Re: [PATCH v10 07/10] OF: Introduce helper function for getting PCI domain_nr

[Rob Herring]

On Mon, Sep 8, 2014 at 9:54 AM, Liviu Dudau <Liviu.Dudau@xxxxxxx> wrote:
> On Mon, Sep 08, 2014 at 03:27:56PM +0100, Rob Herring wrote:
>> On Mon, Sep 8, 2014 at 8:54 AM, Liviu Dudau <Liviu.Dudau@xxxxxxx> wrote:
>> > Add of_pci_get_domain_nr() to retrieve the PCI domain number
>> > of a given device from DT. If the information is not present,
>> > the function can be requested to allocate a new domain number.
>> >
>> > Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
>> > Cc: Arnd Bergmann <arnd@xxxxxxxx>
>> > Cc: Grant Likely <grant.likely@xxxxxxxxxx>
>> > Cc: Rob Herring <robh+dt@xxxxxxxxxx>
>> > Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
>> > Signed-off-by: Liviu Dudau <Liviu.Dudau@xxxxxxx>
>> > ---
>> >  drivers/of/of_pci.c    | 34 ++++++++++++++++++++++++++++++++++
>> >  include/linux/of_pci.h |  7 +++++++
>> >  2 files changed, 41 insertions(+)
>> >
>> > diff --git a/drivers/of/of_pci.c b/drivers/of/of_pci.c
>> > index 8481996..a107edb 100644
>> > --- a/drivers/of/of_pci.c
>> > +++ b/drivers/of/of_pci.c
>> > @@ -89,6 +89,40 @@ int of_pci_parse_bus_range(struct device_node *node, struct resource *res)
>> >  }
>> >  EXPORT_SYMBOL_GPL(of_pci_parse_bus_range);
>> >
>> > +static atomic_t of_domain_nr = ATOMIC_INIT(-1);
>> > +
>> > +/**
>> > + * This function will try to obtain the host bridge domain number by
>> > + * using of_alias_get_id() call with "pci-domain" as a stem. If that
>> > + * fails, a local allocator will be used. The local allocator can
>> > + * be requested to return a new domain_nr if the information is missing
>> > + * from the device tree.
>> > + *
>> > + * @node: device tree node with the domain information
>> > + * @allocate_if_missing: if DT lacks information about the domain nr,
>> > + * allocate a new number.
>> > + *
>> > + * Returns the associated domain number from DT, or a new domain number
>> > + * if DT information is missing and @allocate_if_missing is true. If
>> > + * @allocate_if_missing is false then the last allocated domain number
>> > + * will be returned.
>> > + */
>> > +int of_pci_get_domain_nr(struct device_node *node, bool allocate_if_missing)
>> > +{
>> > +       int domain;
>> > +
>> > +       domain = of_alias_get_id(node, "pci-domain");
>> > +       if (domain == -ENODEV) {
>> > +               if (allocate_if_missing)
>> > +                       domain = atomic_inc_return(&of_domain_nr);
>> > +               else
>> > +                       domain = atomic_read(&of_domain_nr);
>>
>> This function seems a bit broken to me. It is overloaded with too many
>> different outcomes. Think about how this would work if you have
>> multiple PCI buses and a mixture of having pci-domain aliases or not.
>> Aren't domain numbers global? Allocation should then start outside of
>> the range of alias ids.
>>
>> Rob
>>
>
> Rob,
>
> Would this version make more sense?

No.

> int of_pci_get_domain_nr(struct device_node *node, bool allocate_if_missing)
> {
>        int domain;
>
>        domain = of_alias_get_id(node, "pci-domain");
>        if (domain == -ENODEV) {
>                if (allocate_if_missing)
>                        domain = atomic_inc_return(&of_domain_nr);
>                else
>                        domain = atomic_read(&of_domain_nr);
>        } else {
>                /* remember the largest value seen */
>                int d = atomic_read(&of_domain_nr);
>                atomic_set(&of_domain_nr, max(domain, d));
>        }
>
>        return domain;
> }
> EXPORT_SYMBOL_GPL(of_pci_get_domain_nr);
>
> It would still create gaps and possible duplicates, but this is just a number
> and trying to create a new root bus in an existing domain should fail. I have

Is failure okay in that case?

> no clue on how to generate unique values without parsing the DT and filling
> a sparse array with values found there and then checking for allocated values

You really only need to know the maximum value and then start the
non-DT allocations from there.

> on new requests. This function gets called quite a lot and I'm trying not to
> make it too heavy weight.

Generally, nothing should be accessing the same DT value frequently.
It should get cached somewhere.

I don't really understand how domains are used so it's hard to provide
a recommendation here. Do domains even belong in the DT? This function
is just a weird mixture of data retrieval and allocation. I think you
need to separate it into 2 functions.

Rob
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