linux-kernel - [PATCH 4/4] usb: gadget: uvc: retry vb2_reqbufs() with vb_vmalloc_memops if use

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Message-Id: <20260108-uvc-gadget-fix-patch-v1-4-8b571e5033cc@nxp.com>
Date: Thu, 08 Jan 2026 15:43:05 +0800
From: Xu Yang <xu.yang_2@....com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
 Michael Grzeschik <m.grzeschik@...gutronix.de>
Cc: jun.li@....com, imx@...ts.linux.dev, linux-usb@...r.kernel.org, 
 linux-kernel@...r.kernel.org, Xu Yang <xu.yang_2@....com>
Subject: [PATCH 4/4] usb: gadget: uvc: retry vb2_reqbufs() with
 vb_vmalloc_memops if use_sg fail

Based on the reality[1][2] that vb2_dma_sg_alloc() can't alloc buffer with
device DMA limits, those device will always get below error: "swiotlb
buffer is full (sz: 393216 bytes), total 65536 (slots), used 2358 (slots)"
and the uvc gadget function can't work at all.

The videobuf2-dma-sg.c driver doesn't has a formal improve about this issue
till now. To workaround the issue, lets retry vb2_reqbufs() with
vb_vmalloc_memops if it fails to allocate buffer with vb2_dma_sg_memops.

Link[1]: https://lore.kernel.org/linux-media/20230828075420.2009568-1-anle.pan@nxp.com/
Link[2]: https://lore.kernel.org/linux-media/20230914145812.12851-1-hui.fang@nxp.com/
Signed-off-by: Xu Yang <xu.yang_2@....com>
---
 drivers/usb/gadget/function/uvc_queue.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/usb/gadget/function/uvc_queue.c b/drivers/usb/gadget/function/uvc_queue.c
index 21d80322cb6148ed87eb77f453a1f1644e4923ae..586e5524c171f115d98af5dda43fb800466f46d2 100644
--- a/drivers/usb/gadget/function/uvc_queue.c
+++ b/drivers/usb/gadget/function/uvc_queue.c
@@ -182,7 +182,15 @@ int uvcg_alloc_buffers(struct uvc_video_queue *queue,
 {
 	int ret;
 
+retry:
 	ret = vb2_reqbufs(&queue->queue, rb);
+	if (ret < 0 && queue->use_sg) {
+		uvc_trace(UVC_TRACE_IOCTL,
+			  "failed to alloc buffer with sg enabled, try non-sg mode\n");
+		queue->use_sg = 0;
+		queue->queue.mem_ops = &vb2_vmalloc_memops;
+		goto retry;
+	}
 
 	return ret ? ret : rb->count;
 }

-- 
2.34.1