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

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Message-ID: <CAHp75VdOi1rwaKjzowhj0KA-eNNL4NxpiCeqfELFgO_RcnZ-xw@mail.gmail.com>
Date:   Mon, 11 May 2020 16:58:53 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Mark Brown <broonie@...nel.org>,
        Serge Semin <fancer.lancer@...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 4:48 PM Serge Semin
<Sergey.Semin@...kalelectronics.ru> wrote:
>
> On Mon, May 11, 2020 at 12:58:13PM +0100, Mark Brown wrote:
> > On Mon, May 11, 2020 at 05:10:16AM +0300, Serge Semin wrote:
> >
> > > Alas linearizing the SPI messages won't help in this case because the DW DMA
> > > driver will split it into the max transaction chunks anyway.
> >
> > That sounds like you need to also impose a limit on the maximum message
> > size as well then, with that you should be able to handle messages up
> > to whatever that limit is.  There's code for that bit already, so long
> > as the limit is not too low it should be fine for most devices and
> > client drivers can see the limit so they can be updated to work with it
> > if needed.
>
> Hmm, this might work. The problem will be with imposing such limitation through
> the DW APB SSI driver. In order to do this I need to know:
> 1) Whether multi-block LLP is supported by the DW DMA controller.
> 2) Maximum DW DMA transfer block size.
> Then I'll be able to use this information in the can_dma() callback to enable
> the DMA xfers only for the safe transfers. Did you mean something like this when
> you said "There's code for that bit already" ? If you meant the max_dma_len
> parameter, then setting it won't work, because it just limits the SG items size
> not the total length of a single transfer.
>
> 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).

-- 
With Best Regards,
Andy Shevchenko