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

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Message-ID: <20200511134502.hjbu5evkiuh75chr@mobilestation>
Date:   Mon, 11 May 2020 16:45:02 +0300
From:   Serge Semin <Sergey.Semin@...kalelectronics.ru>
To:     Mark Brown <broonie@...nel.org>
CC:     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@...r.kernel.org>, <dmaengine@...r.kernel.org>,
        <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 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?

-Sergey