linux-kernel - Re: [PATCHv4 2/4] serial: imx: add runtime pm support

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Date:	Mon, 17 Aug 2015 17:28:25 -0700
From:	Eduardo Valentin <edubezval@...il.com>
To:	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>,
	Fabio Estevam <festevam@...il.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Linux PM <linux-pm@...r.kernel.org>,
	linux-serial@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	khilman@...nel.org
Subject: Re: [PATCHv4 2/4] serial: imx: add runtime pm support

Bartlomiej,

On Mon, Aug 17, 2015 at 05:40:59PM +0200, Bartlomiej Zolnierkiewicz wrote:
> 
> Hi,
> 
> On Friday, August 14, 2015 09:37:46 PM Eduardo Valentin wrote:
> > This change introduces the runtime pm support on imx serial
> > driver. The objective is to be able to idle the uart
> > port whenever it is not in use while still being able
> > to wake it up when needed. The key changes in this patch are:
> > 1. Move the clock handling to runtime pm. Both, ipg and per,
> > are now handled in the suspend and resume callbacks. Only
> > enabling and disabling the clocks are handled in runtime
> > suspend and resume, so we are able to use runtime pm
> > in IRQ context.
> > 2. Clocks are prepared in probe and unprepared in remove,
> > so we do not need to prepare (may sleep) in runtime pm.
> > 3. We mark the device activity based on uart and console
> > callbacks. Whenever the device is needed and we want to
> > access registers, we runtime_pm_get and then mark its
> > last usage when we are done. This is done also across
> > IRQs and DMA callbacks.
> > 4. We reuse the infrastructure in place for suspend and
> > resume, so we do not need to redo wakeup configuration,
> > or context save and restore.
> > 
> > After this change, the clocks are still sane, in the sense
> > of having balanced clock prepare and enable.
> 
> The clock changes in this patch seem to make this driver
> non-functional with CONFIG_PM=n.  Have you tested your
> changes with CONFIG_PM=n?

This is a valid point. I tried it yes, but maybe because in my
environment I had another user of the pll3_80m I did not see
issues with PM=n. Now after sanitizing my .config, executing
the system with only the uart port / console as user of pll3_80m,
I see the uart console hanging because the uart clocks get gated.

> 
> Generally the driver should not depend on PM support to
> enable its clocks.  We had this issue in few Exynos-specific
> drivers not that long time ago..
> 

No doubt here. I am checking the best way to cover for this case.
Probably I will go for adding the support via the pm_clk*
infrastructure, as already suggested by khilman.

Thanks for spotting this.

BR,

> Best regards,
> --
> Bartlomiej Zolnierkiewicz
> Samsung R&D Institute Poland
> Samsung Electronics
> 

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