Re: [PATCH v4 3/6] of: fix size when dma-range is not used

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Jan 28, 2015 at 03:45:19PM +0000, Rob Herring wrote:
> On Wed, Jan 28, 2015 at 5:05 AM, Catalin Marinas
> <catalin.marinas@arm.com> wrote:
> > On Tue, Jan 27, 2015 at 06:55:15PM +0000, Murali Karicheri wrote:
> >> How about having the logic like this?
> >>
> >>       ret = of_dma_get_range(np, &dma_addr, &paddr, &size);
> >>       if (ret < 0) {
> >>               dma_addr = offset = 0;
> >>               size = dev->coherent_dma_mask + 1;
> >>       } else {
> >>               offset = PFN_DOWN(paddr - dma_addr);
> >>               dev_dbg(dev, "dma_pfn_offset(%#08lx)\n", offset);
> >>       }
> >>
> >>       if (is_power_of_2(size + 1))
> >>               size = size + 1;
> >>       else if (!is_power_of_2(size))
> >>       {
> >>               dev_err(dev, "invalid size\n");
> >>               return;
> >>       }
> >
> > In of_dma_configure(), we currently assume that the default coherent
> > mask is 32-bit. In this thread:
> >
> > http://article.gmane.org/gmane.linux.kernel/1835096
> >
> > we talked about setting the coherent mask based on size automatically.
> > I'm not sure about the size but I think we can assume is 32-bit mask + 1
> > if it is not specified in the DT. Instead of just assuming a default
> > mask, let's assume a default size and create the mask based on this
> > (untested):
> >
> > diff --git a/drivers/of/platform.c b/drivers/of/platform.c
> > index 5b33c6a21807..9ff8d1286b44 100644
> > --- a/drivers/of/platform.c
> > +++ b/drivers/of/platform.c
> > @@ -170,10 +170,10 @@ static void of_dma_configure(struct device *dev)
> >         struct iommu_ops *iommu;
> >
> >         /*
> > -        * Set default dma-mask to 32 bit. Drivers are expected to setup
> > -        * the correct supported dma_mask.
> > +        * Set default size to cover the 32-bit. Drivers are expected to setup
> > +        * the correct size and dma_mask.
> >          */
> > -       dev->coherent_dma_mask = DMA_BIT_MASK(32);
> > +       size = 1ULL << 32;
> >
> >         /*
> >          * Set it to coherent_dma_mask by default if the architecture
> > @@ -185,13 +185,24 @@ static void of_dma_configure(struct device *dev)
> >         ret = of_dma_get_range(dev->of_node, &dma_addr, &paddr, &size);
> >         if (ret < 0) {
> >                 dma_addr = offset = 0;
> > -               size = dev->coherent_dma_mask;
> 
> Are we assuming dma_addr, paddr and size are not touched on error? If
> so, we can get rid of this clause entirely.

We can if we initialise dma_addr and offset to 0 when declared in this
function. The dma_addr and size variables are later passed to the
arch_setup_dma_ops(), so they need to have some sane values independent
of the presence of dma-ranges in the DT.

> >         } else {
> >                 offset = PFN_DOWN(paddr - dma_addr);
> >                 dev_dbg(dev, "dma_pfn_offset(%#08lx)\n", dev->dma_pfn_offset);
> >         }
> >         dev->dma_pfn_offset = offset;
> >
> > +       /*
> > +        * Workaround for DTs setting the size to a mask or 0.
> > +        */
> > +       if (is_power_of_2(size + 1))
> > +               size += 1;
> 
> As I mentioned, I think power of 2 is too restrictive (from a DT
> perspective even though the kernel may require a power of 2 here for
> the mask). Just checking bit0 set should be enough.

The power of 2 was mainly to cover the case where the size is wrongly
written as a mask in the DT. If the size is deliberately not a power of
two and not a full mask, the above check won't change it. With my
proposal, ilog2 gets rid of extra bits in size, only that if the size
was a mask because of DT error, we lose a bit in the coherent_dma_mask.

> Also, we need a WARN here so DTs get fixed.

I agree.

-- 
Catalin