glTexDirectVIV Pointer Becomes Invalid

rikfelton — Wed, 31 Oct 2018 11:50:57 GMT

Hi,

I've written the software for a digital signage product that we're developing which uses the iMX6. The screen content is configurable, and is rendered using OpenGL ES (to an SDL generated window - not using X11). It's possible to set up the screen layout such that is has video on a part of the screen, and I'm handling the video using a combination of ffmpeg's libav and a custom decoder that utilises the iMX6 VPU.

To render the video to the OpenGL ES scene, I initially create a ring buffer of 3 textures and then call glTexDirectVIV for each texture and store the pointer that's returned. Then as the video plays, I have a thread that copies the decoded video frames to the textures using the pointers that I initially received via glTexDirectVIV (I'm using NEON instructions to copy the frames as quickly as possible).

The result is quite impressive, given that it's not a zero copy solution. I'm getting about 60 fps for fairly complex OpenGL ES scenes with a video playing in the middle of it.

I'm experiencing a problem though, where after a period of time (seems fairly random - anywhere from a few seconds to a few hours) the video starts to flicker with black frames. It initially looks like every third frame is black, and then after a while every 2 out of 3 frames, and then it turns completely black - so I'm immediately thinking it's a problem with my ring buffer of 3 textures. I've been through the code trying to diagnose the issue, and the problem doesn't occur if I don't use glTexDirectVIV and just copy the frames using glTexSubImage2D, but that causes the frame rate to drop.

I now have it where I can connect a USB keyboard to the device, and when I hit a certain key it re-calls glTexDirectVIV to refresh the pointers that I have stored for the 3 textures. That's all that happens when I hit the keyboard key. Now when the video starts to flicker, when I hit this key then the flickering immediately stops.

So I've concluded that the GPU must be moving the textures in the GPU memory, thus invalidating the pointers that I have stored by my initial glTexDirectVIV call.

I originally referred to this when I wrote my code:

Computer Vision on i.MX Processors: Video to Texture Streaming (Part 3) - i.MX6 processor

and with regard to glTexDirectVIV, it states "This function should be called just once in your code! (this was my mistake for a long time.....)".

My question is - should I in fact be calling glTexDirectVIV on every video frame before I copy the data, rather than assume the pointers will stay valid? That's what I'm starting to consider, but I'm a little concerned that it might cause a different problem that I'm not aware of, given the statement on that blogspot page.

If I am supposed to only call glTexDirectVIV once initially, how can I get around this problem that I'm seeing where the GPU seems to move the textures and therefore invalidates my stored pointers? Is there a way I can detect that this has happened, or prevent it happening in the first place?

Thanks

Re: glTexDirectVIV Pointer Becomes Invalid

Bio_TICFSL — Thu, 08 Nov 2018 15:20:08 GMT

Hi Rik,

Which GPU version are you using? which BSP? you are not using X11, then I suppose you are using 4.9.88 version, where supposedly fixed all glTexDirectVIV issues.

Re: glTexDirectVIV Pointer Becomes Invalid

rikfelton — Thu, 08 Nov 2018 15:34:14 GMT

Hi,

Thanks for the reply.

I'm using a SOM from one of our suppliers (the RM3 from Blue Chip Technology, RM3 ARM Dual/Quad-Core Cortex A9 Module | Blue Chip Technology). It has the i.MX6 Quad, which I believe has the Vivante GC2000 GPU?

I'm using buildroot to create the system, and the kernel is 4.1.15 which comes from GitHub - bluechiptechnology/linux-4.1.15-bctre3: kernel for RE3 boards based on Freescale Linux 4.1.15

Are you suggesting that if I contact Blue Chip and we use kernel version 4.9.88 or later, then that would solve my problem?

Many thanks,

Rik

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