linux-kernel - Re: [PATCH 3/7] of: introduce of_dma_is

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Date:	Thu, 6 Mar 2014 21:55:56 -0600
From:	Rob Herring <robherring2@...il.com>
To:	Santosh Shilimkar <santosh.shilimkar@...com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Russell King <linux@....linux.org.uk>,
	Olof Johansson <olof@...om.net>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH 3/7] of: introduce of_dma_is_coherent() helper

On Thu, Mar 6, 2014 at 9:44 PM, Santosh Shilimkar
<santosh.shilimkar@...com> wrote:
> On Friday 07 March 2014 11:13 AM, Rob Herring wrote:
>> On Thu, Mar 6, 2014 at 3:19 AM, Santosh Shilimkar
>> <santosh.shilimkar@...com> wrote:
>>> The of_dma_is_coherent() helper parses the given DT device
>>> node to see if the "dma-coherent" property is supported and
>>> returns true or false accordingly.
>>>
>>> For the architectures which are fully dma coherent and don't need per device
>>> property, it can enable CONFIG_ARCH_IS_DMA_COHERENT config option which
>>> enables DMA coherent for all devices by default.
>>
>> This worries me. I killed off arch_is_coherent() for arm. Now we're
>> adding something back. Also, we already have HAVE_GENERIC_DMA_COHERENT
>> which is different, but the names will be confusing. MIPS also has
>> DMA_NONCOHERENT.
>>
> Thanks for comments Rob. I will address them in next version.
> Specifically about ARCH_IS_DMA_COHERENT, I wasn't very comfortable either
> while adding it. But as Arnd mentioned, there is a need to have a way
> for the arch's which are fully coherent to use coherent ops by default.
>
> Am not sure whats the best way to have such support without imposing
> any special updates on such arches.

Thinking about this some more, if the arch is always coherent or
always non-coherent, then the default ops are always fine. In that
case set_arch_dma_coherent_ops is always a nop and of_dma_is_coherent
is a don't care.

Rob
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