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

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Date: Mon, 24 Jun 2024 12:45:45 -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-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>, 
	Singo Chang (張興國) <Singo.Chang@...iatek.com>, 
	"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 Mon, Jun 24, 2024 at 6:29 AM AngeloGioacchino Del Regno
<angelogioacchino.delregno@...labora.com> wrote:
>
> Il 20/06/24 16:39, Jassi Brar ha scritto:
> > On Thu, Jun 20, 2024 at 1:33 AM Jason-JH Lin (林睿祥)
> > <Jason-JH.Lin@...iatek.com> wrote:
> >>
> >> On Wed, 2024-06-19 at 10:38 -0500, Jassi Brar wrote:
> >>>
> >>> External email : Please do not click links or open attachments until
> >>> you have verified the sender or the content.
> >>>   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?
> >>>
> >>
> >> I found that the BUG report only occurred when opening the camera APP.
> >> Maybe something in imgsys_cmdq_sendtask() is too expensive or somewhere
> >> else in imgsys driver.
> >>
> > If you move anything from submit() into startup(), which is once per
> > lifetime of a channel, it will only make imgsys_cmdq_sendtask()
> > cheaper.
> > Btw, the imgsys code is not public, I don't know how it looks.
> >
> >
> >> I'm wondering why this BUG report is not occurred in display use case
> >> which is more frequent than imgsys use case.
> >> Does this mean sleep() is not always called in pm_runtime_get_sync()
> >> and if we can guarantee this sleep() won't be called, then using
> >> pm_runtime_get_sync() in atomic context is OK?
> >>
> > Instead of hacking around, maybe try using startup() and shutdown()
> > which is for such uses? Maybe request/release channel as part of RPM
> > in your client driver if you are worried about the noise?
> >
> >
> >>>> 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.
> >>
> >> I agree with Angelo's point!
> >>
> > Ok, but you or Angelo still don't explain "unacceptable latencies"
> > when your startup() and shutdown() are empty. You just want api
> > changed, which we can but at least do you part and tell me where the
> > bottleneck (unexpected latency) comes from.
> >
>
> "I want numbers" is a sensible request, honestly I would do the same so I totally
> understand that.
>
> Jason, can you please perform latency measurements on 60Hz and *especially* ISP/cam
> cases while "continuously" calling startup() and shutdown() for every power saving
> operation?
>
To be clear, do with the mailbox channel that you do with the clocks
now, because your startup() is literally and shutdown() is practically
empty.
Call shutdown() when no request has come in for a while, so you know
the client has quiesced likely. So try putting request/release in
higher level RPM with autosuspend.

thanks