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:
> >
> >> The current implementation does not allow two different devices to use
> >> a common hwspinlock. This patch set proposes to have, as an option, some
> >> hwspinlocks shared between several users.
> >>
> >> Below is an example that explain the need for this:
> >> 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.
> >>
> >>
> >> The proposed approach does not modify the API, but extends the DT 'hwlocks'
> >> property with a second optional parameter (the first one identifies an
> >> hwlock) that specifies whether an hwlock is requested for exclusive usage
> >> (current behavior) or can be shared between several users.
> >> Examples:
> >> hwlocks = <&hsem 8>; Ref to hwlock #8 for exclusive usage
> >> hwlocks = <&hsem 8 0>; Ref to hwlock #8 for exclusive (0) usage
> >> hwlocks = <&hsem 8 1>; Ref to hwlock #8 for shared (1) usage
> >>
> >> As a constraint, the #hwlock-cells value must be 1 or 2.
> >> In the current implementation, this can have theorically any value but:
> >> - all of the exisiting drivers use the same value : 1.
> >> - the framework supports only one value : 1 (see implementation of
> >> of_hwspin_lock_simple_xlate())
> >> Hence, it shall not be a problem to restrict this value to 1 or 2 since
> >> it won't break any driver.
> >>
> > Hi Fabien,
> >
> > Your series looks good, but it makes me wonder why the hardware locks
> > should be an exclusive resource.
> >
> > How about just making all (specific) locks shared?
>
> Hi Bjorn,
>
> Making all locks shared is a possible implementation (my first
> implementation
> was going this way) but there are some drawbacks we must be aware of:
>
> A/ This theoretically break the legacy behavior (the legacy works with
> exclusive (UNUSED radix tag) usage). As a consequence, an existing driver
> that is currently failing to request a lock (already claimed by another
> user) would now work fine. Not sure that there are such drivers, so this
> point is probably not a real issue.
>
Right, it's possible that a previously misconfigured system now
successfully probes more than one device that uses a particular
spinlock. But such system would be suffering from issues related to e.g.
probe ordering.
So I think we should ignore this issue.
> 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
The case where I can see a
problem with this would be if the two clients somehow would nest their
locking regions.
But generally I think this could consider this an improvement, because
the request_specific() would now be able to acquire its hwlock, with
some additional contention due to the multiple use.
> I could not find any driver using the hwspin_lock_request() API, we
> may decide to remove (or to make deprecated) this API, having
> everything 'shared without any conditions'.
>
It would be nice to have an upstream user of this API.
>
> I can see three options:
> 1- Keep my initial proposition
> 2- Have hwspin_lock_request_specific() using shared locks and
> hwspin_lock_request() using unused (so 'initially' exclusive) locks.
> 3- Have hwspin_lock_request_specific() using shared locks and
> remove/make deprecated hwspin_lock_request().
>
> Just let me know what is your preference.
>
I think we should start with #2 and would like input from e.g. Suman
regarding #3.
Regards,
Bjorn
> BR
>
> Fabien
>
> >
> > Regards,
> > Bjorn
> >
> >> Fabien Dessenne (6):
> >> dt-bindings: hwlock: add support of shared locks
> >> hwspinlock: allow sharing of hwspinlocks
> >> dt-bindings: hwlock: update STM32 #hwlock-cells value
> >> ARM: dts: stm32: Add hwspinlock node for stm32mp157 SoC
> >> ARM: dts: stm32: Add hwlock for irqchip on stm32mp157
> >> ARM: dts: stm32: hwlocks for GPIO for stm32mp157
> >>
> >> .../devicetree/bindings/hwlock/hwlock.txt | 27 +++++--
> >> .../bindings/hwlock/st,stm32-hwspinlock.txt | 6 +-
> >> Documentation/hwspinlock.txt | 10 ++-
> >> arch/arm/boot/dts/stm32mp157-pinctrl.dtsi | 2 +
> >> arch/arm/boot/dts/stm32mp157c.dtsi | 10 +++
> >> drivers/hwspinlock/hwspinlock_core.c | 82 +++++++++++++++++-----
> >> drivers/hwspinlock/hwspinlock_internal.h | 2 +
> >> 7 files changed, 108 insertions(+), 31 deletions(-)
> >>
> >> --
> >> 2.7.4
> >>
