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

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Date:   Fri, 8 May 2020 14:26:04 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        Mark Brown <broonie@...nel.org>
Cc:     Vinod Koul <vkoul@...nel.org>, Viresh Kumar <vireshk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Serge Semin <fancer.lancer@...il.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

+Cc: Mark (question about SPI + DMA workflow)

On Fri, May 08, 2020 at 01:53:02PM +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. A problem may
> happen for multi-block DMA slave transfers, when the slave device buffers
> (for example Tx and Rx FIFOs) depend on each other and have size smaller
> than the block size. In this case writing data to the DMA slave Tx buffer
> may cause the Rx buffer overflow if Rx DMA channel is paused to
> reinitialize the DW DMA controller with a next Rx LLP item. In particular
> We've discovered this problem in the framework of the DW APB SPI device

Mark, do we have any adjustment knobs in SPI core to cope with this?

> working in conjunction with DW DMA. Since there is no comprehensive way to
> fix it right now lets at least print a warning for the first found
> multi-blockless DW DMAC channel. This shall point a developer to the
> possible cause of the problem if one would experience a sudden data loss.

-- 
With Best Regards,
Andy Shevchenko