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

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

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.

>  }
>  
>  int do_dma_probe(struct dw_dma_chip *chip)
> -- 
> 2.26.2
> 

-- 
With Best Regards,
Andy Shevchenko