linux-kernel - Re: [PATCH v5 2/8] firmware: add support for ARM System Control and Power Interface(SCPI) protocol

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Date:	Thu, 30 Jul 2015 23:26:56 +0530
From:	Jassi Brar <jassisinghbrar@...il.com>
To:	Sudeep Holla <sudeep.holla@....com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Liviu Dudau <Liviu.Dudau@....com>,
	Punit Agrawal <Punit.Agrawal@....com>,
	"Jon Medhurst (Tixy)" <tixy@...aro.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Arnd Bergmann <arnd@...db.de>, Olof Johansson <olof@...om.net>,
	Kevin Hilman <khilman@...nel.org>
Subject: Re: [PATCH v5 2/8] firmware: add support for ARM System Control and
 Power Interface(SCPI) protocol

On Wed, Jul 29, 2015 at 6:20 PM, Sudeep Holla <sudeep.holla@....com> wrote:
> On 29/07/15 12:19, Jassi Brar wrote:
>
>> Assuming the former, let me explain. When a client receives a
>> response, it can be sure that the request has already been read by the
>> remote.
>
> Waiting for the response would be too late for few expensive commands
> (e.g setting up external regulators). The remote firmware acknowledges
> Tx by setting status flags and will be ready to accept new commands.
>
No. Polling still happens. If anything, mbox_client_txdone() should
only speed up things.

>> If the protocol specifies every request has some response, the
>
> Not always true there can be few commands without response. The protocol
> specifies that we need check the status flag before sending the new
> command as it's bidirectional, hence polling is recommended (Section 2.2
> Communication flow in the SCPI specification)
>
mbox_client_txdone() will only be called for commands that has some
response. Commands that don't have a response would be completed by
polling.

>> client should assert 'knows_txdone' and call mbox_client_txdone() upon
>> receiving a reply packet.
>
> Since this is not always true and not recommended in the specification,
> I am hesitant to use this option as the firmware can always change their
> internal mechanics without breaking the protocol. We need to ensure we are
> compliant to the spec.
>
I don't see how it could break compliance.

>> So I said,  cl->knows_txdone = false;   is the root of problems you
>
> It could be and won't rule that out. I would prefer using knows_txdone
> and use mbox_client_txdone if feasible, but I can't as the without
> violating the specification.
>
> FYI, I had tried it and ended up with issues in the firmware. The
> argument from the firmware is that we aren't specification compliant,
> so I had to use polling.
>
I am sure you would have copy of that discarded code. Care to share? I
can't imagine how we handle completions locally could affect the
remote. The mbox_client_txdone() is untested so I don't rule out bugs,
otherwise it should only make things better.

Jassi
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