patch "usb: dwc3: gadget: Update chain bit correctly when using sg list" added to usb-linus

This is a note to let you know that I've just added the patch titled

usb: dwc3: gadget: Update chain bit correctly when using sg list

to my usb git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git
in the usb-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From dad2aff3e827b112f27fa5e6f2bf87a110067c3f Mon Sep 17 00:00:00 2001
From: Pratham Pratap <[email protected]>
Date: Mon, 2 Mar 2020 21:44:43 +0000
Subject: usb: dwc3: gadget: Update chain bit correctly when using sg list

If scatter-gather operation is allowed, a large USB request is split into multiple TRBs. For preparing TRBs for sg list, driver iterates over the list and creates TRB for each sg and mark the chain bit to false for the last sg. The current IOMMU driver is clubbing the list of sgs which shares a page boundary into one and giving it to USB driver. With this the number of sgs mapped it not equal to the the number of sgs passed. Because of this USB driver is not marking the chain bit to false since it couldn't iterate to the last sg. This patch addresses this issue by marking the chain bit to false if it is the last mapped sg.

At a practical level, this patch resolves USB transfer stalls seen with adb on dwc3 based db845c, pixel3 and other qcom hardware after functionfs gadget added scatter-gather support around v4.20.

Credit also to Anurag Kumar Vulisha <[email protected]> who implemented a very similar fix to this issue.

[jstultz: Slight tweak to remove sg_is_last() usage, reworked
commit message, minor comment tweak]

---
 drivers/usb/dwc3/gadget.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

index 1b7d2f9cb673..1e00bf2d65a2 100644
@@ -1071,7 +1071,14 @@ static void dwc3_prepare_one_trb_sg(struct dwc3_ep *dep,
 		unsigned int rem = length % maxp;
 		unsigned chain = true;
 
-		if (sg_is_last(s))
+		/*
+		 * IOMMU driver is coalescing the list of sgs which shares a
+		 * page boundary into one and giving it to USB driver. With
+		 * this the number of sgs mapped is not equal to the number of
+		 * sgs passed. So mark the chain bit to false if it isthe last
+		 * mapped sg.
+		 */
+		if (i == remaining - 1)
 			chain = false;
 
 		if (rem && usb_endpoint_dir_out(dep->endpoint.desc) && !chain) {

2.25.1