linux-kernel - Re: [PATCH v3 3/8] of/address: Introduce of_dma_get_max_cpu

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Message-ID: <CAMj1kXFnLEpNTZVq16YgBomkZwwdGsH89OSELsYeiee4P+GLPg@mail.gmail.com>
Date:   Thu, 15 Oct 2020 08:56:40 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Hellwig <hch@....de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:BROADCOM BCM2835 ARM ARCHITECTURE" 
        <linux-rpi-kernel@...ts.infradead.org>,
        Jeremy Linton <jeremy.linton@....com>,
        Linux IOMMU <iommu@...ts.linux-foundation.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH v3 3/8] of/address: Introduce of_dma_get_max_cpu_address()

On Thu, 15 Oct 2020 at 00:03, Rob Herring <robh+dt@...nel.org> wrote:
>
> On Wed, Oct 14, 2020 at 2:12 PM Nicolas Saenz Julienne
> <nsaenzjulienne@...e.de> 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@...e.de>
> >
> > ---
> >
> > 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.


> > +       struct of_range_parser parser;
> > +       phys_addr_t subtree_max_addr;
> > +       struct device_node *child;
> > +       phys_addr_t cpu_end = 0;
> > +       struct of_range range;
> > +       const __be32 *ranges;
> > +       int len;
> > +
> > +       if (!np)
> > +               np = of_root;
> > +
> > +       ranges = of_get_property(np, "dma-ranges", &len);
>
> I'm not really following why you changed the algorithm here. You're
> skipping disabled nodes which is good. Was there some other reason?
>
> > +       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 (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;
> > +}
> > +
> >  /**
> >   * of_dma_is_coherent - Check if device is coherent
> >   * @np:        device node
> > diff --git a/include/linux/of.h b/include/linux/of.h
> > index 481ec0467285..db8db8f2c967 100644
> > --- a/include/linux/of.h
> > +++ b/include/linux/of.h
> > @@ -558,6 +558,8 @@ int of_map_id(struct device_node *np, u32 id,
> >                const char *map_name, const char *map_mask_name,
> >                struct device_node **target, u32 *id_out);
> >
> > +phys_addr_t of_dma_get_max_cpu_address(struct device_node *np);
> > +
> >  #else /* CONFIG_OF */
> >
> >  static inline void of_core_init(void)
> > @@ -995,6 +997,11 @@ static inline int of_map_id(struct device_node *np, u32 id,
> >         return -EINVAL;
> >  }
> >
> > +static inline phys_addr_t of_dma_get_max_cpu_address(struct device_node *np)
> > +{
> > +       return PHYS_ADDR_MAX;
> > +}
> > +
> >  #define of_match_ptr(_ptr)     NULL
> >  #define of_match_node(_matches, _node) NULL
> >  #endif /* CONFIG_OF */
> > --
> > 2.28.0
> >