On Wed, May 27, 2020 at 01:50:21AM +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. 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. > In principal I agree, one nit below. If you are okay with it, feel free to add my Rb tag. > 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; To be on the safer side I would explicitly do it like if (dwc->nollp) /* your nice comment */ = 1; else /* Unlimited */ = 0; type or content of nollp theoretically can be changed and this will affect maximum segments. > } > > int do_dma_probe(struct dw_dma_chip *chip) > -- > 2.26.2 > -- With Best Regards, Andy Shevchenko
