linux-kernel - Re: [PATCH 2/2] mailbox: mtk-cmdq: Move pm_runimte_get and put to mbox_chan

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Date: Wed, 19 Jun 2024 10:38:15 -0500
From: Jassi Brar <jassisinghbrar@...il.com>
To: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
Cc: Jason-JH Lin (林睿祥) <Jason-JH.Lin@...iatek.com>, 
	"linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>, 
	Singo Chang (張興國) <Singo.Chang@...iatek.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"chunkuang.hu@...nel.org" <chunkuang.hu@...nel.org>, Nancy Lin (林欣螢) <Nancy.Lin@...iatek.com>, 
	Project_Global_Chrome_Upstream_Group <Project_Global_Chrome_Upstream_Group@...iatek.com>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, 
	"matthias.bgg@...il.com" <matthias.bgg@...il.com>
Subject: Re: [PATCH 2/2] mailbox: mtk-cmdq: Move pm_runimte_get and put to
 mbox_chan_ops API

On Wed, Jun 19, 2024 at 3:18 AM AngeloGioacchino Del Regno
<angelogioacchino.delregno@...labora.com> wrote:
> Il 18/06/24 17:59, Jassi Brar ha scritto:
.....

> For example, when static content is displayed on screen, the CMDQ mailbox never
> gets shut down, but no communication happens for a relatively long time; the
> overhead of actually shutting down the mailbox and setting it back up would be
> increasing latency in an unacceptable manner.
>
Hmm...  in your driver,  startup() is _empty_   and  shutdown() is all
under a spin-lock with irqs disabled, so that too shouldn't be
expensive. Right?
Then what causes unacceptable latencies?

> This is why I opted for autosuspend - it's only bringing down certain clocks for
> the CMDQ HW, adding up a bit of power saving to the mix which, for some use cases
> such as mobile devices with relatively small batteries, is definitely important.
>
> I'll also briefly (and only briefly) mention that 120Hz displays are already a
> common thing and in this case the gap between TX and ACK is ~8.33ms instead, let
> alone that displays with a framerate of more than 120Hz also do exist even though
> they're less common.
>
I don't know how even busier channels help your point.

> All of the above describes a few of the reasons why autosuspend is a good choice
> here, instead of a shutdown->startup flow.
> And again - I can place some bets that PM would also be applicable to SoCs from
> other vendors as well, with most probably different benefits (but still with some
> power related benefits!) compared to MediaTek.
>
Sure, if some platform _actually_ has a high channel
startup()/shutdown() cost, it may need finer PM control. And I have a
way for that, but even with that I am sure someone from MTK will
realize they didn't need that.

Thanks.