Re: [PATCH v3 6/6] interconnect: Add OPP table support for interconnects

[Saravana Kannan <saravanak@google.com>]

On Tue, Jul 2, 2019 at 11:45 PM Vincent Guittot
<vincent.guittot@linaro.org> wrote:
>
> On Wed, 3 Jul 2019 at 03:10, Saravana Kannan <saravanak@google.com> wrote:
> >
> > Interconnect paths can have different performance points. Now that OPP
> > framework supports bandwidth OPP tables, add OPP table support for
> > interconnects.
> >
> > Devices can use the interconnect-opp-table DT property to specify OPP
> > tables for interconnect paths. And the driver can obtain the OPP table for
> > an interconnect path by calling icc_get_opp_table().
>
> The opp table of a path must come from the aggregation of OPP tables
> of the interconnect providers.

The aggregation of OPP tables of the providers is certainly the
superset of what a path can achieve, but to say that OPPs for
interconnect path should match that superset is an oversimplification
of the reality in hardware.

There are lots of reasons an interconnect path might not want to use
all the available bandwidth options across all the interconnects in
the route.

1. That particular path might not have been validated or verified
   during the HW design process for some of the frequencies/bandwidth
   combinations of the providers.

2. Similarly during parts screening in the factory, some of the
   combinations might not have been screened and can't be guaranteed
   to work.

3. Only a certain set of bandwidth levels might make sense to use from
   a power/performance balance given the device using it. For example:
   - The big CPU might not want to use some of the lower bandwidths
     but the little CPU might want to.
   - The big CPU might not want to use some intermediate bandwidth
     points if they don't save a lot of power compared to a higher
     bandwidth levels, but the little CPU might want to.
   - The little CPU might never want to use the higher set of
     bandwidth levels since they won't be power efficient for the use
     cases that might run on it.

4. It might not make sense from a system level power perspective.
Let's take an example of a path S (source) -> A -> B -> C -> D
(destination).
   - A supports only 2, 5, 7 and 10 GB/s. B supports 1, 2 ... 10 GB/s.
     C supports 5 and 10 GB/s
   - If you combine and list the superset of bandwidth levels
     supported in that path, that'd be 1, 2, 3, ... 10 GB/s.
   - Which set of bandwidth levels make sense will depend on the
     hardware characteristics of the interconnects.
   - If B is the biggest power sink, then you might want to use all 10
     levels.
   - If A is the biggest power sink, then you might want to use all 2,
     5 and 10 GB/s of the levels.
   - If C is the biggest power sink then you might only want to use 5
     and 10 GB/s
   - The more hops and paths you get the more convoluted this gets.

5. The design of the interconnects themselves might have an impact on
which bandwidth levels are used.
   - For example, the FIFO depth between two specific interconnects
     might affect the valid bandwidth levels for a specific path.
   - Say S1 -> A -> B -> D1, S2 -> C -> B -> D1 and S2 -> C -> D2 are
     three paths.
   - If C <-> B FIFO depth is small, then there might be a requirement
     that C and B be closely performance matched to avoid system level
     congestion due to back pressure.
   - So S2 -> D1 path can't use all the bandwidth levels supported by
     C-B combination.
   - But S2 -> D2 can use all the bandwidth levels supported by C.
   - And S1 -> D1 can use all the levels supported by A-B combination.

These are just some of the reasons I could recollect in a few minutes.
These are all real world cases I had to deal with in the past several
years of dealing with scaling interconnects. I'm sure vendors and SoCs
I'm not familiar with have other good reasons I'm not aware of.

Trying to figure this all out by aggregating OPP tables of
interconnect providers just isn't feasible nor is it efficient. The
OPP tables for an interconnect path is describing the valid BW levels
supported by that path and verified in hardware and makes a lot of
sense to capture it clearly in DT.

> So such kind of OPP table should be at
> provider level but not at path level.

They can also use it if they want to, but they'll probably want to use
a frequency OPP table.


