linux-kernel - Re: [PATCH 2/3] dt-bindings: mtd: atmel-quadspi: add an optional property 'dmacap,memcpy'

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Message-ID: <20180103115116.GG5603@sirena.org.uk>
Date:   Wed, 3 Jan 2018 11:51:16 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>
Cc:     Rob Herring <robh@...nel.org>,
        Ludovic Desroches <ludovic.desroches@...rochip.com>,
        computersforpeace@...il.com, dwmw2@...radead.org, richard@....at,
        boris.brezillon@...e-electrons.com, marek.vasut@...il.com,
        linux-mtd@...ts.infradead.org, vigneshr@...com,
        linux@...linux.org.uk, linux-kernel@...r.kernel.org,
        linux-spi@...r.kernel.org, devicetree@...r.kernel.org,
        nicolas.ferre@...rochip.com, radu.pirea@...rochip.com
Subject: Re: [PATCH 2/3] dt-bindings: mtd: atmel-quadspi: add an optional
 property 'dmacap,memcpy'

On Wed, Dec 27, 2017 at 10:40:00PM +0100, Cyrille Pitchen wrote:
> Le 27/12/2017 à 00:23, Rob Herring a écrit :
> > On Sun, Dec 24, 2017 at 05:36:05AM +0100, Cyrille Pitchen wrote:

> >> +Optional properties:
> >> +- dmacap,memcpy:  Reserve a DMA channel to perform DMA memcpy() between the
> >> +                  system memory and the QSPI mapped memory.

> > How is this a h/w property? Why would I not want to always enable DMA if 
> > possible?

> The number of DMA channels is limited for a given SoC. This number may be
> lower than the number of enabled controllers (spi, i2c, qspi, aes, sha,
> des, sdmmc, usart, ...).

> So we use a DT property to explicitly tell the matching drivers to request
> and reserved the DMA channels they need. This policy is not driver or even
> SoC specific but board specific. It's very common to reserved DMA channels
> for the most used or most performance dependent controllers, setting the
> relevant properties in the device-tree then restricting remaining
> controllers to their PIO mode.

Why can't we just time share the DMA channels at runtime, why do we need
this static allocation?  

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