Re: [PATCH] usb: gadget: uvc: limit isoc_sg to super speed gadgets

From: Michael Grzeschik <mgr@pengutronix.de>
To: linux-usb@vger.kernel.org
Cc: balbi@kernel.org, laurent.pinchart@ideasonboard.com,
	kernel@pengutronix.de, linux-media@vger.kernel.org
Subject: Re: [PATCH] usb: gadget: uvc: limit isoc_sg to super speed gadgets
Date: Tue, 11 Oct 2022 23:32:56 +0200	[thread overview]
Message-ID: <20221011213256.GI27626@pengutronix.de> (raw)
In-Reply-To: <20221011205707.1603017-1-m.grzeschik@pengutronix.de>

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On Tue, Oct 11, 2022 at 10:57:07PM +0200, Michael Grzeschik wrote:
>The overhead of preparing sg data is high for transfers with limited
>payload. When transferring isoc over high-speed usb the maximum payload
>is rather small which is a good argument no to use sg. This patch is
>changing the uvc_video_encode_isoc_sg encode function only to be used
>for super speed gadgets.
>
>Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
>---
> drivers/usb/gadget/function/uvc_video.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
>diff --git a/drivers/usb/gadget/function/uvc_video.c b/drivers/usb/gadget/function/uvc_video.c
>index bb037fcc90e69e..5081eb3bc5484c 100644
>--- a/drivers/usb/gadget/function/uvc_video.c
>+++ b/drivers/usb/gadget/function/uvc_video.c
>@@ -448,6 +448,9 @@ static void uvcg_video_pump(struct work_struct *work)
>  */
> int uvcg_video_enable(struct uvc_video *video, int enable)
> {
>+	struct uvc_device *uvc = video->uvc;
>+	struct usb_composite_dev *cdev = uvc->func.config->cdev;
>+	struct usb_gadget *gadget = cdev->gadget;
> 	unsigned int i;
> 	int ret;
>
>@@ -479,9 +482,11 @@ int uvcg_video_enable(struct uvc_video *video, int enable)
> 	if (video->max_payload_size) {
> 		video->encode = uvc_video_encode_bulk;
> 		video->payload_size = 0;
>-	} else
>-		video->encode = video->queue.use_sg ?
>+	} else {
>+		video->encode = (video->queue.use_sg &&
>+				 !(gadget->speed <= USB_SPEED_HIGH)) ?

I also came up with the following Idea:

-                                !(gadget->speed <= USB_SPEED_HIGH)) ?
+                                video->req_size > 4096) ?

Would this threshold of 4096 make sense? What should be preferred?

> 			uvc_video_encode_isoc_sg : uvc_video_encode_isoc;
>+	}
>
> 	video->req_int_count = 0;
>
>-- 
>2.30.2
>
>
>

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