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 18:44:14 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
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 04:45:02PM +0300, Serge Semin wrote:
> On Mon, May 11, 2020 at 12:58:13PM +0100, Mark Brown wrote:

> > 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.

There is a constraint enumeration interface in the DMA API which you
should be able to extend for this if it doesn't already support what you
need.

> 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.

You can set max_transfer_size and/or max_message_size in the SPI driver
- you should be able to do this on probe.

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