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

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Date:	Thu, 9 Jun 2016 13:47:10 -0700
From:	Hoan Tran <hotran@....com>
To:	"Prakash, Prashanth" <pprakash@...eaurora.org>
Cc:	Ashwin Chaugule <ashwin.chaugule@...aro.org>,
	Jassi Brar <jassisinghbrar@...il.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	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>
Subject: Re: [PATCH v3] mailbox: pcc: Support HW-Reduced Communication
 Subspace type 2

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.

Thanks
Hoan

>
>>
>> Hoan,
>> Even if we are not using platform notification, we still need to clear the doorbell
>> interrupt bit in the PCC interrupt handler (Section14.2.2 and 14.4).  I didn't see
>> clearing the doorbell interrupt bit in this patch (and platform is supposed to set
>> it again when it is sending the interrupt again). Did I miss it? or is it intentionally
>> left out to avoid the race that Ashwin mentioned above?
>>
>
> The PCC client driver is supposed to do that. Which mean, the
> mbox_chan_received_data() function should clear it.
>
> Thanks
> Hoan
>
>>
>> Thanks,
>> Prashanth
>>
>>