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    SubjectRe: [PATCH v2 4/6] dmaengine: dw: Print warning if multi-block is unsupported
    +Cc: Mark (question about SPI + DMA workflow)

    On Fri, May 08, 2020 at 01:53:02PM +0300, Serge Semin wrote:
    > Multi-block support provides a way to map the kernel-specific SG-table so
    > the DW DMA device would handle it as a whole instead of handling the
    > SG-list items or so called LLP block items one by one. So if true LLP
    > list isn't supported by the DW DMA engine, then soft-LLP mode will be
    > utilized to load and execute each LLP-block one by one. A problem may
    > happen for multi-block DMA slave transfers, when the slave device buffers
    > (for example Tx and Rx FIFOs) depend on each other and have size smaller
    > than the block size. In this case writing data to the DMA slave Tx buffer
    > may cause the Rx buffer overflow if Rx DMA channel is paused to
    > reinitialize the DW DMA controller with a next Rx LLP item. In particular
    > We've discovered this problem in the framework of the DW APB SPI device

    Mark, do we have any adjustment knobs in SPI core to cope with this?

    > working in conjunction with DW DMA. Since there is no comprehensive way to
    > fix it right now lets at least print a warning for the first found
    > multi-blockless DW DMAC channel. This shall point a developer to the
    > possible cause of the problem if one would experience a sudden data loss.

    --
    With Best Regards,
    Andy Shevchenko


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