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SubjectRe: [PATCH v3 10/10] dmaengine: dw: Initialize max_sg_nents with nollp flag
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@baikalelectronics.ru>
> > Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
> > Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Rob Herring <robh+dt@kernel.org>
> > Cc: linux-mips@vger.kernel.org
> > Cc: devicetree@vger.kernel.org
> >
> > ---
> >
> > 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
>
>

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