Re: [PATCH v2 06/22] PCI/P2PDMA: Attempt to set map_type if it has not been set

[Christoph Hellwig]

On Thu, May 13, 2021 at 04:31:47PM -0600, Logan Gunthorpe wrote:
> Attempt to find the mapping type for P2PDMA pages on the first
> DMA map attempt if it has not been done ahead of time.
> 
> Previously, the mapping type was expected to be calculated ahead of
> time, but if pages are to come from userspace then there's no
> way to ensure the path was checked ahead of time.
> 
> With this change it's no longer invalid to call pci_p2pdma_map_sg()
> before the mapping type is calculated so drop the WARN_ON when that
> is the case.

Why?