linux-kernel - Re: DMABOUNCE in pci-rcar

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Message-ID: <532B0B14.7040600@codethink.co.uk>
Date:	Thu, 20 Mar 2014 16:36:52 +0100
From:	Ben Dooks <ben.dooks@...ethink.co.uk>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Arnd Bergmann <arnd@...db.de>, linux-sh@...r.kernel.org,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Magnus Damm <damm@...nsource.se>,
	Ben Dooks <ben-linux@...ff.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Simon Horman <horms+renesas@...ge.net.au>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: DMABOUNCE in pci-rcar

On 20/03/14 16:26, Russell King - ARM Linux wrote:
> On Thu, Mar 20, 2014 at 04:04:36PM +0100, Ben Dooks wrote:
>> On 26/02/14 20:57, Arnd Bergmann wrote:
>>> On Wednesday 26 February 2014 12:48:17 Bjorn Helgaas wrote:
>>>> On Mon, Feb 24, 2014 at 4:00 AM, Arnd Bergmann <arnd@...db.de> wrote:
>>>>> Hi Magnus,
>>>>>
>>>>> I noticed during randconfig testing that you enabled DMABOUNCE for the
>>>>> pci-rcar-gen2 driver as posted in this patch https://lkml.org/lkml/2014/2/5/30
>>>>>
>>>>> I didn't see the original post unfortunately, but I fear we have to
>>>>> revert it and come up with a better solution, ...
>>>>
>>>> Sounds like I should drop the following patches from my pci/host-rcar
>>>> branch for now?
>>>>
>>>>     PCI: rcar: Add DMABOUNCE support
>>>>     PCI: rcar: Enable BOUNCE in case of HIGHMEM
>>>>     PCI: rcar: Make the Kconfig dependencies more generic
>>>
>>> Sounds good to me. The last patch is actually fine, but you'll have to
>>> fix the context to apply it without the other two.
>>
>> As a note, if we now boot a Lager with DT on 3.14-rc3 series the USB
>> controllers no longer work with full 2GiB RAM enabled in the device
>> tree.
>>
>> Could we work around this by having 1GiB of memory defined in the
>> 32bit memory and then add the rest of the 3GiB from the >32bit
>> area via LPAE? Will the kernel ever try to allocate DMA memory from
>> anything >32bit?
>
> Ignore highmem.  What is the actual hardware restriction concerning memory
> it can access?

If the window is set to 2GiB it can only access 2GiB aligned to 2GiB
boundary, however DRAM spans a 2GiB boundary (being at 1-3GiB)


-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius
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