Re: [PATCH v3 3/8] of/address: Introduce of_dma_get_max_cpu_address()

[Nicolas Saenz Julienne]

On Thu, 2020-10-15 at 08:56 +0200, Ard Biesheuvel wrote:
> On Thu, 15 Oct 2020 at 00:03, Rob Herring <robh+dt@xxxxxxxxxx> wrote:
> > On Wed, Oct 14, 2020 at 2:12 PM Nicolas Saenz Julienne
> > <nsaenzjulienne@xxxxxxx> wrote:
> > > Introduce of_dma_get_max_cpu_address(), which provides the highest CPU
> > > physical address addressable by all DMA masters in the system. It's
> > > specially useful for setting memory zones sizes at early boot time.
> > > 
> > > Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>
> > > 
> > > ---
> > > 
> > > Changes since v2:
> > >  - Use PHYS_ADDR_MAX
> > >  - return phys_dma_t
> > >  - Rename function
> > >  - Correct subject
> > >  - Add support to start parsing from an arbitrary device node in order
> > >    for the function to work with unit tests
> > > 
> > >  drivers/of/address.c | 42 ++++++++++++++++++++++++++++++++++++++++++
> > >  include/linux/of.h   |  7 +++++++
> > >  2 files changed, 49 insertions(+)
> > > 
> > > diff --git a/drivers/of/address.c b/drivers/of/address.c
> > > index eb9ab4f1e80b..b5a9695aaf82 100644
> > > --- a/drivers/of/address.c
> > > +++ b/drivers/of/address.c
> > > @@ -1024,6 +1024,48 @@ int of_dma_get_range(struct device_node *np, const struct bus_dma_region **map)
> > >  }
> > >  #endif /* CONFIG_HAS_DMA */
> > > 
> > > +/**
> > > + * of_dma_get_max_cpu_address - Gets highest CPU address suitable for DMA
> > > + * @np: The node to start searching from or NULL to start from the root
> > > + *
> > > + * Gets the highest CPU physical address that is addressable by all DMA masters
> > > + * in the system (or subtree when np is non-NULL). If no DMA constrained device
> > > + * is found, it returns PHYS_ADDR_MAX.
> > > + */
> > > +phys_addr_t __init of_dma_get_max_cpu_address(struct device_node *np)
> > > +{
> > > +       phys_addr_t max_cpu_addr = PHYS_ADDR_MAX;
> > 
> > One issue with using phys_addr_t is it may be 32-bit even though the
> > DT is 64-bit addresses. LPAE capable system with LPAE disabled. Maybe
> > the truncation is fine here? Maybe not.
> > 
> 
> PHYS_ADDR_MAX is the max addressable CPU address on the system, and so
> it makes sense to use it for the return type, and for the preliminary
> return value: this is actually what /prevents/ truncation, because we
> will only overwrite max_cpu_addr if the new u64 value is lower.
> 

Actually I now see how things might go south.

> > > +       if (ranges && len) {
> > > +               of_dma_range_parser_init(&parser, np);
> > > +               for_each_of_range(&parser, &range)
> > > +                       if (range.cpu_addr + range.size > cpu_end)
> > > +                               cpu_end = range.cpu_addr + range.size;

If cpu_end hits 0x1_00000000, it'll overflow to 0. This is possible on 32-bit
systems (LPAE or not). And something similar might happen on LPAE disabled
systems.

I could add some extra logic, something like:

	/* We overflowed */
	if (cpu_end < range.cpu_addr)
		cpu_end = PHYS_ADDR_MAX;

Which is not perfect but will cover most sensible cases.

Or simply deal internally in u64s, and upon returning, check if "max_cpu_addr"
falls higher than PHYS_ADDR_MAX.

> > > +
> > > +               if (max_cpu_addr > cpu_end)
> > > +                       max_cpu_addr = cpu_end;
> > > +       }
> > > +
> > > +       for_each_available_child_of_node(np, child) {
> > > +               subtree_max_addr = of_dma_get_max_cpu_address(child);
> > > +               if (max_cpu_addr > subtree_max_addr)
> > > +                       max_cpu_addr = subtree_max_addr;
> > > +       }
> > > +
> > > +       return max_cpu_addr;
> > > +}

Regards,
Nicolas

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