Hi Daniel, My apologies for the delayed replies; been out of town for the last couple of days.
On Fri, Apr 20, 2018 at 5:54 PM, Daniel Glöckner <daniel...@gmx.net> wrote: > for some reason I feel like buffer_queue in cx88-vbi.c should not be > calling cx8800_start_vbi_dma as it is also called a few lines further > down in start_streaming. > > Devin, can you check if it helps to remove that line and if VBI still > works afterwards? So I've commented out that line in buffer_queue, and so far haven't been able to reproduce the issue, and it does look like VBI is working as expected (captions are being rendered in VLC). This doesn't suggest I've done exhaustive testing by any means, but it's certainly a good sign. I've seen drivers in the past which start the main data pump when buffer_queue() or buffer_prepare() is called (whether it be to start a DMA engine in the case of PCI or start URB submission in the case of USB devices). However it's not clear that's required, in particular with VB2 which will automatically call start_streaming() in the case where read() is used. If I had to guess, I suspect the origin of starting DMA that early was probably oriented around users who wanted to simply run "cat /dev/video0 > out.mpeg" without having to explicitly issue a bunch of V4L ioctl() calls beforehand. It's worth noting that we're also doing it in the buffer_queue() routine for video and not just VBI. Presumably we would want to drop both cases. Hans, you did the VB2 conversion and have obviously been through this exercise with a number of other drivers. Any thoughts on whether we can drop the starting of DMA engine in buffer_queue()? On a related note, a quick review of the start/stop logic for DMA in that driver suggests the calls might not be properly balanced. Looks like portions of the core logic are also duplicated between stop_streaming() and stop_video_dma() (which is only ever used if CONFIG_PM is defined). It feels like it could probably use some review/cleanup, although I'm loathed to touch such a mature driver for fear of breaking something subtle. Thanks, Devin -- Devin J. Heitmueller - Kernel Labs http://www.kernellabs.com
