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 13:29:13 +0200
From: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
To: Jassi Brar <jassisinghbrar@...il.com>,
 Jason-JH Lin (林睿祥) <Jason-JH.Lin@...iatek.com>
Cc: "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

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?


Thanks,
Angelo