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

[Robin Murphy <robin.murphy@arm.com>]

On 12/06/2026 3:31 pm, Matt Evans wrote:
> Hi Kevin, Pranjal, (+Robin, hi!)

Oh hey there! :)

> 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?

Indeed, looking at the history it seems like that was overlooked, but it 
worked out at the time since the only selector of NEED_SG_DMA_FLAGS was 
PCI_P2PDMA as you say. If we're now generalising then moving the 
explicit 64BIT dependency to NEED_SG_DMA_FLAGS itself sounds like the 
right thing to do.

Cheers,
Robin.