-Saravana

>
> >
> > Signed-off-by: Saravana Kannan <saravanak@google.com>
> > ---
> >  drivers/interconnect/core.c  | 27 ++++++++++++++++++++++++++-
> >  include/linux/interconnect.h |  7 +++++++
> >  2 files changed, 33 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/interconnect/core.c b/drivers/interconnect/core.c
> > index 871eb4bc4efc..881bac80bc1e 100644
> > --- a/drivers/interconnect/core.c
> > +++ b/drivers/interconnect/core.c
> > @@ -47,6 +47,7 @@ struct icc_req {
> >   */
> >  struct icc_path {
> >         size_t num_nodes;
> > +       struct opp_table *opp_table;
> >         struct icc_req reqs[];
> >  };
> >
> > @@ -313,7 +314,7 @@ struct icc_path *of_icc_get(struct device *dev, const char *name)
> >  {
> >         struct icc_path *path = ERR_PTR(-EPROBE_DEFER);
> >         struct icc_node *src_node, *dst_node;
> > -       struct device_node *np = NULL;
> > +       struct device_node *np = NULL, *opp_node;
> >         struct of_phandle_args src_args, dst_args;
> >         int idx = 0;
> >         int ret;
> > @@ -381,10 +382,34 @@ struct icc_path *of_icc_get(struct device *dev, const char *name)
> >                 dev_err(dev, "%s: invalid path=%ld\n", __func__, PTR_ERR(path));
> >         mutex_unlock(&icc_lock);
> >
> > +       opp_node = of_parse_phandle(np, "interconnect-opp-table", idx);
> > +       if (opp_node) {
> > +               path->opp_table = dev_pm_opp_of_find_table_from_node(opp_node);
> > +               of_node_put(opp_node);
> > +       }
> > +
> > +
> >         return path;
> >  }
> >  EXPORT_SYMBOL_GPL(of_icc_get);
> >
> > +/**
> > + * icc_get_opp_table() - Get the OPP table that corresponds to a path
> > + * @path: reference to the path returned by icc_get()
> > + *
> > + * This function will return the OPP table that corresponds to a path handle.
> > + * If the interconnect API is disabled, NULL is returned and the consumer
> > + * drivers will still build. Drivers are free to handle this specifically, but
> > + * they don't have to.
> > + *
> > + * Return: opp_table pointer on success. NULL is returned when the API is
> > + * disabled or the OPP table is missing.
> > + */
> > +struct opp_table *icc_get_opp_table(struct icc_path *path)
> > +{
> > +       return path->opp_table;
> > +}
> > +
> >  /**
> >   * icc_set_bw() - set bandwidth constraints on an interconnect path
> >   * @path: reference to the path returned by icc_get()
> > diff --git a/include/linux/interconnect.h b/include/linux/interconnect.h
> > index dc25864755ba..0c0bc55f0e89 100644
> > --- a/include/linux/interconnect.h
> > +++ b/include/linux/interconnect.h
> > @@ -9,6 +9,7 @@
> >
> >  #include <linux/mutex.h>
> >  #include <linux/types.h>
> > +#include <linux/pm_opp.h>
> >
> >  /* macros for converting to icc units */
> >  #define Bps_to_icc(x)  ((x) / 1000)
> > @@ -28,6 +29,7 @@ struct device;
> >  struct icc_path *icc_get(struct device *dev, const int src_id,
> >                          const int dst_id);
> >  struct icc_path *of_icc_get(struct device *dev, const char *name);
> > +struct opp_table *icc_get_opp_table(struct icc_path *path);
> >  void icc_put(struct icc_path *path);
> >  int icc_set_bw(struct icc_path *path, u32 avg_bw, u32 peak_bw);
> >
> > @@ -49,6 +51,11 @@ static inline void icc_put(struct icc_path *path)
> >  {
> >  }
> >
> > +static inline struct opp_table *icc_get_opp_table(struct icc_path *path)
> > +{
> > +       return NULL;
> > +}
> > +
> >  static inline int icc_set_bw(struct icc_path *path, u32 avg_bw, u32 peak_bw)
> >  {
> >         return 0;
> > --
> > 2.22.0.410.gd8fdbe21b5-goog
> >