Hi, John Stultz <john.stultz@xxxxxxxxxx> writes: > From: Anurag Kumar Vulisha <anurag.kumar.vulisha@xxxxxxxxxx> > > 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 + 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@xxxxxxxxxxxxxxx> > Cc: Yang Fei <fei.yang@xxxxxxxxx> > Cc: Thinh Nguyen <thinhn@xxxxxxxxxxxx> > Cc: Tejas Joglekar <tejas.joglekar@xxxxxxxxxxxx> > Cc: Andrzej Pietrasiewicz <andrzej.p@xxxxxxxxxxxxx> > Cc: Jack Pham <jackp@xxxxxxxxxxxxxx> > Cc: Todd Kjos <tkjos@xxxxxxxxxx> > Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> > Cc: Linux USB List <linux-usb@xxxxxxxxxxxxxxx> > Cc: stable <stable@xxxxxxxxxxxxxxx> > Signed-off-by: Anurag Kumar Vulisha <anurag.kumar.vulisha@xxxxxxxxxx> > [jstultz: Add note to end of commit message on specific issue this resovles] > Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx> > --- > 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? -- balbi
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