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

[Vincent Guittot <vincent.guittot@linaro.org>]

On Tue, 16 Jul 2019 at 02:56, Saravana Kannan <saravanak@google.com> wrote:
>
> On Mon, Jul 15, 2019 at 1:16 AM Vincent Guittot
> <vincent.guittot@linaro.org> wrote:
> >
> > On Tue, 9 Jul 2019 at 21:03, Saravana Kannan <saravanak@google.com> wrote:
> > >
> > > On Tue, Jul 9, 2019 at 12:25 AM Vincent Guittot
> > > <vincent.guittot@linaro.org> wrote:
> > > >
> > > > On Sun, 7 Jul 2019 at 23:48, Saravana Kannan <saravanak@google.com> wrote:
> > > > >
> > > > > On Thu, Jul 4, 2019 at 12:12 AM Vincent Guittot
> > > > > <vincent.guittot@linaro.org> wrote:
> > > > > >
> > > > > > On Wed, 3 Jul 2019 at 23:33, Saravana Kannan <saravanak@google.com> wrote:
> > > > > > >
> > > > > > > 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.
> > > > > >
> > > > > > All these constraint are provider's constraints and not consumer's one
> > > > > >
> > > > > > The consumer asks for a bandwidth according to its needs and then the
> > > > > > providers select the optimal bandwidth of each interconnect after
> > > > > > aggregating all the request and according to what OPP have been
> > > > > > validated
> > > > >
> > > > > Not really. The screening can be a consumer specific issue. The
> > > > > consumer IP itself might have some issue with using too low of a
> > > > > bandwidth or bandwidth that's not within some range. It should not be
> > > >
> > > > How can an IP ask for not enough bandwidth ?
> > > > It asks the needed bandwidth based on its requirements
> > >
> > > The "enough bandwidth" is not always obvious. It's only for very
> > > simple cases that you can calculate the required bandwidth. Even for
> > > cases that you think might be "obvious/easy" aren't always easy.
> > >
> > > For example, you'd think a display IP would have a fixed bandwidth
> > > requirement for a fixed resolution screen. But that's far from the
> > > truth. It can also change as the number of layers change per frame.
> > > For video decoder/encoder, it depends on how well the frames compress
> > > with a specific compression scheme.
> > > So the "required" bandwidth is often a heuristic based on the IP
> > > frequency or traffic measurement.
> > >
> > > But that's not even the point I was making in this specific "bullet".
> > >
> > > A hardware IP might be screen/verified with only certain bandwidth
> > > levels. Or it might have hardware bugs that prevent it from using
> > > lower bandwidths even though it's technically sufficient. We need a
> > > way to capture that per path. This is not even a fictional case. This
> > > has been true multiple times over widely used IPs.
> >
> > here you are mixing HW constraint on the soc and OPP screening with
> > bandwidth request from consumer
> > ICC framework is about getting bandwidth request not trying to fix
> > some HW/voltage dependency of the SoC
> >
> > >
> > > > > the provider's job to take into account all the IP that might be
> > > > > connected to the interconnects. If the interconnect HW itself didn't
> > > >
> > > > That's not what I'm saying. The provider knows which bandwidth the
> > > > interconnect can provide as it is the ones which configures it. So if
> > > > the interconnect has a finite number of bandwidth point based probably
> > > > on the possible clock frequency and others config of the interconnect,
> > > > it selects the best final config after aggregating the request of the
> > > > consumer.
> > >
> > > I completely agree with this. What you are stating above is how it
> > > should work and that's the whole point of the interconnect framework.
> > >
> > > But this is orthogonal to the point I'm making.
> >
> > It's not orthogonal because you want to add a OPP table pointer in the
> > ICC path structure to fix your platform HW constraint whereas it's not
> > the purpose of the framework IMO
> >
> > >
> > > > > change, the provider driver shouldn't need to change. By your
> > > > > definition, a provider driver will have to account for all the
> > > > > possible bus masters that might be connected to it across all SoCs.
> > > >
> > > > you didn't catch my point
> > >
> > > Same. I think we are talking over each other. Let me try again.
> > >
> > > You are trying to describe how and interconnect provider and framework
> > > should work. There's no disagreement there.
> > >
> > > My point is that consumers might not want to or can not always use all
> > > the available bandwidth levels offered by the providers. There can be
> > > many reasons for that (which is what I listed in my earlier emails)
> > > and we need a good and generic way to capture that so that everyone
> > > isn't trying to invent their own property.
> >
> > And my point is that you want to describe some platform or even UCs
> > specific constraint in the ICC framework which is not the place to do.
> >
> > If the consumers might not want to use all available bandwidth because
> > this is not power efficient as an example, this should be describe
> > somewhere else to express  that there is a shared power domain
> > between some devices and we shoudl ensure that all devices in this
> > power domain should use the  Optimal Operating Point (optimal freq for
> > a voltage)
>
> My patch series has nothing to do with shared power domains. I think
> the examples have made it amply clear.

