Re: [RFC][PATCH 0/5] Functional dependencies between devices

[Tomeu Vizoso <tomeu.vizoso@collabora.com>]

On 14 January 2016 at 02:52, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> On Tuesday, October 27, 2015 04:24:14 PM Rafael J. Wysocki wrote:
>> Hi All,
>>
>> As discussed in the recent "On-demand device probing" thread and in a Kernel
>> Summit session earlier today, there is a problem with handling cases where
>> functional dependencies between devices are involved.
>>
>> What I mean by a "functional dependency" is when the driver of device B needs
>> both device A and its driver to be present and functional to be able to work.
>> This implies that the driver of A needs to be working for B to be probed
>> successfully and it cannot be unbound from the device before the B's driver.
>> This also has certain consequences for power management of these devices
>> (suspend/resume and runtime PM ordering).
>>
>> So I want to be able to represent those functional dependencies between devices
>> and I'd like the driver core to track them and act on them in certain cases
>> where they matter.  The argument for doing that in the driver core is that
>> there are quite a few distinct use cases related to that, they are relatively
>> hard to get right in a driver (if one wants to address all of them properly)
>> and it only gets worse if multiplied by the number of drivers potentially
>> needing to do it.  Morever, at least one case (asynchronous system suspend/resume)
>> cannot be handled in a single driver at all, because it requires the driver of A
>> to wait for B to suspend (during system suspend) and the driver of B to wait for
>> A to resume (during system resume).
>>
>> My idea is to represent a supplier-consumer dependency between devices (or
>> more precisely between device+driver combos) as a "link" object containing
>> pointers to the devices in question, a list node for each of them and some
>> additional information related to the management of those objects, ie.
>> something like:
>>
>> struct device_link {
>>       struct device *supplier;
>>       struct list_head supplier_node;
>>       struct device *consumer;
>>       struct list_head consumer_node;
>>       <flags, status etc>
>> };
>>
>> In general, there will be two lists of those things per device, one list
>> of links to consumers and one list of links to suppliers.
>>
>> In that picture, links will be created by calling, say:
>>
>> int device_add_link(struct device *me, struct device *my_supplier, unsigned int flags);
>>
>> and they will be deleted by the driver core when not needed any more.  The
>> creation of a link should also cause dpm_list and the list used during shutdown
>> to be reordered if needed.
>>
>> In principle, it seems usefult to consider two types of links, one created
>> at device registration time (when registering the second device from the linked
>> pair, whichever it is) and one created at probe time (of the consumer device).
>> I'll refer to them as "permanent" and "probe-time" links, respectively.
>>
>> The permanent links (created at device registration time) will stay around
>> until one of the linked devices is unregistered (at which time the driver
>> core will drop the link along with the device going away).  The probe-time
>> ones will be dropped (automatically) at the consumer device driver unbind time.
>>
>> There's a question about what if the supplier device is being unbound before
>> the consumer one (for example, as a result of a hotplug event).  My current
>> view on that is that the consumer needs to be force-unbound in that case too,
>> but I guess I may be persuaded otherwise given sufficiently convincing
>> arguments.  Anyway, there are reasons to do that, like for example it may
>> help with the synchronization.  Namely, if there's a rule that suppliers
>> cannot be unbound before any consumers linked to them, than the list of links
>> to suppliers for a consumer can only change at its registration/probe or
>> unbind/remove times (which simplifies things quite a bit).
>>
>> With that, the permanent links existing at the probe time for a consumer
>> device can be used to check whether or not to defer the probing of it
>> even before executing its probe callback.  In turn, system suspend
>> synchronization should be a matter of calling device_pm_wait_for_dev()
>> for all consumers of a supplier device, in analogy with dpm_wait_for_children(),
>> and so on.
>>
>> Of course, the new lists have to be stable during those operations and ensuring
>> that is going to be somewhat tricky (AFAICS right now at least), but apart from
>> that the whole concept looks reasonably straightforward to me.
>>
>
> What follows is my prototype implementation of this.  It took some time
> to develop (much more than I was hoping for), but here it goes at last.
>
> The first patch rearranges the code around __device_release_driver() a bit
> to prepare it for the next one.
>
> The second patch introduces the actual device links mechanics, but without
> system suspend/resume and runtime PM support which are added by the subsequent
> patches.
>
> This hasn't been really tested yet (apart from checking that it doesn't break
> things when device links are not in used, which would be rather embarrassing),
> but at this time I'd really like you to have a look and tell me what you think
> (especially if you see a reason why this is not going to work).

Hi Rafael,

have given a quick look and I have 2 questions for now:

- Why deferring the probe if a supplier isn't ready? Seems like quite
a bit of a waste to keep iterating that list until all suppliers have
probed. If we know that a supplier is needed at a given time, why not
probe it right away?

- When were you thinking of calling device_link_add for permanent links?

I also wonder if we could find clearer names for supplier_links and
consumer_links, as it wasn't immediately clear to me what those lists
contained. Maybe just "consumers" and "suppliers"?

Thanks,

Tomeu