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

[Valentine <valentine.barshak@cogentembedded.com>]

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.

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

Thanks,
Val.