Re: [RFC PATCH 00/28] Removing struct page from P2PDMA

Logan Gunthorpe
2019-06-24T16:53:38Z

On 2019-06-24 7:55 a.m., Jason Gunthorpe wrote:
> On Mon, Jun 24, 2019 at 03:50:24PM +0200, Christoph Hellwig wrote:
>> On Mon, Jun 24, 2019 at 10:46:41AM -0300, Jason Gunthorpe wrote:
>>> BTW, it is not just offset right? It is possible that the IOMMU can
>>> generate unique dma_addr_t values for each device?? Simple offset is
>>> just something we saw in certain embedded cases, IIRC.
>>
>> Yes, it could. If we are trying to do P2P between two devices on
>> different root ports and with the IOMMU enabled we'll generate
>> a new bus address for the BAR on the other side dynamically everytime
>> we map.
>
> Even with the same root port if ACS is turned on could behave like this.

Yup.

> It is only a very narrow case where you can take shortcuts with
> dma_addr_t, and I don't think shortcuts like are are appropriate for
> the mainline kernel..

I don't think it's that narrow and it opens up a lot of avenues for
system design that people are wanting to go. If your high speed data
path can avoid the root complex and CPU, you can design a system which a
much smaller CPU and fewer lanes directed at the CPU.

Logan