RE: [EXT] Re: [PATCH] media: videobuf2-dma-sg: limit the sg segment size

Hui Fang
2023-09-06T08:16:42Z

On Wed, Sep 5, 2023 at 12:44 AM Tomasz Figa <tfiga@chromium.org> wrote:
>
> I see. I guess the mapping is done by the USB gadget controller driver? Could
> you point us to the exact driver that's used?
>
> Just to clarify, swiotlb should only be needed in the very extreme fallback case,
> because of the performance impact of the memory copy back and forth. The
> right approach would depend on the DMA capabilities of your device, though.

[ 138.493943][ T2104] Call trace:
[ 138.497090][ T2104] vb2_dma_sg_alloc+0x2ec/0x2fc
[ 138.501808][ T2104] __vb2_queue_alloc+0x224/0x724
[ 138.506608][ T2104] vb2_core_reqbufs+0x374/0x528
[ 138.511320][ T2104] vb2_reqbufs+0xe0/0xf4
[ 138.515428][ T2104] uvcg_alloc_buffers+0x18/0x34
[ 138.520159][ T2104] uvc_v4l2_reqbufs+0x38/0x54
[ 138.524703][ T2104] v4l_reqbufs+0x68/0x80
[ 138.528820][ T2104] __video_do_ioctl+0x370/0x4dc
[ 138.533535][ T2104] video_usercopy+0x43c/0xb38
[ 138.538076][ T2104] video_ioctl2+0x18/0x28
[ 138.542272][ T2104] v4l2_ioctl+0x6c/0x84
[ 138.546291][ T2104] __arm64_sys_ioctl+0xa8/0xe4
[ 138.550928][ T2104] invoke_syscall+0x58/0x114
[ 138.555389][ T2104] el0_svc_common+0x88/0xfc
[ 138.559755][ T2104] do_el0_svc+0x2c/0xb8
[ 138.563776][ T2104] el0_svc+0x2c/0xa4
[ 138.567544][ T2104] el0t_64_sync_handler+0x68/0xb4
[ 138.572434][ T2104] el0t_64_sync+0x1a4/0x1a8
[ 138.576803][ T2104] Code: 17ffffcb 928002b3 d4210000 17ffffc8 (d4210000)
[ 138.583598][ T2104] ---[ end trace 0000000000000000 ]---

Also, below should explain why vb2_dma_sg_alloc is used.
We tested on 8mp with use dwc3 controller.

In drivers/usb/dwc3/gadget.c:
dwc->gadget->sg_supported = true;

In drivers/usb/gadget/function/uvc_queue.c
if (cdev->gadget->sg_supported) {
queue->queue.mem_ops = &vb2_dma_sg_memops;
queue->use_sg = 1;
} else {
queue->queue.mem_ops = &vb2_vmalloc_memops;
}

BRs,
Fang Hui