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

[Jack Pham <jackp@codeaurora.org>]

Hi Anurag,

On Fri, Jun 07, 2019 at 09:49:59AM +0300, Felipe Balbi wrote:
> Anurag Kumar Vulisha <anuragku@xilinx.com> writes:
> >>> 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.
> >>
> >>isn't this a bug in the scatterlist mapping code? Why doesn't it keep
> >>SG_END?
> >>
> >
> > Thanks for providing your comment.
> >
> > I don't think it is a bug, instead I feel some enhancement needs to be done in
> > dma-mapping code.
> >
> > SG_END represents the last sg entry in the sglist and it is correctly getting
> > set to the last sg entry.
> >
> > The issue happens only when 2 or more sg entry pages are merged into
> > contiguous dma-able address and sg_is_last() is used to find the last sg entry
> > with valid dma address.
> 
> Right, and that's something that's bound to happen. I'm arguing that, perhaps,
> dma API should move SG_END in case entries are merged.
> 
> > I think that along with sg_is_last() a new flag (SG_DMA_END) and function
> > (something like sg_dma_is_last() ) needs to be added into dma-mapping code for
> > identifying the last valid sg entry with valid dma address. So that we can
> > make use of that function instead of sg_is_last().
> 
> Sure, propose a patch to DMA API.

I'm curious if this was ever resolved. I just ran into this exact issue
with Android ADB which uses 16KB buffers, along with f_fs supporting
S/G since 5.0, combined with our IOMMU which performs this merging
behavior, so it resulted in a single TRB getting queued with CHN=1 and
LST=0 and thus the transfer never completes. Your initial patch resolves
the issue for me, but upon revisiting this discussion I couldn't tell if
you had attempted to patch DMA API instead as per Felipe's suggestion.

Thanks,
Jack
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