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

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Message-ID: <20200521014705.ha7mpxoio4gitjox@mobilestation>
Date:   Thu, 21 May 2020 04:47:05 +0300
From:   Serge Semin <Sergey.Semin@...kalelectronics.ru>
To:     Vinod Koul <vkoul@...nel.org>
CC:     Serge Semin <fancer.lancer@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        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 5/6] dmaengine: dw: Introduce max burst length hw
 config

On Tue, May 19, 2020 at 10:37:14PM +0530, Vinod Koul wrote:
> On 17-05-20, 22:38, Serge Semin wrote:
> > On Fri, May 15, 2020 at 12:09:50PM +0530, Vinod Koul wrote:
> > > On 12-05-20, 22:12, Andy Shevchenko wrote:
> > > > On Tue, May 12, 2020 at 05:08:20PM +0300, Serge Semin wrote:
> > > > > On Fri, May 08, 2020 at 02:41:53PM +0300, Andy Shevchenko wrote:
> > > > > > On Fri, May 08, 2020 at 01:53:03PM +0300, Serge Semin wrote:

[nip]

> > > > > > 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.
> > > > > 
> > > > > There is no need to define minimum if such limit doesn't exists except a
> > > > > natural 1. Moreover it doesn't exist for all DMA controllers seeing noone has
> > > > > added such capability into the generic DMA subsystem so far.
> > > > 
> > > > There is a contract between provider and consumer about DMA resource. That's
> > > > why both sides should participate in fulfilling it. Theoretically it may be a
> > > > hardware that doesn't support minimum burst available in DMA by a reason. For
> > > > such we would need minimum to be provided as well.
> > > 
> > > Agreed and if required caps should be extended to tell consumer the
> > > minimum values supported.
> > 
> > Sorry, it's not required by our hardware. Is there any, which actually has such
> > limitation? (minimum burst length)
> 
> IIUC the idea is that you will tell maximum and minimum values supported
> and client can pick the best value. Esp in case of slave transfers
> things like burst, msize are governed by client capability and usage. So
> exposing the set to pick from would make sense

Agreed. I'll add min_burst capability.

-Sergey

> 
> -- 
> ~Vinod