Tomasz, Thank you for your answers. learned a lot. BR. Ning. 在 2018-11-22四的 16:06 +0900，Tomasz Figa写道： > Hi Ning, > > On Thu, Nov 22, 2018 at 11:52 AM Zhang, Ning A <ning.a.zhang@xxxxxxxx > m> 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. I met this issue :( > > > 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. yes, if there are multiple user cases, then this is not optional. > > > > > 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. this may be alternative choice. > > > Best regards, > Tomasz
