Re: [PATCH] iommu: Don't reserve IOVA when address and size are zero

[Ashish Mhetre]

On 11/23/2023 4:43 PM, Robin Murphy wrote:
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>
>
> On 2023-11-23 6:12 am, Ashish Mhetre wrote:
> > When the bootloader/firmware doesn't setup the framebuffers, their
> > address and size are zero in "iommu-addresses" property. If we intend to
> > use display driver in kernel without framebuffer then it's causing
> > the display IOMMU mappings to fail as IOVA is reserved with size and
> > address as zero.
>
> Can you clarify the problem there? Looking at the code in
> iova_reserve_iommu_regions() I'm guessing it's that "region->start +
> region->length - 1" underflows so reserve_iova() actually ends up
> reserving the entire valid IOVA space?

Yes, that's the problem which lead to dma_map call failures from
display driver. I don't have the logs handy to pin-point the exact
function which failed as this issue was seen before few months.

> > An ideal solution would be firmware removing the "iommu-addresses"
> > property and corresponding "memory-region" if display is not present.
> > But the kernel should be able to handle this by checking for size and
> > address of IOVA and skipping the IOVA reservation if both are 0.
>
> Surely it doesn't make sense to reserve a 0-length region at *any* base
> address? The symptom above wouldn't be quite the same if the base was
> nonzero, but corrupting the rbtree with an entry where pfn_hi < pfn_lo
> would definitely not be good either.
Agreed, we should restrict reservation for 0-length region at any base.
I will update the condition in next version.


> > Fixes: a5bf3cfce8cb ("iommu: Implement of_iommu_get_resv_regions()")
> > Signed-off-by: Ashish Mhetre <amhetre@xxxxxxxxxx>
> > ---
> >   drivers/iommu/of_iommu.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
> > index 157b286e36bf..150ef65d357a 100644
> > --- a/drivers/iommu/of_iommu.c
> > +++ b/drivers/iommu/of_iommu.c
> > @@ -255,6 +255,10 @@ void of_iommu_get_resv_regions(struct device 
> > *dev, struct list_head *list)
> >                               size_t length;
> >
> >                               maps = of_translate_dma_region(np, 
> > maps, &iova, &length);
> > +                             if (iova == 0 && length == 0) {
> > +                                     dev_dbg(dev, "Skipping IOVA 
> > reservation as address and size are zero\n");
>
> FWIW I'd be inclined to log a visible warning that firmware is giving us
> nonsense.
Okay, I'll replace dev_dbg() with dev_warn() in next version.


> Thanks,
> Robin.
>
> > + continue;
> > +                             }
> >                               type = iommu_resv_region_get_type(dev, 
> > &phys, iova, length);
> >
> >                               region = iommu_alloc_resv_region(iova, 
> > length, prot, type,