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Subject: Re: [PATCH v3 2/4] of/platform: Add functional dependency link from
 DT bindings
To:     Saravana Kannan <saravanak@google.com>,
        Rob Herring <robh+dt@kernel.org>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        devicetree@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        David Collins <collinsd@codeaurora.org>,
        Android Kernel Team <kernel-team@android.com>
References: <20190702004811.136450-1-saravanak@google.com>
 <20190702004811.136450-3-saravanak@google.com>
 <CAL_JsqLdvDpKB=iV6x3eTr2F4zY0bxU-Wjb+JeMjj5rdnRc-OQ@mail.gmail.com>
 <CAGETcx_i9353aRFbJXNS78EvqwmU-2-xSBJ+ySZX1gjjHpz_cg@mail.gmail.com>
From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <9e75b3dd-380b-c868-728f-46379e53bc11@gmail.com>
Date:   Mon, 15 Jul 2019 07:26:39 -0700
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HiRob,

Sorry for such a late reply...


On 7/1/19 8:25 PM, Saravana Kannan wrote:
> On Mon, Jul 1, 2019 at 6:32 PM Rob Herring <robh+dt@kernel.org> wrote:
>>
>> On Mon, Jul 1, 2019 at 6:48 PM Saravana Kannan <saravanak@google.com> wrote:
>>>
>>> Add device-links after the devices are created (but before they are
>>> probed) by looking at common DT bindings like clocks and
>>> interconnects.
>>>
>>> Automatically adding device-links for functional dependencies at the
>>> framework level provides the following benefits:
>>>
>>> - Optimizes device probe order and avoids the useless work of
>>>   attempting probes of devices that will not probe successfully
>>>   (because their suppliers aren't present or haven't probed yet).
>>>
>>>   For example, in a commonly available mobile SoC, registering just
>>>   one consumer device's driver at an initcall level earlier than the
>>>   supplier device's driver causes 11 failed probe attempts before the
>>>   consumer device probes successfully. This was with a kernel with all
>>>   the drivers statically compiled in. This problem gets a lot worse if
>>>   all the drivers are loaded as modules without direct symbol
>>>   dependencies.
>>>
>>> - Supplier devices like clock providers, interconnect providers, etc
>>>   need to keep the resources they provide active and at a particular
>>>   state(s) during boot up even if their current set of consumers don't
>>>   request the resource to be active. This is because the rest of the
>>>   consumers might not have probed yet and turning off the resource
>>>   before all the consumers have probed could lead to a hang or
>>>   undesired user experience.
>>>
>>>   Some frameworks (Eg: regulator) handle this today by turning off
>>>   "unused" resources at late_initcall_sync and hoping all the devices
>>>   have probed by then. This is not a valid assumption for systems with
>>>   loadable modules. Other frameworks (Eg: clock) just don't handle
>>>   this due to the lack of a clear signal for when they can turn off
>>>   resources. This leads to downstream hacks to handle cases like this
>>>   that can easily be solved in the upstream kernel.
>>>
>>>   By linking devices before they are probed, we give suppliers a clear
>>>   count of the number of dependent consumers. Once all of the
>>>   consumers are active, the suppliers can turn off the unused
>>>   resources without making assumptions about the number of consumers.
>>>
>>> By default we just add device-links to track "driver presence" (probe
>>> succeeded) of the supplier device. If any other functionality provided
>>> by device-links are needed, it is left to the consumer/supplier
>>> devices to change the link when they probe.
>>>
>>> Signed-off-by: Saravana Kannan <saravanak@google.com>
>>> ---
>>>  drivers/of/Kconfig    |  9 ++++++++
>>>  drivers/of/platform.c | 52 +++++++++++++++++++++++++++++++++++++++++++
>>>  2 files changed, 61 insertions(+)
>>>
>>> diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
>>> index 37c2ccbefecd..7c7fa7394b4c 100644
>>> --- a/drivers/of/Kconfig
>>> +++ b/drivers/of/Kconfig
>>> @@ -103,4 +103,13 @@ config OF_OVERLAY
>>>  config OF_NUMA
>>>         bool
>>>
>>> +config OF_DEVLINKS
>>
>> I'd prefer this not be a config option. After all, we want one kernel
>> build that works for all platforms.
> 
> We need a lot more changes before one kernel build can work for all
> platforms. At least until then, I think we need this. Lot less chance
> of breaking existing platforms before all the missing pieces are
> created.
> 
>> A kernel command line option to disable might be useful for debugging.
> 
> Or we can have a command line to enable this for platforms that want
> to use it and have it default off.

