Re: [PATCH 1/2] mux: add mux_chip_resume() function

[Thomas Richard <thomas.richard@bootlin.com>]

On 9/4/24 11:32, Peter Rosin wrote:
> Hi!
> 
> 2024-09-04 at 11:18, Thomas Richard wrote:
>> On 9/3/24 15:22, Peter Rosin wrote:
>>> Hi!
>>>
>>> Sorry for being unresponsive. And for first writing this in the older v4
>>> thread instead of here.
>>>
>>> 2024-06-13 at 15:07, Thomas Richard wrote:
>>>> The mux_chip_resume() function restores a mux_chip using the cached state
>>>> of each mux.
>>>>
>>>> Signed-off-by: Thomas Richard <thomas.richard@bootlin.com>
>>>> ---
>>>>  drivers/mux/core.c         | 29 +++++++++++++++++++++++++++++
>>>>  include/linux/mux/driver.h |  1 +
>>>>  2 files changed, 30 insertions(+)
>>>>
>>>> diff --git a/drivers/mux/core.c b/drivers/mux/core.c
>>>> index 78c0022697ec..0858cacae845 100644
>>>> --- a/drivers/mux/core.c
>>>> +++ b/drivers/mux/core.c
>>>> @@ -215,6 +215,35 @@ void mux_chip_free(struct mux_chip *mux_chip)
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(mux_chip_free);
>>>>  
>>>> +/**
>>>> + * mux_chip_resume() - restores the mux-chip state
>>>> + * @mux_chip: The mux-chip to resume.
>>>> + *
>>>> + * Restores the mux-chip state.
>>>> + *
>>>> + * Return: Zero on success or a negative errno on error.
>>>> + */
>>>> +int mux_chip_resume(struct mux_chip *mux_chip)
>>>> +{
>>>> +	int ret, i;
>>>> +
>>>> +	for (i = 0; i < mux_chip->controllers; ++i) {
>>>> +		struct mux_control *mux = &mux_chip->mux[i];
>>>> +
>>>> +		if (mux->cached_state == MUX_CACHE_UNKNOWN)
>>>> +			continue;
>>>> +
>>>> +		ret = mux_control_set(mux, mux->cached_state);
>>>
>>> mux_control_set() is an internal helper. It is called from
>>> __mux_control_select() and mux_control_deselect() (and on init...)
>>>
>>> In all those cases, there is no race to reach the mux_control_set()
>>> function, by means of the mux->lock semaphore (or the mux not being
>>> "published" yet).
>>>
>>> I fail to see how resume is safe when mux->lock is ignored?
>>
>> I think I should use mux_control_select() to use the lock.
>> If I ignore the lock, I could have a cache coherence issue.
>>
>> I'll send a new version which use mux_control_select().
>> But if I use mux_control_select(), I have to clean the cache before to
>> call it, if not nothing happen [1].
>>
>> [1]
>> https://elixir.bootlin.com/linux/v6.11-rc6/source/drivers/mux/core.c#L319
> 
> No, calling mux_control_select() in resume context is not an
> option IIUC. That would dead-lock if there is a long-time client
> who has locked the mux in some desired state.

Yes, I didn't thought about it.

> 
> I see no trivial solution to integrate suspend/resume, and do
> not have enough time to think about what a working solutions
> would look like. Sorry.
> 

We maybe have a solution.
Please let me know if it's relevant or not for you:

- Add a get operation in struct mux_control_ops.

- Implement mux_chip_suspend() which reads the state of each mux using
the get operation, and store it in a hardware_state variable (stored in
the mux_control struct).

- The mux_chip_resume uses the hardware_state value to restore all muxes
using mux_control_set().
So if a mux is locked with a long delay, there is no dead-lock.

- If the get operation is not defined, mux_chip_suspend() and
mux_chip_resume() do nothing (maybe a warning or info message could be
useful).

Regards,

Thomas