Re: [RFC][PATCH 2/2] usb: dwc3: gadget: Correct the logic for finding last SG entry

[Felipe Balbi <balbi@kernel.org>]

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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 + 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?

-- 
balbi

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