Re: [PATCH] ARM: dma-api: fix max_pfn off-by-one error in __dma_supported()

[Chen-Yu Tsai]

On Wed, Dec 11, 2019 at 9:41 PM Robin Murphy <robin.murphy@xxxxxxx> wrote:
>
> On 11/12/2019 10:41 am, Chen-Yu Tsai wrote:
> > From: Chen-Yu Tsai <wens@xxxxxxxx>
> >
> > max_pfn, as set in arch/arm/mm/init.c:
> >
> >      static void __init find_limits(unsigned long *min,
> >                                  unsigned long *max_low,
> >                                  unsigned long *max_high)
> >      {
> >           *max_low = PFN_DOWN(memblock_get_current_limit());
> >           *min = PFN_UP(memblock_start_of_DRAM());
> >           *max_high = PFN_DOWN(memblock_end_of_DRAM());
> >      }
> >
> > with memblock_end_of_DRAM() pointing to the next byte after DRAM. As
> > such, max_pfn points to the PFN after the end of DRAM.
> >
> > Thus when using max_pfn to check DMA masks, we should subtract one
> > when checking DMA ranges against it.
> >
> > Commit 8bf1268f48ad ("ARM: dma-api: fix off-by-one error in
> > __dma_supported()") fixed the same issue, but missed this spot.
> >
> > This issue was found while working on the sun4i-csi v4l2 driver on the
> > Allwinner R40 SoC. On Allwinner SoCs, DRAM is offset at 0x40000000,
> > and we are starting to use of_dma_configure() with the "dma-ranges"
> > property in the device tree to have the DMA API handle the offset.
> >
> > In this particular instance, dma-ranges was set to the same range as
> > the actual available (2 GiB) DRAM. The following error appeared when
> > the driver attempted to allocate a buffer:
> >
> >      sun4i-csi 1c09000.csi: Coherent DMA mask 0x7fffffff (pfn 0x40000-0xc0000)
> >      covers a smaller range of system memory than the DMA zone pfn 0x0-0xc0001
> >      sun4i-csi 1c09000.csi: dma_alloc_coherent of size 307200 failed
> >
> > Fixing the off-by-one error makes things work.
> >
> > Fixes: 11a5aa32562e ("ARM: dma-mapping: check DMA mask against available memory")
> > Fixes: 9f28cde0bc64 ("ARM: another fix for the DMA mapping checks")
> > Fixes: ab746573c405 ("ARM: dma-mapping: allow larger DMA mask than supported")
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Signed-off-by: Chen-Yu Tsai <wens@xxxxxxxx>
> > ---
> >   arch/arm/mm/dma-mapping.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> > index e822af0d9219..f4daafdbac56 100644
> > --- a/arch/arm/mm/dma-mapping.c
> > +++ b/arch/arm/mm/dma-mapping.c
> > @@ -227,12 +227,12 @@ static int __dma_supported(struct device *dev, u64 mask, bool warn)
> >        * Translate the device's DMA mask to a PFN limit.  This
> >        * PFN number includes the page which we can DMA to.
> >        */
> > -     if (dma_to_pfn(dev, mask) < max_dma_pfn) {
> > +     if (dma_to_pfn(dev, mask) < max_dma_pfn - 1) {
>
> I think this correction actually wants to happen a couple of lines up in
> the definition:
>
>         unsigned long max_dma_pfn = min(max_pfn, arm_dma_pfn_limit);
>
> max_pfn is indeed an exclusive limit, but AFAICS arm_dma_pfn_limit is
> inclusive, so none of these "+1"s and "-1"s can be entirely right for
> both cases.

You're absolutely right. I'll fix it and send a v2 out.

Thanks

ChenYu

> Robin.
>
> >               if (warn)
> >                       dev_warn(dev, "Coherent DMA mask %#llx (pfn %#lx-%#lx) covers a smaller range of system memory than the DMA zone pfn 0x0-%#lx\n",
> >                                mask,
> >                                dma_to_pfn(dev, 0), dma_to_pfn(dev, mask) + 1,
> > -                              max_dma_pfn + 1);
> > +                              max_dma_pfn);
> >               return 0;
> >       }
> >
> >