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

[Matt Evans <matt@ozlabs.org>]

Hi Kevin, Pranjal, (+Robin, hi!)

On 12/06/2026 04:39, Tian, Kevin wrote:
>> From: Pranjal Shrivastava <praan@google.com>
>> Sent: Friday, June 12, 2026 2:38 AM
>>
>> On Wed, Jun 10, 2026 at 04:43:15PM +0100, Matt Evans wrote:
>>> --- 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?
> 
> seems that comment is stale. According to the commit msg:
> 
> " it would make vfio-pci only available if CONFIG_ZONE_DEVICE is
> present (e.g. 64-bit systems), "
> 
> so it sounds a redundant dependency hence is removed.

This was intentional.  In practice there is still a dependency on 64BIT
for PCI_P2PDMA, but it is because of ZONE_DEVICE (and mem hotplug).  The
key need is PCI_P2PDMA_CORE is available on !64BIT for VFIO, but I
didn't see a requirement from PCI_P2PDMA itself (as opposed to its
dependencies).  If I've missed one, I can put it back...

But NEED_SG_DMA_FLAGS doesn't smell quite right; I see from comments in

  af2880ec44021 ("scatterlist: add dedicated config for DMA flags")

that it assumes 64BIT, but it seems to be missing a "depends on 64BIT".

Robin -- should that depend on 64BIT?


Cheers,


Matt