lkml.org 
[lkml]   [2018]   [Mar]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH 2/8] PCI: Add pci_find_common_upstream_dev()


On 29/03/18 05:44 AM, Christian König wrote:
> Am 28.03.2018 um 21:53 schrieb Logan Gunthorpe:
>>
>> On 28/03/18 01:44 PM, Christian König wrote:
>>> Well, isn't that exactly what dma_map_resource() is good for? As far as
>>> I can see it makes sure IOMMU is aware of the access route and
>>> translates a CPU address into a PCI Bus address.
>>> I'm using that with the AMD IOMMU driver and at least there it works
>>> perfectly fine.
>> Yes, it would be nice, but no arch has implemented this yet. We are just
>> lucky in the x86 case because that arch is simple and doesn't need to do
>> anything for P2P (partially due to the Bus and CPU addresses being the
>> same). But in the general case, you can't rely on it.
>
> Well, that an arch hasn't implemented it doesn't mean that we don't have
> the right interface to do it.

Yes, but right now we don't have a performant way to check if we are
doing P2P or not in the dma_map_X() wrappers. And this is necessary to
check if the DMA ops in use support it or not. We can't have the
dma_map_X() functions do the wrong thing because they don't support it yet.

> Devices integrated in the CPU usually only "claim" to be PCIe devices.
> In reality their memory request path go directly through the integrated
> north bridge. The reason for this is simple better throughput/latency.

These are just more reasons why our patchset restricts to devices behind
a switch. And more mess for someone to deal with if they need to relax
that restriction.

Logan

\
 
 \ /
  Last update: 2018-03-29 17:46    [W:0.100 / U:0.212 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site