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 <anurag.kumar.vulisha@xilinx.com>
who implemented a very similar fix to this issue.

Cc: Felipe Balbi <balbi@kernel.org>
Cc: Yang Fei <fei.yang@intel.com>
Cc: Thinh Nguyen <thinhn@synopsys.com>
Cc: Tejas Joglekar <tejas.joglekar@synopsys.com>
Cc: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Cc: Jack Pham <jackp@codeaurora.org>
Cc: Todd Kjos <tkjos@google.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Linux USB List <linux-usb@vger.kernel.org>
Cc: stable <stable@vger.kernel.org> #4.20+
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>
[jstultz: Slight tweak to remove sg_is_last() usage, reworked
          commit message, minor comment tweak]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Link: https://lore.kernel.org/r/20200302214443.55783-1-john.stultz@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/dwc3/gadget.c

@@ -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) {