GstBuffer from NvBuffer dmabuf

[Jakowlew]

Hi, I want to zero-copy pass dma buffer (NvBuffer) to GstBuffer for video streaming.
While scrolling this forum I found poorpong_nvmm sample application. After some tweaks it works on my side, but I’m struggling with gst_buffer_new_wrapped_full call in feed_function.

From sample it’s

buffer = gst_buffer_new_wrapped_full(flags,
                                             data,
                                             par.nv_buffer_size,
                                             0,
                                             par.nv_buffer_size,
                                             user_data,
                                             notify_to_destroy);

and then

gst_buffer_map (buffer, &map, GST_MAP_WRITE);
memcpy(map.data, par.nv_buffer, par.nv_buffer_size);
gst_buffer_unmap(buffer, &map);

What is nv_buffer_size? On my side it’s always equal to 1008 bytes. I assume, this is size of NvBuffer internal struct, or smth like that. Definitely not actual buffer size. Why does the above code works?

According to docs i rewrote the following call as

GstBuffer *buffer = gst_buffer_new_wrapped_full(
            (GstMemoryFlags)0,
            par.nv_buffer,
            par.psize[0],
            par.offset[0],
            par.psize[0],
            user_data,
            notify_to_destroy
        );

and skipped gst_buffer_map with memcpy calls.
Looks like everything is working as intended.

So, the question is: what is nv_buffer_size and why code from poorpong_nvmm actually works?

[DaneLLL]

Hi,
This sample should be similar to your use-case:
Opencv gpu mat into GStreamer without downloading to cpu - #15 by DaneLLL

Please take a look.

[Honey_Patouceul]

@DaneLLL you’re pointing to the same topic as @Jakowlew mentionned where I provided that weird sample.

@Jakowlew, this was just a one evening trial that I posted for forum users, but not sure that it can be a good template unless this works fine with your case.

The main point would be the memory flags, where I was using NVMM memory instead of system memory (that you’re using with (GstMemoryFlags)0 ).
It is usual AFAIK that buffer size in NVMM reports a lower size than actual buffer.

Probably @DaneLLL or someone else more skilled than me would be able to give more details about the reported size and actual operation

[DaneLLL]

Hi,

@Jakowlew

NvBuffer internal struct

You are correct. It is internal structure of NvBuffer.

Thank Honey Patouceul for pointing out the query.