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

[Marek Behún]

On Fri, 30 Sep 2022 14:46:06 +0100
Robin Murphy <robin.murphy at arm.com> wrote:

> 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.

OK I enabled CONFIG_DMA_API_DEBUG and now am getting a null pointer
dereference. I managed to isolate the bug to a specifc line in mvneta
driver:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/ethernet/marvell/mvneta.c#n2591

I put debug printfs (pr_err("  a %i\n", __LINE__)) into the
mvneta_rx_hwbm() function.
The pr_err after the call to dma_sync_single_range_for_cpu() prints,
but the pr_err after skb_put_data() does not print.

Attaching console output.

Marek
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