Hi Felipe,
-----Original Message----- From: Felipe Balbi balbif@gmail.com On Behalf Of Felipe Balbi Sent: Thursday, January 23, 2020 12:56 PM To: John Stultz john.stultz@linaro.org; lkml linux-kernel@vger.kernel.org Cc: Anurag Kumar Vulisha anuragku@xilinx.com; Felipe Balbi felipe.balbi@linux.intel.com; Yang Fei fei.yang@intel.com; Thinh Nguyen thinhn@synopsys.com; Tejas Joglekar tejas.joglekar@synopsys.com; Andrzej Pietrasiewicz andrzej.p@collabora.com; Jack Pham jackp@codeaurora.org; Todd Kjos tkjos@google.com; Greg KH gregkh@linuxfoundation.org; Linux USB List linux-usb@vger.kernel.org; stable stable@vger.kernel.org; John Stultz john.stultz@linaro.org Subject: Re: [RFC][PATCH 2/2] usb: dwc3: gadget: Correct the logic for finding last SG entry
Hi,
John Stultz john.stultz@linaro.org writes:
From: Anurag Kumar Vulisha anurag.kumar.vulisha@xilinx.com
As a process of preparing TRBs usb_gadget_map_request_by_dev() is called from dwc3_prepare_trbs() for mapping the request. This will call dma_map_sg() if req->num_sgs are greater than 0. dma_map_sg() will map the sg entries in sglist and return the number of mapped SGs. As a part of mapping, some sg entries having contigous memory may be merged together into a single sg (when IOMMU used). So, the number of mapped sg entries may not be equal to the number of orginal sg entries in the request (req->num_sgs).
As a part of preparing the TRBs, dwc3_prepare_one_trb_sg() iterates over the sg entries present in the sglist and calls sg_is_last() to identify whether the sg entry is last and set IOC bit for the last sg entry. The sg_is_last() determines last sg if SG_END is set in sg->page_link. When IOMMU used, dma_map_sg() merges 2 or more sgs into a single sg and it doesn't retain the page_link properties. Because of this reason the sg_is_last() may not find SG_END and thus resulting in IOC bit never getting set.
For example:
Consider a request having 8 sg entries with each entry having a length of 4096 bytes. Assume that sg1 & sg2, sg3 & sg4, sg5 & sg6, sg7 & sg8 are having contigous memory regions.
Before calling dma_map_sg(): sg1-->sg2-->sg3-->sg4-->sg6-->sg7-->sg8 dma_length: 4K 4K 4K 4K 4K 4K 4K SG_END: False False False False False False True num_sgs = 8 num_mapped_sgs = 0
The dma_map_sg() merges sg1 & sg2 memory regions into sg1->dma_address. Similarly sg3 & sg4 into sg2->dma_address, sg5 & sg6 into the sg3->dma_address and sg6 & sg8 into sg4->dma_address. Here the memory regions are merged but the page_link properties like SG_END are not retained into the merged sgs.
After calling dma_map_sg(); sg1-->sg2-->sg3-->sg4-->sg6-->sg7-->sg8 dma_length: 8K 8K 8K 8K 0K 0K 0K SG_END: False False False False False False True num_sgs = 8 num_mapped_sgs = 4
After calling dma_map_sg(), sg1,sg2,sg3,sg4 are having dma_length of 8096 bytes each and remaining sg4,sg5,sg6,sg7 are having 0 bytes of dma_length.
After dma_map_sg() is performed dma_perpare_trb_sg() iterates on all sg entries and sets IOC bit only for the sg8 (since sg_is_last() returns true only for sg8). But after calling dma_map_sg() the valid data are present only till sg4 and the IOC bit should be set for sg4 TRB only (which is not happening in the present code)
The above mentioned issue can be fixed by determining last sg based on the req->num_queued_sgs instead of sg_is_last(). If (req->num_queued_sgs + req->1) is equal to req->num_mapped_sgs, then this sg is the last sg. In the above example, the dwc3 driver has already queued 3 sgs (upto sg3), so the num_queued_sgs = 3. On preparing the next sg (i.e sg4), check for last sg (num_queued_sgs + 1) == num_mapped_sgs becomes true. So, the driver sets IOC bit for sg4. This patch does the same.
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.
Cc: Felipe Balbi felipe.balbi@linux.intel.com 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 Signed-off-by: Anurag Kumar Vulisha anurag.kumar.vulisha@xilinx.com [jstultz: Add note to end of commit message on specific issue this resovles] Signed-off-by: John Stultz john.stultz@linaro.org
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 1edce3bbb55c..30a80bc97cfe 100644 --- a/drivers/usb/dwc3/gadget.c +++ b/drivers/usb/dwc3/gadget.c @@ -1068,7 +1068,7 @@ static void dwc3_prepare_one_trb_sg(struct
dwc3_ep *dep,
unsigned int rem = length % maxp; unsigned chain = true;
if (sg_is_last(s))
if ((req->num_queued_sgs + 1) == req-request.num_mapped_sgs)
This is probably a bug on DMA API. If it combines pages from scatter-list, then it should also move the last SG so sg_is_last() continues to work.
I had asked author to discuss this with DMA API maintainers. Can you do that?
I was stuck with other tasks, so couldn't discuss with DMA maintainers on this. I am sorry for that.
Hi John, Thanks for bringing this patch back . Please let me know if I can help you with anything on this. If you want, I am ready to start working on this.
Thanks, Anurag Kumar Vulisha