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

[Paul Walmsley]

Hi

On Thu, 13 Nov 2014, Tony Lindgren wrote:

> * Tomi Valkeinen <tomi.valkeinen@xxxxxx> [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.

- Paul
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