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

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Date:	Mon, 27 Jun 2016 23:32 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Hoan Tran <hotran@....com>
Cc:	Jassi Brar <jassisinghbrar@...il.com>,
	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

On Monday, June 27, 2016 11:27:42 AM Hoan Tran wrote:
> 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 ?

Yes.

Thanks,
Rafael