Date:   Wed, 20 May 2020 14:41:22 -0600
From:   Jeffrey Hugo <jhugo@...eaurora.org>
To:     bbhatt@...eaurora.org
Cc:     manivannan.sadhasivam@...aro.org, linux-arm-msm@...r.kernel.org,
        hemantk@...eaurora.org, linux-kernel@...r.kernel.org,
        linux-kernel-owner@...r.kernel.org
Subject: Re: [PATCH v3 2/7] bus: mhi: core: Introduce independent voting
 mechanism

On 5/20/2020 1:44 PM, bbhatt@...eaurora.org wrote:
> On 2020-05-20 12:06, Jeffrey Hugo wrote:
>> On 5/20/2020 12:43 PM, bbhatt@...eaurora.org wrote:
>>> On 2020-05-20 09:54, Jeffrey Hugo wrote:
>>>> On 5/18/2020 2:03 PM, Bhaumik Bhatt wrote:
>>>>> Allow independent votes from clients such that they can choose to vote
>>>>> for either the device or the bus or both. This helps in cases where 
>>>>> the
>>>>> device supports autonomous low power mode wherein it can move to M2
>>>>> state without the need to notify the host. Clients can also vote 
>>>>> only to
>>>>> keep the underlying bus active without having the device in M0 
>>>>> state to
>>>>> support offload use cases.
>>>>>
>>>>> Signed-off-by: Bhaumik Bhatt <bbhatt@...eaurora.org>
>>>>> ---
>>>>
>>>> I wonder, why doesn't this fit with runtimePM?
>>> Hi Jeff,
>>>
>>> Can you elaborate?
>>>
>>> In short, with this patch, MHI just wants to give controller the 
>>> option to
>>> choose the vote type so we can implement autonomous low power mode 
>>> entries
>>> on both host and device.
>>
>> So, you are attempting to manage the power mode of the device.  The
>> standard mechanism to do so in Linux is runtime pm.
>>
>> https://elixir.bootlin.com/linux/latest/source/Documentation/driver-api/pm/devices.rst 
>>
>>
>> I'm no runtime pm expert, but it feels like your whole voting
>> mechanism, etc is just reimplemeting that.  Reimplementing the wheel,
>> when its been a standard thing that the majority of the kernel uses is
>> not usually acceptable.
>>
>> IMO, you need some sort of justification why runtime pm is not
>> applicable for you, because I'm willing to bet Mani/Greg are going to
>> ask the same.
> I think we can look at the patch as simply expanding the scope of what 
> already exists.
> 
> The client here has been calling mhi_device_get/put/sync APIs to gain 
> device vote and with
> new features yet to come in, this introductory change is only 
> re-purposing what voting
> means going forward. i.e. allowing individual bus and device votes.
> 
> If you're suggesting using runtimePM APIs to replace the newly 
> introduced bus vote, it
> would be kind of overkill here IMO. Is that what you were getting at? 
> Because currently,
> we just have controllers use runtimePM and provide callbacks to them.
> 
> If you have ideas, we can discuss them.

Ultimately, yes I think I am suggesting replacing this API with the 
runtime pm API.

As near as I can tell, if I'm a device driver on some other bus, and I 
want to keep my device alive because I'm doing a DMA to it or something, 
I would call pm_runtime_get(), and when I have confirmation my activity 
is done, I would use pm_runtime_put().

As near as I can tell, this is already plumbed into the bus framework, 
such that the MHI bus would get a callback when the device driver does 
that.  In the MHI bus callback, you would be able to route the request 
as needed.

So, with this API (that has no consumers currently), I call 
pm_runtime_get() and also mhi_device_get()?

-- 
Jeffrey Hugo
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.

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