Re: [PATCH 0/6] hwspinlock: allow sharing of hwspinlocks

[Fabien DESSENNE <fabien.dessenne@st.com>]

Hi


On 06/08/2019 11:30 PM, Suman Anna wrote:
> On 8/6/19 1:21 PM, Bjorn Andersson wrote:
>> On Tue 06 Aug 10:38 PDT 2019, Suman Anna wrote:
>>
>>> Hi Fabien,
>>>
>>> On 8/5/19 12:46 PM, Bjorn Andersson wrote:
>>>> On Mon 05 Aug 01:48 PDT 2019, Fabien DESSENNE wrote:
>>>>
>>>>> On 01/08/2019 9:14 PM, Bjorn Andersson wrote:
>>>>>> On Wed 13 Mar 08:50 PDT 2019, Fabien Dessenne wrote:
>> [..]
>>>>> B/ This would introduce some inconsistency between the two 'request' API
>>>>> which are hwspin_lock_request() and hwspin_lock_request_specific().
>>>>> hwspin_lock_request() looks for an unused lock, so requests for an exclusive
>>>>> usage. On the other side, request_specific() would request shared locks.
>>>>> Worst the following sequence can transform an exclusive usage into a shared
>>>>>
>>>> There is already an inconsistency in between these; as with above any
>>>> system that uses both request() and request_specific() will be suffering
>>>> from intermittent failures due to probe ordering.
>>>>
>>>>> one:
>>>>>     -hwspin_lock_request() -> returns Id#0 (exclusive)
>>>>>     -hwspin_lock_request() -> returns Id#1 (exclusive)
>>>>>     -hwspin_lock_request_specific(0) -> returns Id#0 and makes Id#0 shared
>>>>> Honestly I am not sure that this is a real issue, but it's better to have it
>>>>> in mind before we take ay decision
>>> Wouldn't it be actually simpler to just introduce a new specific API
>>> variant for this, similar to the reset core for example (it uses a
>>> separate exclusive API), without having to modify the bindings at all.
>>> It is just a case of your driver using the right API, and the core can
>>> be modified to use the additional tag semantics based on the API. It
>>> should avoid any confusion with say using a different second cell value
>>> for the same lock in two different nodes.
>>>
>> But this implies that there is an actual need to hold these locks
>> exclusively. Given that they are (except for the raw case) all wrapped
>> by Linux locking primitives there shouldn't be a problem sharing a lock
>> (except possibly for the raw case).
> Yes agreed, the HWLOCK_RAW and HWLOCK_IN_ATOMIC cases are unprotected. I
> am still trying to understand better the usecase to see if the same lock
> is being multiplexed for different protection contexts, or if all of
> them are protecting the same context.


Here are two different examples that explain the need for changes.
In both cases the Linux clients are talking to a single entity on the
remote-side.

Example 1:
     exti: interrupt-controller@5000d000 {
         compatible = "st,stm32mp1-exti", "syscon";
         interrupt-controller;
         #interrupt-cells = <2>;
         reg = <0x5000d000 0x400>;
         hwlocks = <&hsem 1>;
     };
The two drivers (stm32mp1-exti and syscon) refer to the same hwlock.
With the current hwspinlock implementation, only the first driver succeeds
in requesting (hwspin_lock_request_specific) the hwlock. The second request
fails.
Here, we really need to share the hwlock between the two drivers.
Note: hardware spinlock support for regmap was 'recently' introduced in 4.15
see https://lore.kernel.org/patchwork/patch/845941/



Example 2:
Here it is more a question of optimization : we want to save the number of
hwlocks used to protect resources, using an unique hwlock to protect all
pinctrl resources:
         pinctrl: pin-controller@50002000 {
             compatible = "st,stm32mp157-pinctrl";
             ranges = <0 0x50002000 0xa400>;
             hwlocks = <&hsem 0 1>;

         pinctrl_z: pin-controller-z@54004000 {
             compatible = "st,stm32mp157-z-pinctrl";
             ranges = <0 0x54004000 0x400>;
             pins-are-numbered;
             hwlocks = <&hsem 0 1>;

>
>> I agree that we shouldn't specify this property in DT - if anything it
>> should be a variant of the API.


If we decide to add a 'shared' API, then, what about the generic regmap 
driver?

In the context of above example1, this would require to update the 
regmap driver.

But would this be acceptable for any driver using syscon/regmap?


I think it is better to keep the existing API (modifying it so it always 
allows

hwlocks sharing, so no need for bindings update) than adding another API.



>>
>>> If you are sharing a hwlock on the Linux side, surely your driver should
>>> be aware that it is a shared lock. The tag can be set during the first
>>> request API, and you look through both tags when giving out a handle.
>>>
>> Why would the driver need to know about it?
> Just the semantics if we were to support single user vs multiple users
> on Linux-side to even get a handle. Your point is that this may be moot
> since we have protection anyway other than the raw cases. But we need to
> be able to have the same API work across all cases.
>
> So far, it had mostly been that there would be one user on Linux
> competing with other equivalent peer entities on different processors.
> It is not common to have multiple users since these protection schemes
> are usually needed only at the lowest levels of a stack, so the
> exclusive handle stuff had been sufficient.
>
>>> Obviously, the hwspin_lock_request() API usage semantics always had the
>>> implied additional need for communicating the lock id to the other peer
>>> entity, so a realistic usage is most always the specific API variant. I
>>> doubt this API would be of much use for the shared driver usage. This
>>> also implies that the client user does not care about specifying a lock
>>> in DT.
>>>
>> Afaict if the lock are shared then there shouldn't be a problem with
>> some clients using the request API and others request_specific(). As any
>> collisions would simply mean that there are more contention on the lock.
>>
>> With the current exclusive model that is not possible and the success of
>> the request_specific will depend on probe order.
>>
>> But perhaps it should be explicitly prohibited to use both APIs on the
>> same hwspinlock instance?
> Yeah, they are meant to be complimentary usage, though I doubt we will
> ever have any realistic users for the generic API if we haven't had a
> usage so far. I had posted a concept of reserved locks long back [1] to
> keep away certain locks from the generic requestor, but dropped it since
> we did not have an actual use-case needing it.
>
> regards
> Suman
>
> [1] https://lwn.net/Articles/611944/
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