Re: [PATCH 11/14] PCI/P2PDMA: dma_map P2PDMA map requests that traverse the host bridge

[Logan Gunthorpe]

On 2019-07-25 12:58 p.m., Jason Gunthorpe wrote:
> On Mon, Jul 22, 2019 at 05:08:56PM -0600, Logan Gunthorpe wrote:
>> Any requests that traverse the host bridge will need to be mapped into
>> the IOMMU, so call dma_map_sg() inside pci_p2pdma_map_sg() when
>> appropriate.
>>
>> Similarly, call dma_unmap_sg() inside pci_p2pdma_unmap_sg().
>>
>> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
>>  drivers/pci/p2pdma.c | 31 ++++++++++++++++++++++++++++++-
>>  1 file changed, 30 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
>> index 5f43f92f9336..76f51678342c 100644
>> +++ b/drivers/pci/p2pdma.c
>> @@ -830,8 +830,22 @@ int pci_p2pdma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
>>  		int nents, enum dma_data_direction dir, unsigned long attrs)
>>  {
>>  	struct dev_pagemap *pgmap = sg_page(sg)->pgmap;
>> +	struct pci_dev *client;
>> +	int dist;
>> +
>> +	client = find_parent_pci_dev(dev);
>> +	if (WARN_ON_ONCE(!client))
>> +		return 0;
>>  
>> -	return __pci_p2pdma_map_sg(pgmap, dev, sg, nents);
>> +	dist = upstream_bridge_distance(pgmap->pci_p2pdma_provider,
>> +					client, NULL);
>> +	if (WARN_ON_ONCE(dist & P2PDMA_NOT_SUPPORTED))
>> +		return 0;
>> +
>> +	if (dist & P2PDMA_THRU_HOST_BRIDGE)
>> +		return dma_map_sg_attrs(dev, sg, nents, dir, attrs);
> 
> IIRC at this point the SG will have struct page references to the BAR
> memory - so (all?) the IOMMU drivers are able to handle P2P setup in
> this case?

Yes. The IOMMU drivers refer to the physical address for BAR which they
can get from the struct page. And this works fine today.

> Didn't you also have a patch to remove the struct page's for the BAR
> memory?  

Well that was an RFC that's not going anywhere in it's current form. I'd
still like to remove struct pages but that's going to take a while and
when that happens, this will have to be reworked. Probably to use
dma_map_resource() instead but will depend on the form the removal takes.

Logan