Hi Fabien,

On 8/5/19 12:46 PM, Bjorn Andersson wrote:

On Mon 05 Aug 01:48 PDT 2019, Fabien DESSENNE wrote:

quoted

On 01/08/2019 9:14 PM, Bjorn Andersson wrote:

quoted

On Wed 13 Mar 08:50 PDT 2019, Fabien Dessenne wrote:

quoted

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.

Help me understand the problem that you are trying to solve here. Is
this a case of you having two clients on Linux-side needing to use the
same lock but still requiring the arbitration with software running on
some other remote processor? Are they talking to the same entity on the
remote-side or different peers.

I see the series is all about getting a handle so that they can use the
API, and is the expected usage that the same entity will lock and unlock
before the other driver can lock it.

quoted

quoted

quoted

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.

quoted

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.

quoted

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.

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.

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.

regards
Suman

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.

quoted

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.

quoted

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

quoted

BR

Fabien

quoted

Regards,
Bjorn

quoted

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

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