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

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Message-ID: <20171226232330.trljk2ofyvu4ly3n@rob-hp-laptop>
Date:   Tue, 26 Dec 2017 17:23:30 -0600
From:   Rob Herring <robh@...nel.org>
To:     Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>
Cc:     computersforpeace@...il.com, dwmw2@...radead.org, richard@....at,
        boris.brezillon@...e-electrons.com, marek.vasut@...il.com,
        linux-mtd@...ts.infradead.org, broonie@...nel.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 Sun, Dec 24, 2017 at 05:36:05AM +0100, Cyrille Pitchen wrote:
> The optional 'dmacap,memcpy' DT property tells the Atmel QSPI controller
> driver to reserve some DMA channel then to use it to perform DMA
> memcpy() during data transfers. This feature relies on the generic
> bounce buffer helper from spi-nor.c.
> 
> Signed-off-by: Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>
> ---
>  Documentation/devicetree/bindings/mtd/atmel-quadspi.txt | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/mtd/atmel-quadspi.txt b/Documentation/devicetree/bindings/mtd/atmel-quadspi.txt
> index b93c1e2f25dd..002d3f0a445b 100644
> --- a/Documentation/devicetree/bindings/mtd/atmel-quadspi.txt
> +++ b/Documentation/devicetree/bindings/mtd/atmel-quadspi.txt
> @@ -12,6 +12,10 @@ Required properties:
>  - #address-cells: Should be <1>.
>  - #size-cells:    Should be <0>.
>  
> +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?

Furthermore, you are reusing a property, but giving it a different 
meaning. The current definition is an indication whether a DMA 
controller supports memcpy operations or not. It is not a flag for 
clients to use memcpy channels.

Why don't you use "dmas" property to point to the DMA controller.

> +
>  Example:
>  
>  spi@...20000 {
> @@ -24,6 +28,7 @@ spi@...20000 {
>  	#size-cells = <0>;
>  	pinctrl-names = "default";
>  	pinctrl-0 = <&pinctrl_spi0_default>;
> +	dmacap,memcpy;
>  
>  	m25p80@0 {
>  		...
> -- 
> 2.11.0
> 
> --
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