise it would be best to retrieve the instance created
by the camera manager.

> +	std::string devicePath = de->lookupSysfsPath(deviceNode_) + "/device";
> +
> +	char *realPath = realpath(devicePath.c_str(), nullptr);
> +	if (!realPath)
> +		LOG(V4L2, Fatal) << "Can not resolve path for " << devicePath;
> +
> +	std::string path{ realPath };

std::string::string(nullptr) has undefined behaviour. You should return
{} in the error case above.

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

-- 
Regards,

Laurent Pinchart