linux-kernel - Re: [PATCH v3] mailbox: pcc: Support HW-Reduced Communication Subspace type 2

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Date:	Mon, 27 Jun 2016 11:27:42 -0700
From:	Hoan Tran <hotran@....com>
To:	Jassi Brar <jassisinghbrar@...il.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Ashwin Chaugule <ashwin.chaugule@...aro.org>,
	Robert Moore <robert.moore@...el.com>,
	Alexey Klimov <alexey.klimov@....com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux acpi <linux-acpi@...r.kernel.org>, Loc Ho <lho@....com>,
	Duc Dang <dhdang@....com>,
	"Prakash, Prashanth" <pprakash@...eaurora.org>
Subject: Re: [PATCH v3] mailbox: pcc: Support HW-Reduced Communication
 Subspace type 2

Hi Jassi and Rafael,

On Wed, Jun 15, 2016 at 9:19 AM, Prakash, Prashanth
<pprakash@...eaurora.org> wrote:
>
>
> On 6/9/2016 4:43 PM, Hoan Tran wrote:
>> Hi Prashanth,
>>
>> On Thu, Jun 9, 2016 at 3:25 PM, Prakash, Prashanth
>> <pprakash@...eaurora.org> wrote:
>>>
>>> On 6/9/2016 2:47 PM, Hoan Tran wrote:
>>>> Hi Ashwin and Prashanth,
>>>>
>>>> On Wed, Jun 8, 2016 at 5:41 PM, Hoan Tran <hotran@....com> wrote:
>>>>> Hi Prashanth,
>>>>>
>>>>>
>>>>> On Wed, Jun 8, 2016 at 5:32 PM, Prakash, Prashanth
>>>>> <pprakash@...eaurora.org> wrote:
>>>>>> On 6/8/2016 10:24 AM, Hoan Tran wrote:
>>>>>>> Hi Ashwin,
>>>>>>>
>>>>>>> On Wed, Jun 8, 2016 at 5:18 AM, Ashwin Chaugule
>>>>>>> <ashwin.chaugule@...aro.org> wrote:
>>>>>>>> + Prashanth (Can you please have a look as well?)
>>>>>>>>
>>>>>>>> On 31 May 2016 at 15:35, Hoan Tran <hotran@....com> wrote:
>>>>>>>>> Hi Ashwin,
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> Sorry about the delay. I'm in the middle of switching jobs and
>>>>>>>> locations, so its been a bit crazy lately.
>>>>>>> It's ok and hope you're doing well.
>>>>>>>
>>>>>>>> I dont have any major
>>>>>>>> concerns with this code, although there could be subtle issues with
>>>>>>>> this IRQ thing. In this patchset, your intent is to add support for
>>>>>>>> PCC subspace type 2. But you're also adding support for tx command
>>>>>>>> completion which is not specific to Type 2. We could support that even
>>>>>>>> in Type 1. Hence I wanted to separate the two, not just for review,
>>>>>>>> but also the async IRQ completion has subtle issues esp. in the case
>>>>>>>> of async platform notification, where you could have a PCC client in
>>>>>>>> the OS writing to the cmd bit and the platform sending an async
>>>>>>>> notification by writing to some bits in the same 8byte address as the
>>>>>>>> cmd bit. So we need some mutual exclusivity there, otherwise the OS
>>>>>>>> and platform could step on each other. Perhaps Prashanth has better
>>>>>>>> insight into this.
>>>>>>> I think, this mutual exclusivity could be in another patch.
>>>>>> Ashwin,
>>>>>> Sorry, I am not sure how we can prevent platform and OSPM from stepping on
>>>>>> each other.  There is a line is spec that says "all operations on status field
>>>>>> must be made using interlocked operations", but not sure what these
>>>>>> interlocked operation translates to.
>>>>> Yes, I had the same question about how to prevent it.
>>>> For platform notification, if the hardware doesn't support interlocked
>>>> operations. I think we can use a workaround that, platform triggers
>>>> interrupt to OSPM without touching status field. The OSPM PCC client
>>>> will decide what to do with this interrupt. For example, OSPM sends a
>>>> consumer command to check it.
>>> How do we decide which platform can support this interlocked operation?
>>> and how do we decide between a completion notification and platform
>>> notification?
>> Truly, we should follow the specification. But I don't know if there's
>> any hardware support this interlocked operation.
>> For the decide between a completion notification and platform notification
>>  - Completion notification: Bit "Command Complete" is set.
>>  - Platform notification: Bit "Command Complete" is not set.
>>
>>> I think the ACPI spec on platform notification is quite ambiguous and it is
>>> best to get the necessary clarification and/or correction before implementing
>>> anything related to platform notification.
>> Agreed, a clarification inside ACPI Specification is needed
> This patch look good to me, as it doesn't deal with platform notification.
> We can try to get some clarification from spec side before handling the platform
> notification pieces.
>
> Reviewed-by: Prashanth Prakash <pprakash@...eaurora.org>

Do you have plan to apply this patch ?

Thanks
Hoan

>
> Thanks,
> Prashanth
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