REGRESSION in 6.0-rc7 caused by patch "ARM/dma-mapping: use dma-direct unconditionally"

[Robin Murphy]

On 2022-09-30 14:10, Marek Behún wrote:
> Hello Linus, Arnd, Robin and Christoph,
> 
> I just bisected a regression on Turris Omnia (Armada 385), wherein the
> system hangs shortly after init is run, to commit
> 
>    ae626eb97376 ("ARM/dma-mapping: use dma-direct unconditionally")
>    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ae626eb97376
> 
> In order to fix the regression, I had to revert this commit and
> subsequent 3 commits:
>    ae626eb97376 ("ARM/dma-mapping: use dma-direct unconditionally")
>    42998ef08aba ("ARM/dma-mapping: drop .dma_supported for IOMMU ops")
>    d563bccfa35b ("ARM/dma-mapping: consolidate IOMMU ops callbacks")
>    4136ce90f079 ("ARM/dma-mapping: merge IOMMU ops")
> in reverse order, of course:
>    git revert 4136ce90f079
>    git revert d563bccfa35b
>    git revert 42998ef08aba
>    git revert ae626eb97376
> 
> Christoph, Robin, since you are the authors of these commits, do you
> have any idea what could be happening? Are we able to fix this without
> reverting those commits, before 6.0?

"hangs shortly after init" isn't much to go on. Are any errors logged? 
Possibly some driver is sat waiting for a DMA transfer to complete, that 
has somehow got the wrong address or lost coherency so never gets seen, 
but without at least being able to narrow it down to the affected driver 
it's hard to do much more than vague guessing.

Robin.