Hi YP, Em Fri, 2 Jul 2021 18:16:49 +0800 YP WU <yp.wu@xxxxxxxxxxxx> escreveu: > Hello, dvb frontend maintainer: > We have an internal discussion about how to implement DVBS blind scan. > Currently, we can't see any structs and IOCTL commands related to DVBS blind scan in Linux-dvb framework. > So we want to confirm if Linux-dvb framework have inherent flow of DVBS blind scan or not. > If the answer is yes, could you tell us how to work in Linux-dvb. > If the answer is no, do you have any suggestions or design scenario for DVBS blind scan? Right now, the DVB core lacks support for doing blind scan, basically because no driver currently implements it. Right now, a couple of userspace programs implement blind scan, but at the hard way. I discussed that in the past with another developer that wanted to implement blind scans, but he ended working with something else. - There is an API on V4L2 that it is somewhat similar to blind scan. It is meant to auto-tune FM radio channels: https://linuxtv.org/downloads/v4l-dvb-apis-new/userspace-api/v4l/vidioc-s-hw-freq-seek.html The way it works there is that the tuning parameters are set at the ioctl, together with the scan direction. When this ioctl is called, the tuner starts seeking the next channel on a given frequency range. I suspect that a similar way could be implemented at DTV side, using dvbv5 API. This would work by adding a new DTV command that would start to blind scan a single channel. Something like: DTV_BLIND_SCAN_UP Subsequent calls would find the next channels. On other words, userspace would send a series of properties to setup the initial frequency and the digital TV standard, and, instead of using DTV_TUNE, the parameters will end with DTV_BLIND_SCAN_UP. So, a blind scan properties would be something like: DTV_FREQUENCY = 573000000 DTV_DELIVERY_SYSTEM = SYS_DVBC ... # other properties if needed DTV_BLIND_SCAN_UP ioctl(FE_SET_PROPERTY) The frontend will then seek for valid frequency >= DTV_FREQUENCY. While a new channel is found and the frequency is below to the highest FE frequency, it would keep doing: DTV_FREQUENCY = <previous freq> + <previous_channel_bandwidth) DTV_DELIVERY_SYSTEM = SYS_DVBC ... # other properties if needed DTV_BLIND_SCAN_UP ioctl(FE_SET_PROPERTY) - Now, the userspace application needs to know when the blind scan finds the next channel, or if it finished without finding anything. The normal way the DVB subsystem works is that FE_SET_PROPERTY is an async operation: it shouldn't block until a command is completed. So, making userspace to simply wait for it to complete is not a good idea. So, the best is to use the poll() ioctl, E.g.: Userspace (or userspace thread) can call poll() system call, which is meant to make userspace to sleep until an async event (read/write/error) happens. Right now, the core already implements poll() support: it returns EPOLLIN | EPOLLRDNORM | EPOLLPRI if the frontend status changes. As we have empty flags, all the DVB core need is to add some flags that are specific to the blind scan. So, let's say we add two new flags at fe_status: FE_SCAN_FOUND_CHANNEL FE_SCAN_DONE The userspace will then call poll() waiting for the Once userspace gets aware that a blind scan happened, as a call to poll() waiting for a POLLIN event. it will then read the FE status. If it returns FE_SCAN_DONE, the blind scan finished. Otherwise, it can call FE_GET_PROPERTY in order to get the tuned frequencies and the the modulation parameters for the detected channel. - So, in summary, I guess we just need to add a new DTV property, and two flags to fe_status to make such functionality supported by the DVB core. Thanks, Mauro
