Hi Ning, On Thu, Nov 22, 2018 at 11:52 AM Zhang, Ning A <ning.a.zhang@xxxxxxxxx> wrote: > > Hello everyone > > when we need to setup media pipeline, eg, for camera capture, media-ctl > needs to be called multiple time to setup media link and subdev > formats, or similar code in a single application. this will use > multiple IOCTLs on "/dev/mediaX" and "/dev/v4l2-subdevY". > > to setup media pipeline in userspace requires to fully understanding > the topology of the media stack. but the fact is only media driver > developer could know how to setup media pipeline. each time driver > updates, this would break userspace application if application > engineers don't know this change. That's obviously a bug in the driver. Kernel interfaces must not change in a way that are not compatible with the userspace. > In this case, if a IOCTL is designed > to setup media pipeline, no need to update applications, after driver > is updated. > > this will not only benefit for design a single IOCTL, this also helps > to hide the detail of media pipeline, by load a binary blob which holds > information about how to setup pipeline, or hide it in bootloader/ACPI > tables/device tree, etc. Media pipeline configuration is specific to the use case. If you hardcode it in the driver or bootloader, the user will not be able to use any other use case than the hardcoded blob, which is unacceptable for Linux drivers. Instead, it sounds like your userspace should be designed in a way that the media topology configuration is loaded from a configuration file that you could either get from your kernel driver developer or just maintain yourself based on any changes the media developers do. Of course that's unrelated to the backwards compatibility issue, which should not happen normally. The configuration file would be helpful for handling future extensions and new hardware platforms. > > another benefit is save time for setup media pipeline, if there is a > PKI like "time for open camera". as my test, this will saves hundreds > of milliseconds. For this problem, the proper solution would be to create an ioctl that can aggregate setting multiple parts of the topology in one go. For example, V4L2 has VIDIOC_S_CTRL for setting a control, but there is also VIDIOC_S_EXT_CTRLS, which lets you set multiple controls in one call. Something like VIDIOC_S_EXT_CTRLS for configuring the media topology would solve the performance problem. Best regards, Tomasz
