Re: [PATCH 08/16] PCI/P2PDMA: Introduce helpers for dma_map_sg implementations

[Logan Gunthorpe]

On 2021-05-02 6:50 p.m., John Hubbard wrote:
> On 4/8/21 10:01 AM, Logan Gunthorpe wrote:
>> Add pci_p2pdma_map_segment() as a helper for simple dma_map_sg()
>> implementations. It takes an scatterlist segment that must point to a
>> pci_p2pdma struct page and will map it if the mapping requires a bus
>> address.
>>
>> The return value indicates whether the mapping required a bus address
>> or whether the caller still needs to map the segment normally. If the
>> segment should not be mapped, -EREMOTEIO is returned.
>>
>> This helper uses a state structure to track the changes to the
>> pgmap across calls and avoid needing to lookup into the xarray for
>> every page.
>>
> 
> OK, coming back to this patch, after seeing how it is used later in
> the series...
> 
>> Also add pci_p2pdma_map_bus_segment() which is useful for IOMMU
>> dma_map_sg() implementations where the sg segment containing the page
>> differs from the sg segment containing the DMA address.
>>
>> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
>> ---
>>   drivers/pci/p2pdma.c       | 65 ++++++++++++++++++++++++++++++++++++++
>>   include/linux/pci-p2pdma.h | 21 ++++++++++++
>>   2 files changed, 86 insertions(+)
>>
>> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
>> index 38c93f57a941..44ad7664e875 100644
>> --- a/drivers/pci/p2pdma.c
>> +++ b/drivers/pci/p2pdma.c
>> @@ -923,6 +923,71 @@ void pci_p2pdma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg,
>>   }
>>   EXPORT_SYMBOL_GPL(pci_p2pdma_unmap_sg_attrs);
>>   
>> +/**
>> + * pci_p2pdma_map_segment - map an sg segment determining the mapping type
>> + * @state: State structure that should be declared on the stack outside of
>> + *	the for_each_sg() loop and initialized to zero.
> 
> Silly fine point for the docs here: it doesn't actually have to be on
> the stack, so I don't think you need to write that constraint in the
> documentation. It just has be be somehow allocated and zeroed.

Yeah, that's true, but there's really no reason it would ever not be
allocated on the stack.

> 
>> + * @dev: DMA device that's doing the mapping operation
>> + * @sg: scatterlist segment to map
>> + * @attrs: dma mapping attributes
>> + *
>> + * This is a helper to be used by non-iommu dma_map_sg() implementations where
>> + * the sg segment is the same for the page_link and the dma_address.
>> + *
>> + * Attempt to map a single segment in an SGL with the PCI bus address.
>> + * The segment must point to a PCI P2PDMA page and thus must be
>> + * wrapped in a is_pci_p2pdma_page(sg_page(sg)) check.
> 
> Should this be backed up with actual checks in the function, that
> the prerequisites are met?

I think that would be unnecessary. All callers are going to call this
inside an is_pci_p2pdma_page() check, otherwise it would slow down the
fast path.

>> + *
>> + * Returns 1 if the segment was mapped, 0 if the segment should be mapped
>> + * directly (or through the IOMMU) and -EREMOTEIO if the segment should not
>> + * be mapped at all.
>> + */
>> +int pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state,
>> +			   struct device *dev, struct scatterlist *sg,
>> +			   unsigned long dma_attrs)
>> +{
>> +	if (state->pgmap != sg_page(sg)->pgmap) {
>> +		state->pgmap = sg_page(sg)->pgmap;
>> +		state->map = pci_p2pdma_map_type(state->pgmap, dev, dma_attrs);
>> +		state->bus_off = to_p2p_pgmap(state->pgmap)->bus_offset;
>> +	}
> 
> I'll quote myself from patch 9, because I had a comment there that actually
> was meant for this patch:
> 
> Is it worth putting this stuff on the caller's stack? I mean, is there a
> noticeable performance improvement from caching the state? Because if
> it's invisible, then simplicity is better. I suspect you're right, and
> that it *is* worth it, but it's good to know for real.

Yeah, I responded to this in another email.

> 
>> +
>> +	switch (state->map) {
>> +	case PCI_P2PDMA_MAP_BUS_ADDR:
>> +		sg->dma_address = sg_phys(sg) + state->bus_off;
>> +		sg_dma_len(sg) = sg->length;
>> +		sg_mark_pci_p2pdma(sg);
>> +		return 1;
>> +	case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
>> +		return 0;
>> +	default:
>> +		return -EREMOTEIO;
>> +	}
>> +}
>> +
>> +/**
>> + * pci_p2pdma_map_bus_segment - map an sg segment pre determined to
>> + *	be mapped with PCI_P2PDMA_MAP_BUS_ADDR
> 
> Or:
> 
>   * pci_p2pdma_map_bus_segment - map an SG segment that is already known
>   * to be mapped with PCI_P2PDMA_MAP_BUS_ADDR
> 
> Also, should that prerequisite be backed up with checks in the function?

No, this function only really exists for the needs of iommu_dma_map_sg().

>> + * @pg_sg: scatterlist segment with the page to map
>> + * @dma_sg: scatterlist segment to assign a dma address to
>> + *
>> + * This is a helper for iommu dma_map_sg() implementations when the
>> + * segment for the dma address differs from the segment containing the
>> + * source page.
>> + *
>> + * pci_p2pdma_map_type() must have already been called on the pg_sg and
>> + * returned PCI_P2PDMA_MAP_BUS_ADDR.
> 
> Another prerequisite, so same question: do you think that the code should
> also check that this prerequisite is met?

Again, no, simply because it's this way because of what's required by
iommu_dma.

Logan