It is amazing how much documentation you can find for Video4Linux2 - and none of it actually explains what Video4Linux is.
First, Video4Linux2 is a Linux driver framework. Framework drivers don't actually control devices directly. Instead, they provide an abstract model of some class of device, in this case video devices for applications to use. Driver frameworks provide three main benefits:
- Provide a unified API for applications to use with a very wide range of physical devices, whether connected by USB, PCIe, MIPI, Ethernet or other type of data transport bus
- In the kernel, frameworks contain the type of code that is needed in almost all of the device drivers of a particular class, thus greatly reducing the volume of disparate code
- In the kernel, frameworks provide a blueprint for writing new lower level drivers that actually control the hardware, thus simplifying driver development.
So, the V4L2 driver is a high-level driver that drives the UVC driver, that drives the USB driver that might be driving an even lower-level hardware driver.
This Matryoshka model is very common in the Linux kernel driver tree. V4L2 is one of the more complex examples because some camera devices require accessing large groups of sub-devices in several layers that control the camera and route the output of the camera among various components such as image processors.
You can still access the UVC driver directly from userspace using a device file and "ioctl" system calls, without going through the V4L2 driver, and you can still access the underlying USB driver directly from userspace using it's device file and "ioctl"s.
Being a general framework that provides functionality common to a wide range of devices, V4L2 doesn't provide you with the all of the functionality that the UVC driver could provide (assuming that your device actually provides more UVC functionality than is required to support V4L2).
So if you were to have a UVC device that does provide all of the functions specified in the UVC specification, then in order to actually use some of these functions you would need to access the UVC driver directly through a device file and "ioctl" system calls, assuming that the Linux kernel UVC driver in fact supports all of the UVC specification.
However, the meaning of
The uvcvideo driver implementation is adherent only to the V4L2 API
is that in the current Linux kernel UVC driver in fact does not provide any more UVC function support than is needed for V4L2.
V4L2 support in the kernel does not by itself provide UVC or USB support or lower-level USB hardware support.