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

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.


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.

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

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
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'.


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.

BR

Fabien

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