Re: [PATCH RFC 0/2] ARM: shmobile: lager: Enable DMA bounce for PCI

[Ben Dooks <ben.dooks@codethink.co.uk>]

On 29/01/14 11:15, Valentine wrote:
> On 01/29/2014 12:03 PM, Ben Dooks wrote:
>> On 29/01/14 06:45, Simon Horman wrote:
>>> On Tue, Jan 28, 2014 at 10:53:31PM +0400, Valentine Barshak wrote:
>>>> This enables DMA bounce for PCI since the controller does
>>>> not support more than 2G PCI-AHB memory window.
>>>> The problems with DMA transfers can be observed when
>>>> setting 2G/2G user/kernel memory split model
>>>> (CONFIG_VMSPLIT_2G=y)
>>>> These patches help to avoid it.
>>>
>>> Are these patches compatible with other user/kernel splits?
>>
>> PS, the bridge is only actually capable of seeing 1GiB of
>> RAM due to alignment issues in the window. You can have either
>> 0x4..0x8 or 0x8..0xc but not /both/. If you open the window to
>> 2GiB then you can see either 0x0..0x8 or 0x8..0xF range.
>>
>
> I don't think this is relater to the user/kernel space memory split.
>
> Currently the R-Car Gen2 PCI driver uses 0x40000000 - 0x7fffffff
> PCI-AHB region. We can set it to 0x00000000 - 0x7fffffff,
> but there's no RAM below 0x40000000 so no DMA access to that area is
> actually legal from the PCI USB host driver.
> So the change wouldn't give us much.

Well, there's IO areas but probably not wanting to DMA to them anyway.

> The 31-bit DMA mask takes care of forbidding any DMA transfers
> to the area above 0x7fffffff.

I will add this to our current patch series and see if it helps.

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