linux-kernel - Re: [Nouveau] [PATCH v2 2/3] drm/ttm: introduce dma cache sync helpers

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Date:	Fri, 27 Jun 2014 08:17:59 +0900
From:	Alexandre Courbot <gnurou@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Stéphane Marchesin <stephane.marchesin@...il.com>,
	Lucas Stach <l.stach@...gutronix.de>,
	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	"nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	Ben Skeggs <bskeggs@...hat.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Nouveau] [PATCH v2 2/3] drm/ttm: introduce dma cache sync helpers

On Fri, Jun 27, 2014 at 1:10 AM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Thu, Jun 26, 2014 at 11:53:20PM +0900, Alexandre Courbot wrote:
>> We don't plan to rely on CMA for too long. IOMMU support is on the way
>> and should make our life easier, although no matter the source of
>> memory, we will still have the issue of the lowmem mappings.
>
> When it comes to DMA memory, talking about lowmem vs highmem is utterly
> meaningless.
>
> The lowmem/highmem split is entirely a software concept and is completely
> adjustable.  An extreme example is that you can boot any platform with
> more than 32MB of memory with 32MB of lowmem and the remainder as
> highmem.

True, but isn't it also the case that all lowmem is already mapped in
the kernel address space, and that re-mapping this memory with
different cache settings (e.g. by creating a WC mapping for user-space
to write into) is undefined on ARM and must be avoided? That is the
issue I was referring to.
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