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

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Message-ID: <DB9PR04MB9284A0CDB1FC7CAADE0A394F87EFA@DB9PR04MB9284.eurprd04.prod.outlook.com>
Date:   Wed, 6 Sep 2023 08:16:29 +0000
From:   Hui Fang <hui.fang@....com>
To:     Tomasz Figa <tfiga@...omium.org>
CC:     Anle Pan <anle.pan@....com>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "mchehab@...nel.org" <mchehab@...nel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jindong Yue <jindong.yue@....com>,
        Xuegang Liu <xuegang.liu@....com>
Subject: RE: [EXT] Re: [PATCH] media: videobuf2-dma-sg: limit the sg segment
 size

On Wed, Sep 5, 2023 at 12:44 AM Tomasz Figa <tfiga@...omium.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