Re: [PATCH v2 4/7] dma: of: introduce of_dma_is_coherent() helper

From: Rob Herring <robherring2-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
To: Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>
Cc: Santosh Shilimkar
	<santosh.shilimkar-l0cyMroinI0@public.gmane.org>,
	"linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org"
	<linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org>,
	"devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Magnus Damm <magnus.damm-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Linus Walleij
	<linus.walleij-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>,
	Grant Likely
	<grant.likely-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>,
	Rob Herring <robh+dt-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	Russell King <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>,
	Olof Johansson <olof-nZhT3qVonbNeoWH0uzbU5w@public.gmane.org>
Subject: Re: [PATCH v2 4/7] dma: of: introduce of_dma_is_coherent() helper
Date: Fri, 28 Feb 2014 09:14:19 -0600	[thread overview]
Message-ID: <CAL_JsqL6-hoD4F7+adPhsOJXeM5f+wpEuBWg_6AWqj3k2v0NEw@mail.gmail.com> (raw)
In-Reply-To: <10040083.PbPEQQPCjl@wuerfel>

On Fri, Feb 28, 2014 at 3:39 AM, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
> On Thursday 27 February 2014 16:17:49 Santosh Shilimkar wrote:
>> +
>> +/**
>> + * of_dma_is_coherent - Check if device is coherent
>> + * @np:        device node
>> + *
>> + * It returns true if "dma-coherent" property was found
>> + * for this device in DT.
>> + */
>> +bool of_dma_is_coherent(struct device_node *np)
>> +{
>> +       struct device_node *node = np;
>> +
>> +       while (node) {
>> +               if (of_property_read_bool(node, "dma-coherent")) {
>> +                       of_node_put(node);
>> +                       return true;
>> +               }
>> +               node = of_get_next_parent(node);
>> +       }
>> +       return false;
>> +}
>> +EXPORT_SYMBOL_GPL(of_dma_is_coherent);
>>
>
> This won't work on architectures that are always coherent and
> did not need 'dma-coherent' properties before, such as IBM
> Power servers.
>
> That said, I think the property makes sense, and we already have
> platforms using it (highbank is the one I'm aware of).
>
> We probably need ways to override this function in both ways:
> "always coherent" (powerpc, x86), and "never coherent" (keystone
> without LPAE) from platform code, and it would be nice to put
> either option into DT in a global location as well. We may have
> to go through a few iterations of this patch to get the best
> algorithm, but I think the interface is good at least.

I know Will D was not a fan of this property. Primarily I believe
because you may need to describe more than just a boolean in more
complex bus topologies.

Effectively, highbank is always coherent. It was only PCI that is
non-coherent, but I can safely say PCI will never be enabled at this
point. There are no designs with PCI beyond 1 or 2 validation boards
(total boards, not designs), and getting PCI to work was quite hacky
due to only a 1MB window. The other masters are programmable, but only
the coherent path is used as the non-coherent path actually has some
issues. I had expected the opposite believing the ACP port would
actually have issues which is also why I made it configurable.

Rob
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