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. The soft-LLP mode of the DMA transactions execution might not work well for some DMA consumers like SPI due to its Tx and Rx buffers inter-dependency. Let's expose the nollp flag indicating the soft-LLP mode by means of the max_sg_nents capability, so the DMA consumer would be ready to somehow workaround errors caused by such mode being utilized. Signed-off-by: Serge Semin <Sergey.Semin@xxxxxxxxxxxxxxxxxxxx> Cc: Alexey Malahov <Alexey.Malahov@xxxxxxxxxxxxxxxxxxxx> Cc: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx> Cc: Arnd Bergmann <arnd@xxxxxxxx> Cc: Rob Herring <robh+dt@xxxxxxxxxx> Cc: linux-mips@xxxxxxxxxxxxxxx Cc: devicetree@xxxxxxxxxxxxxxx --- Changelog v3: - This is a new patch created as a result of the discussion with Vinud and Andy in the framework of DW DMA burst and LLP capabilities. --- drivers/dma/dw/core.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/drivers/dma/dw/core.c b/drivers/dma/dw/core.c index 29c4ef08311d..b850eb7fd084 100644 --- a/drivers/dma/dw/core.c +++ b/drivers/dma/dw/core.c @@ -1054,6 +1054,15 @@ static void dwc_caps(struct dma_chan *chan, struct dma_slave_caps *caps) struct dw_dma_chan *dwc = to_dw_dma_chan(chan); caps->max_burst = dwc->max_burst; + + /* + * It might be crucial for some devices to have the hardware + * accelerated multi-block transfers supported, aka LLPs in DW DMAC + * notation. So if LLPs are supported then max_sg_nents is set to + * zero which means unlimited number of SG entries can be handled in a + * single DMA transaction, otherwise it's just one SG entry. + */ + caps->max_sg_nents = dwc->nollp; } int do_dma_probe(struct dw_dma_chip *chip) -- 2.26.2