Given the fragility of the current boot sequence (without this patch set)
and the potential breakage of existing systems, I think that if we choose
to accept this patch set that it should first bake in the -next tree for
at least one major release cycle.  Maybe even two major release cycles.

-Frank


> 
>>> +       bool "Device links from DT bindings"
>>> +       help
>>> +         Common DT bindings like clocks, interconnects, etc represent a
>>> +         consumer device's dependency on suppliers devices. This option
>>> +         creates device links from these common bindings so that consumers are
>>> +         probed only after all their suppliers are active and suppliers can
>>> +         tell when all their consumers are active.
>>> +
>>>  endif # OF
>>> diff --git a/drivers/of/platform.c b/drivers/of/platform.c
>>> index 04ad312fd85b..a53717168aca 100644
>>> --- a/drivers/of/platform.c
>>> +++ b/drivers/of/platform.c
>>> @@ -61,6 +61,57 @@ struct platform_device *of_find_device_by_node(struct device_node *np)
>>>  EXPORT_SYMBOL(of_find_device_by_node);
>>>
>>>  #ifdef CONFIG_OF_ADDRESS
>>> +static int of_link_binding(struct device *dev, char *binding, char *cell)
>>
>> Under CONFIG_OF_ADDRESS seems like a strange location.
> 
> Yeah, but the rest of the file seems to be under this. So I'm not
> touching that. I can probably move this function further down (close
> to platform populate) if you want that.
>>
>>> +{
>>> +       struct of_phandle_args sup_args;
>>> +       struct platform_device *sup_dev;
>>> +       unsigned int i = 0, links = 0;
>>> +       u32 dl_flags = DL_FLAG_AUTOPROBE_CONSUMER;
>>> +
>>> +       while (!of_parse_phandle_with_args(dev->of_node, binding, cell, i,
>>> +                                          &sup_args)) {
>>> +               i++;
>>> +               sup_dev = of_find_device_by_node(sup_args.np);
>>> +               if (!sup_dev)
>>> +                       continue;
>>> +               if (device_link_add(dev, &sup_dev->dev, dl_flags))
>>> +                       links++;
>>> +               put_device(&sup_dev->dev);
>>> +       }
>>> +       if (links < i)
>>> +               return -ENODEV;
>>> +       return 0;
>>> +}
>>> +
>>> +/*
>>> + * List of bindings and their cell names (use NULL if no cell names) from which
>>> + * device links need to be created.
>>> + */
>>> +static char *link_bindings[] = {
>>
>> const
> 
> Ack
> 
>>
>>> +#ifdef CONFIG_OF_DEVLINKS
>>> +       "clocks", "#clock-cells",
>>> +       "interconnects", "#interconnect-cells",
>>
>> Planning to add others?
> 
> Not in this patch.
> 
> Regulators are the other big missing piece that I'm aware of now but
> they need a lot of discussion (see email from David and my reply).
> 
> Not sure what other resources are shared where they can be "turned
> off" and cause devices set up at boot to fail. For example, I don't
> think interrupts need functional dependency tracking because they
> aren't really turned off by consumer 1 in a way that breaks things for
> consumer 2. Just masked and the consumer 2 can unmask and use it once
> it probes.
> 
> I'm only intimately familiar with clocks, interconnects and regulators
> (to some extent). I'm open to adding other supplier categories in
> future patches as I educate myself of those or if other people want to
> add support for more categories.
> 
> -Saravana
> 
>>> +#endif
>>> +};
>>> +
>>> +static int of_link_to_suppliers(struct device *dev)
>>> +{
>>> +       unsigned int i = 0;
>>> +       bool done = true;
>>> +
>>> +       if (unlikely(!dev->of_node))
>>> +               return 0;
>>> +
>>> +       for (i = 0; i < ARRAY_SIZE(link_bindings) / 2; i++)
>>> +               if (of_link_binding(dev, link_bindings[i * 2],
>>> +                                       link_bindings[i * 2 + 1]))
>>> +                       done = false;
>>> +
>>> +       if (!done)
>>> +               return -ENODEV;
>>> +       return 0;
>>> +}
>>> +
>>>  /*
>>>   * The following routines scan a subtree and registers a device for
>>>   * each applicable node.
>>> @@ -524,6 +575,7 @@ static int __init of_platform_default_populate_init(void)
>>>         if (!of_have_populated_dt())
>>>                 return -ENODEV;
>>>
>>> +       platform_bus_type.add_links = of_link_to_suppliers;
>>>         /*
>>>          * Handle certain compatibles explicitly, since we don't want to create
>>>          * platform_devices for every node in /reserved-memory with a
>>> --
>>> 2.22.0.410.gd8fdbe21b5-goog
>>>
>