linux-kernel - Re: [PATCH v2 1/2] mailbox: add async request mechanism to empower controllers w/ hw queues

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Message-ID: <a7274a6e-1da3-47f2-8725-b0c534bf6608@linaro.org>
Date: Fri, 18 Oct 2024 08:49:28 +0100
From: Tudor Ambarus <tudor.ambarus@...aro.org>
To: Jassi Brar <jassisinghbrar@...il.com>
Cc: krzk@...nel.org, alim.akhtar@...sung.com, mst@...hat.com,
 javierm@...hat.com, tzimmermann@...e.de, bartosz.golaszewski@...aro.org,
 luzmaximilian@...il.com, sudeep.holla@....com, conor.dooley@...rochip.com,
 bjorn@...osinc.com, ulf.hansson@...aro.org,
 linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, marcan@...can.st, neal@...pa.dev,
 alyssa@...enzweig.io, broonie@...nel.org, andre.draszik@...aro.org,
 willmcvicker@...gle.com, peter.griffin@...aro.org, kernel-team@...roid.com,
 vincent.guittot@...aro.org, daniel.lezcano@...aro.org
Subject: Re: [PATCH v2 1/2] mailbox: add async request mechanism to empower
 controllers w/ hw queues

Hi, Jassi,

Thanks for the review!

On 10/18/24 5:17 AM, Jassi Brar wrote:
> On Thu, Oct 17, 2024 at 11:36 AM Tudor Ambarus <tudor.ambarus@...aro.org> wrote:
>>
>> Current form of the mailbox framework doesn't allow controllers to benefit
>> of their hardware queue capabilities as the framework handles a single
>> active request at a time.
>>
>> The active request is considered completed when TX completes. But it seems
>> that TX is not in direct relation with RX,
>>
> Correct, and it is not meant to be.
> You are assuming there is always an RX in response to a TX, which is

Not really. If there's no response expected, clients can set req->rx to
NULL. Then the controllers know that no response is expected and can
complete the request when TX completes.

> not the case. Many platforms just send a message and only need to know
> when it is sent. Many platforms only listen for incoming messages.

these 2 cases are covered with the req approach.

> Many platforms have TX and RX but not as parts of one exchange. In

I don't think I understand this case. Is it related to what you describe
below?

> fact, only minority of platforms expect RX after each TX. Btw, what if

Right, I noticed.

> some platform sends only and always after each receive? For these

This case is covered as well with the req approach. One just needs to
serialize the requests:

ret = mbox_send_request(dc->mbox_chan, req1);
ret = mbox_wait_request(ret, req1->wait);
if (ret)
	return ret;

// req1 completed, send req2
ret = mbox_send_request(dc->mbox_chan, req2);
ret = mbox_wait_request(ret, req2->wait);
if (ret)
	return ret;
	
This shall work regardless if the client expects a response or not. If
no response is expected, but just a TX completion, then the client can
set req->rx = NULL.

> reasons, it is left to the user to tie an incoming RX to some previous
> TX, or not.

It's possible I haven't covered all the cases, but I'm willing to
understand them and come up with a new version with your help, where I
address all the concerns.

Cheers,
ta