It's far from being clear why a consumer doesn't want to use some
bandwidth level TBH

Do you have a real example ?

>
> > ICC framework describes the bandwidth request that are expressed by
> > the consumers for the current running state of their IP but it doesn't
> > reflect the fact that on platform A, the consumer should use bandwidth
> > X because it will select a voltage level of a shared power domain that
> > is optimized for the other devices B, C ... . It's up to the provider
> > to know HW details of the bus that it drives and to make such
> > decision;  the consumer should always request the same
>
> The change to ICC framework is practically just this. I don't have any
> future changes planned for the ICC framework. This is the entirety of
> it.
>
> +       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);
> +       }
>
> It's quite a stretch and bit hyperbolic to say this one change is
> getting ICC framework to do all the things you claim above.
>

So I clearly don't see the benefit of adding this opp_table field in
icc_path struct because I'm still convinced that the consumer doesn't
have to get a bandwidth table like that.
If the consumer already get a bandwidth value and it should in order
to call icc_set or even in order to select one element in your table,
then it should directly set it with icc_set and let the provider
aggregate and choose the real one

> It's literally a simple helper function so that the consumer doesn't
> have to make assumptions about indices and it's a bit more explicit
> about which OPP table of the device (a device can have multiple OPP
> tables) corresponds to which ICC path.
>
> Going by your extreme argument, one can also claim that it's not the
> ICC framework's job to make it easy for consumers to figure out the
> source/destination endpoints or give them names and delete the
> interconnect and interconnect-names properties. That's clearly just as
> absurd a claim.
>
>
> -Saravana
>
> > > > > That's not good design nor is it scalable.
> > > > >
> > > > > > >
> > > > > > > 2. Similarly during parts screening in the factory, some of the
> > > > > > >    combinations might not have been screened and can't be guaranteed
> > > > > > >    to work.
> > > > > >
> > > > > > As above, it's the provider's job to select the final bandwidth
> > > > > > according to its constraint
> > > > >
> > > > > Same reply as above.
> > > > >
> > > > > > >
> > > > > > > 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.
> > > > > >
> > > > > > These example are quite vague about the reasons why little might never
> > > > > > want to use higher bandwidth.
> > > > >
> > > > > How is it vague? I just said because of power/performance balance.
> > > > >
> > > > > > But then, if little doesn't ask high bandwidth it will not use them.
> > > > >
> > > > > If you are running a heuristics based algorithm to pick bandwidth,
> > > > > this is how it'll know NOT to use some of the bandwidth levels.
> > > >
> > > > so you want to set a bandwidth according to the cpu frequency which is
> > > > what has been proposed in other thread
> > >
> > > Nope, that's just one heuristic. Often times it's based on hardware
> > > monitors measuring interconnect activity. If you go look at the SDM845
> > > in a Pixel 3, almost nothing is directly tied to the CPU frequency.
> > >
> > > Even if you are scaling bandwidth based on other hardware
> > > measurements, you might want to avoid some bandwidth level provided by
> > > the interconnect providers because it's suboptimal.
> > >
> > > For example, when making bandwidth votes to accommodate the big CPUs,
> > > you might never want to use some of the lower bandwidth levels because
> > > they are not power efficient for any CPU frequency or any bandwidth
> > > level. Because at those levels the memory/interconnect is so slow that
> > > it has a non-trivial utilization increase (because the CPU is
> > > stalling) of the big CPUs.
> > >
> > > Again, this is completely different from what the providers/icc
> > > framework does. Which is, once the request is made, they aggregate and
> > > set the actual interconnect frequencies correctly.
> > >
> > > > >
> > > > > > >
> > > > > > > 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.
> > > > > > >
> > > > > >
> > > > > > All the examples above makes sense but have to be handle by the
> > > > > > provider not the consumer. The consumer asks for a bandwidth according
> > > > > > to its constraints. Then the provider which is the driver that manages
> > > > > > the interconnect IP, should manage all this hardware and platform
> > > > > > specific stuff related to the interconnect IP in order to set the
> > > > > > optimal bandwidth that fit both consumer constraint and platform
> > > > > > specific configuration.
> > > > >
> > > > > Sure, but the provider itself can have interconnect properties to
> > > > > indicate which other interconnects it's tied to. And the provider will
> > > > > still need the interconnect-opp-table to denote which bandwidth levels
> > > > > are sensible to use with each of its connections.
> > >
> > > You seem to have missed this comment.
> > >
> > > Thanks,
> > > Saravana
> > >
> > > > > So in some instances the interconnect-opp-table covers the needs of
> > > > > purely consumers and in some instances purely providers. But in either
> > > > > case, it's still needed to describe the hardware properly.
> > > > >
> > > > > -Saravana
> > > > >
> > > > > > > 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
> > > > > > > > >
> > > > > >
> > > > > > --
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> > > > > >