linux-kernel - Re: [PATCH 4/4] arm: dts: omap3-gta04: Add static configuration for devconf1 register

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Message-ID: <20141114155823.GV26481@atomide.com>
Date:	Fri, 14 Nov 2014 07:58:23 -0800
From:	Tony Lindgren <tony@...mide.com>
To:	Grazvydas Ignotas <notasas@...il.com>
Cc:	Paul Walmsley <paul@...an.com>,
	Mark Rutland <mark.rutland@....com>,
	devicetree@...r.kernel.org,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	pawel.moll@....com, ijc+devicetree@...lion.org.uk,
	"Dr. H. Nikolaus Schaller" <hns@...delico.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	robh+dt@...nel.org, Tero Kristo <t-kristo@...com>,
	Tomi Valkeinen <tomi.valkeinen@...com>, bcousson@...libre.com,
	galak@...eaurora.org, Grant Likely <grant.likely@...aro.org>,
	Marek Belisko <marek@...delico.com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 4/4] arm: dts: omap3-gta04: Add static configuration for
 devconf1 register

* Grazvydas Ignotas <notasas@...il.com> [141114 06:38]:
> On Fri, Nov 14, 2014 at 1:58 AM, Tony Lindgren <tony@...mide.com> wrote:
> > * Paul Walmsley <paul@...an.com> [141113 15:01]:
> >> Hi
> >>
> >> On Thu, 13 Nov 2014, Tony Lindgren wrote:
> >>
> >> > * Tomi Valkeinen <tomi.valkeinen@...com> [141113 03:33]:
> >> > > On 12/11/14 17:02, Tony Lindgren wrote:
> >> > >
> >> > > >> And, with a quick grep, I see CONTROL_DEVCONF1 touched in multiple
> >> > > >> places in the kernel. I wonder if adding a pinmux entry for it could
> >> > > >> cause some rather odd problems.
> >> > > >
> >> > > > They can all use pinctrl-single no problem.
> >> > >
> >> > > Can, but don't. That's my worry. If we touch the DEVCONF1 via pinmux,
> >> > > and we have code in mach-omap2 that also touch DEVCONF1, without any
> >> > > knowledge (and locking) between those...
> >> >
> >> > Hmm yeah the McBSP clock mux could be racy as the mux register for
> >> > McBSP is treated as a clock. This register muxes the clock between
> >> > external pin and internal clock. Considering that this should be
> >> > selectable at board level as the external clock probably needs to be
> >> > used if level shifters are being used, it should be really handled by
> >> > pinctrl-single.
> >> >
> >> > The other use for hsmmc.c and pdata-quirks.c for the one time mux for
> >> > MMC clock from the MMC clock pin. That can be done with pinctrl-single
> >> > from the MMC driver too for DT based booting.
> >> >
> >> > Then we just have the save and restore of the registers for
> >> > off-idle.
> >> >
> >> > > So _maybe_ that's not an issue, as the pinmux config we have here is
> >> > > fixed, and done once at boot time, and maybe the code in mach-omap2 that
> >> > > touch DEVCONF1 is also ran just once and not at the same time as the
> >> > > pinmux. But I don't know if that's so.
> >> >
> >> > It seems we could just do a read-only check for McBSP in the clock
> >> > code for the mux register, or even completely drop that code from
> >> > cclock3xxx_data.c and start using the pinctrl for that mux.
> >> >
> >> > Paul & Tero, got any comments here?
> >>
> >> It's best to move all of the SCM register reads/writes to an SCM IP block
> >> driver.  This driver would be the only entity that would touch the SCM IP
> >> block registers - no other code on the system would touch it (perhaps
> >> aside from anything needed for early init).  The SCM driver would enforce
> >> mutual exclusion via a spinlock, so concurrent SCM register modifications
> >> wouldn't flake out.  Then the SCM driver would register clocks with the
> >> CCF, register pins with the pinctrl subsystem, etc. etc.
> >
> > We actually do have that with pinctrl-single + syscon. We certainly
> > need to implement more Linux framework drivers for the SCM registers.
> > Things like regulators, clocks, and PHYs, but they should use
> > pinctrl-single + syscon. See the the pbias-regulator.c for example.
> >
> > Looking at the McBSP clock handling, threre's yet more handling of
> > the same DEVCONF1 mux register in omap2_mcbsp_set_clks_src that gets
> > alled from omap_mcbsp_dai_set_dai_sysclk.
> >
> > To me it seems that if we handle the DEVCONF with pinctrl-single, we
> > don't need most of the McBSP fck code or the omap2_mcbsp_set_clks_src.
> > Having the mux register as the clock enable register is not nice..
> > Who knows what the clock coming from the external pin might be :)
> 
> How will audio do dynamic muxing without that code?
> The pin must be remuxed back to internal clock when audio stops, or
> else PM breaks.

There's a standard way of dealing with that already available. We can
just have runtime_pm functions call pinctrl_pm_select_sleep_state()
and pinctrl_pm_select_default_state().

Regards,

Tony
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