linux-kernel - Re: [PATCH v2 5/6] dmaengine: dw: Introduce max burst length hw config

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Date:   Fri, 8 May 2020 14:41:53 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
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 5/6] dmaengine: dw: Introduce max burst length hw
 config

On Fri, May 08, 2020 at 01:53:03PM +0300, Serge Semin wrote:
> IP core of the DW DMA controller may be synthesized with different
> max burst length of the transfers per each channel. According to Synopsis
> having the fixed maximum burst transactions length may provide some
> performance gain. At the same time setting up the source and destination
> multi size exceeding the max burst length limitation may cause a serious
> problems. In our case the system just hangs up. In order to fix this
> lets introduce the max burst length platform config of the DW DMA
> controller device and don't let the DMA channels configuration code
> exceed the burst length hardware limitation. Depending on the IP core
> configuration the maximum value can vary from channel to channel.
> It can be detected either in runtime from the DWC parameter registers
> or from the dedicated dts property.

I'm wondering what can be the scenario when your peripheral will ask something
which is not supported by DMA controller?

Peripheral needs to supply a lot of configuration parameters specific to the
DMA controller in use (that's why we have struct dw_dma_slave).
So, seems to me the feasible approach is supply correct data in the first place.

If you have specific channels to acquire then you probably need to provide a
custom xlate / filter functions. Because above seems a bit hackish workaround
of dynamic channel allocation mechanism.

But let's see what we can do better. Since maximum is defined on the slave side
device, it probably needs to define minimum as well, otherwise it's possible
that some hardware can't cope underrun bursts.

Vinod, what do you think?

-- 
With Best Regards,
Andy Shevchenko