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Date: Fri, 7 Mar 2014 12:18:48 +0800
From: Santosh Shilimkar <santosh.shilimkar@ti.com>
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To: Rob Herring <robherring2@gmail.com>
CC: Arnd Bergmann <arnd@arndb.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
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        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Russell King <linux@arm.linux.org.uk>, Olof Johansson <olof@lixom.net>,
        Grant Likely <grant.likely@linaro.org>,
        Rob Herring <robh+dt@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Linus Walleij <linus.walleij@linaro.org>
Subject: Re: [PATCH 3/7] of: introduce of_dma_is_coherent() helper
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On Friday 07 March 2014 11:55 AM, Rob Herring wrote:
> On Thu, Mar 6, 2014 at 9:44 PM, Santosh Shilimkar
> <santosh.shilimkar@ti.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@ti.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.
> 
Hmmm.. I guess you are right. In that case we can drop the need of
config option.

Regards,
Santosh