linux-kernel - [PATCH 1/3] usb: dwc3: gadget: Fix TRB reclaim logic for short transfers and ZLPs

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Message-Id: <20250621-dwc3-fix-gadget-mtp-v1-1-a45e6def71bb@leica-geosystems.com>
Date: Sat, 21 Jun 2025 21:07:14 +0200
From: Johannes Schneider <johannes.schneider@...ca-geosystems.com>
To: Thinh Nguyen <Thinh.Nguyen@...opsys.com>, 
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Pengutronix Kernel Team <kernel@...gutronix.de>, 
 linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org, 
 bsp-development.geo@...ca-geosystems.com, 
 Johannes Schneider <johannes.schneider@...ca-geosystems.com>
Subject: [PATCH 1/3] usb: dwc3: gadget: Fix TRB reclaim logic for short
 transfers and ZLPs

Commit 61440628a4ff ("usb: dwc3: gadget: Cleanup SG handling") updated
the TRB reclaim path to use the TRB CHN (Chain) bit to determine whether
a TRB was part of a chain. However, this inadvertently changed the
behavior of reclaiming the final TRB in some scatter-gather or short
transfer cases.

In particular, if the final TRB did not have the CHN bit set, the
cleanup path could incorrectly skip clearing the HWO (Hardware Own)
bit, leaving stale TRBs in the ring. This resulted in broken data
transfer completions in userspace, notably for MTP over FunctionFS.

Fix this by unconditionally clearing the HWO bit during TRB reclaim,
regardless of the CHN bit state. This restores correct behavior
especially for transfers that require ZLPs or end on non-CHN TRBs.

Fixes: 61440628a4ff ("usb: dwc3: gadget: Cleanup SG handling")
Signed-off-by: Johannes Schneider <johannes.schneider@...ca-geosystems.com>
---
 drivers/usb/dwc3/gadget.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
index 321361288935db4b773cd06235a16670a6adda1a..99fbd29d8f46d30df558ceb23d2afe7187b4244c 100644
--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c
@@ -3516,7 +3516,7 @@ static int dwc3_gadget_ep_reclaim_completed_trb(struct dwc3_ep *dep,
 	 * We're going to do that here to avoid problems of HW trying
 	 * to use bogus TRBs for transfers.
 	 */
-	if (chain && (trb->ctrl & DWC3_TRB_CTRL_HWO))
+	if (trb->ctrl & DWC3_TRB_CTRL_HWO)
 		trb->ctrl &= ~DWC3_TRB_CTRL_HWO;

 	/*

-- 
2.34.1