Re: [PATCH v5 2/2] PCI: dwc: Add support for 64-bit MSI target address

[Christoph Hellwig]

On Fri, Sep 09, 2022 at 02:47:19PM +0100, Robin Murphy wrote:
> On 2022-09-09 14:29, Christoph Hellwig wrote:
> > On Thu, Aug 25, 2022 at 06:50:25PM +0000, Will McVicker wrote:
> > > Since not all devices require a 32-bit MSI address, add support to the
> > > PCIe host driver to allow setting the DMA mask to 64-bits if the 32-bit
> > > allocation fails. This allows kernels to disable ZONE_DMA32 and bounce
> > > buffering (swiotlb) without risking not being able to get a 32-bit address
> > > during DMA allocation.
> > 
> > Umm.  You can't just disable ZONE_DMA32.  Linux absolutely requires a
> > 32-bit dma mask to work, it is in fact the implicit default.
> 
> Eh, it's behind CONFIG_EXPERT, which makes it enough of a "I think I know
> what I'm doing and accept responsibility for picking up the pieces if it
> breaks" thing.

Seem like indeed on arm64 there is a way to disable it.  The x86 model
is to just select it unconditionally, which I think is the right way
if we don't want to get into completely random failures.