linux-kernel - Re: [PATCH v2 4/6] dmaengine: dw: Print warning if multi-block is unsupported

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Date:   Mon, 11 May 2020 21:25:35 +0300
From:   Serge Semin <Sergey.Semin@...kalelectronics.ru>
To:     Mark Brown <broonie@...nel.org>
CC:     Serge Semin <fancer.lancer@...il.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Vinod Koul <vkoul@...nel.org>,
        Viresh Kumar <vireshk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Arnd Bergmann <arnd@...db.de>,
        Rob Herring <robh+dt@...nel.org>, <linux-mips@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        dmaengine <dmaengine@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/6] dmaengine: dw: Print warning if multi-block is
 unsupported

On Mon, May 11, 2020 at 06:48:00PM +0100, Mark Brown wrote:
> On Mon, May 11, 2020 at 04:58:53PM +0300, Andy Shevchenko wrote:
> > On Mon, May 11, 2020 at 4:48 PM Serge Semin
> 
> > > So the question is of how to export the multi-block LLP flag from DW DMAc
> > > driver. Andy?
> 
> > I'm not sure I understand why do you need this being exported. Just
> > always supply SG list out of single entry and define the length
> > according to the maximum segment size (it's done IIRC in SPI core).
> 
> If there's a limit from the dmaengine it'd be a bit cleaner to export
> the limit from the DMA engine (and it'd help with code reuse for clients
> that might work with other DMA controllers without needing to add custom
> compatibles for those instantiations).

Right. I've already posted a patch which exports the max segment size from the
DW DMA controller driver. The SPI core will get the limit in the spi_map_buf()
method by calling the dma_get_max_seg_size() function. The problem I
described concerns of how to determine whether to apply the solution Andy
suggested, since normally if DW DMA controller has true multi-block LLP
supported the workaround isn't required. So in order to solve the problem in a
generic way the easiest way would be to somehow get the noLLP flag from the DW
DMAC private data and select a one-by-one SG entries submission algorithm
instead of the normal one... On the other hand we could just implement a
flag-based quirks in the DW APB SSI driver and determine whether the LLP
problem exists for the platform-specific DW APB SSI controller.

-Sergey