Re: [PATCH v3 1/9] PCI/P2PDMA: Add CONFIG_PCI_P2PDMA_CORE

[Pranjal Shrivastava <praan@google.com>]

On Wed, Jun 10, 2026 at 04:43:15PM +0100, Matt Evans wrote:
> The P2PDMA code currently provides two features under the same
> CONFIG_PCI_P2PDMA option:
> 
>  1.  Locate providers via pcim_p2pdma_provider()
>  2.  Manage actual P2P DMA
> 
> Some drivers (such as vfio-pci) depend on 1, without having a hard
> dependency on 2.
> 
> A future commit expands the use of DMABUF in vfio-pci for non-P2P
> scenarios, relying on pcim_p2pdma_provider() always being present.  If
> that depended on CONFIG_PCI_P2PDMA, it would make vfio-pci only
> available if CONFIG_ZONE_DEVICE is present (e.g. 64-bit systems), even
> when P2P is not needed.
> 
> To resolve this, introduce CONFIG_PCI_P2PDMA_CORE and refactor the
> basic provider functionality into a new p2pdma_core.c file.  This is
> available even if the CONFIG_PCI_P2PDMA feature is disabled (or
> unavailable due to !CONFIG_ZONE_DEVICE).  Then, drivers can enable any
> additional P2P features with the original CONFIG_PCI_P2PDMA (available
> when CONFIG_ZONE_DEVICE is set).
> 
> Signed-off-by: Matt Evans <matt@ozlabs.org>
> ---
>  MAINTAINERS                |   2 +-
>  drivers/pci/Kconfig        |  10 ++--
>  drivers/pci/Makefile       |   1 +
>  drivers/pci/p2pdma.c       | 109 ++--------------------------------
>  drivers/pci/p2pdma.h       |  29 +++++++++
>  drivers/pci/p2pdma_core.c  | 118 +++++++++++++++++++++++++++++++++++++
>  include/linux/pci-p2pdma.h |  24 ++++----
>  include/linux/pci.h        |   2 +-
>  8 files changed, 174 insertions(+), 121 deletions(-)
>  create mode 100644 drivers/pci/p2pdma.h
>  create mode 100644 drivers/pci/p2pdma_core.c
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index c2c6d79275c6..b21523b3bd8b 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -20617,7 +20617,7 @@ B:	https://bugzilla.kernel.org
>  C:	irc://irc.oftc.net/linux-pci
>  T:	git git://git.kernel.org/pub/scm/linux/kernel/git/pci/pci.git
>  F:	Documentation/driver-api/pci/p2pdma.rst
> -F:	drivers/pci/p2pdma.c
> +F:	drivers/pci/p2pdma*
>  F:	include/linux/pci-p2pdma.h
>  
>  PCI POWER CONTROL
> diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
> index 33c88432b728..59d70bc84cc9 100644
> --- a/drivers/pci/Kconfig
> +++ b/drivers/pci/Kconfig
> @@ -206,11 +206,7 @@ config PCIE_TPH
>  config PCI_P2PDMA
>  	bool "PCI peer-to-peer transfer support"
>  	depends on ZONE_DEVICE
> -	#
> -	# The need for the scatterlist DMA bus address flag means PCI P2PDMA
> -	# requires 64bit
> -	#
> -	depends on 64BIT
> +	select PCI_P2PDMA_CORE
>  	select GENERIC_ALLOCATOR
>  	select NEED_SG_DMA_FLAGS
>  	help

Nit: Did we drop depends on 64BIT intentionally here? I guess the full
PCI_P2PDMA stack still selects NEED_SG_DMA_FLAGS? IIRC, NEED_SG_DMA_FLAGS
doesn't select 64BIT?

With the nit (and Bjorn's comments addressed)
Reviewed-by: Pranjal Shrivastava <praan@google.com>

Thanks,
Praan