Re: [PATCH v3 10/10] dmaengine: dw: Initialize max_sg_nents with nollp flag

[Serge Semin]

On Thu, May 28, 2020 at 05:56:30PM +0300, Andy Shevchenko wrote:
> 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.

Agree. Though I don't like formatting you suggested. If I add my nice comment
between if-statement and assignment the the former will be look detached from
the if-statement, which seems a bit ugly. So I'd leave the comment above the
whole if-else statement, especially seeing I've already mentioned there about
the unlimited number of SG entries there.

	/*
	 * 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.
	 */
 	if (dwc->nollp)
 		caps->max_sg_nents = 1;
 	else
 		caps->max_sg_nents = 0;

-Sergey

> 
> >  }
> >  
> >  int do_dma_probe(struct dw_dma_chip *chip)
> > -- 
> > 2.26.2
> > 
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>