linux-kernel - Re: [PATCH v2 3/9] arm: twr-k70f120m: clock driver for Kinetis SoC

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Date:	Wed, 15 Jul 2015 19:34:09 +0200 (CEST)
From:	Paul Osmialowski <pawelo@...g.net.pl>
To:	Linus Walleij <linus.walleij@...aro.org>
cc:	Paul Osmialowski <pawelo@...g.net.pl>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Jiri Slaby <jslaby@...e.cz>, Kumar Gala <galak@...eaurora.org>,
	Mark Rutland <mark.rutland@....com>,
	Michael Turquette <mturquette@...libre.com>,
	Pawel Moll <pawel.moll@....com>,
	Rob Herring <robh+dt@...nel.org>,
	Russell King <linux@....linux.org.uk>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Vinod Koul <vinod.koul@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, linux-clk@...r.kernel.org,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	dmaengine@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Paul Bolle <pebolle@...cali.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Uwe Kleine-Koenig <u.kleine-koenig@...gutronix.de>,
	Anson Huang <b20788@...escale.com>,
	Frank Li <Frank.Li@...escale.com>,
	Jingchang Lu <jingchang.lu@...escale.com>,
	Rob Herring <r.herring@...escale.com>,
	Yuri Tikhonov <yur@...raft.com>,
	Sergei Poselenov <sposelenov@...raft.com>,
	Alexander Potashev <aspotashev@...raft.com>
Subject: Re: [PATCH v2 3/9] arm: twr-k70f120m: clock driver for Kinetis SoC

Hi Linus,

I had some discussion with an expert here and now I see drawbacks of using 
struct for regs, I'll turn it into defines as you suggested.

On Wed, 15 Jul 2015, Paul Osmialowski wrote:

> Hi Linus,
> 
> Thanks for all of your comments, I'll consider them during my works on the 
> next iteration. However, I have doubts about this one:
> 
> On Tue, 14 Jul 2015, Linus Walleij wrote:
> 
> > On Tue, Jun 30, 2015 at 2:27 PM, Paul Osmialowski <pawelo@...g.net.pl> wrote:
> > 
> > > Based on K70P256M150SF3RM.pdf K70 Sub-Family Reference Manual, Rev. 3.
> > >
> > > Signed-off-by: Paul Osmialowski <pawelo@...g.net.pl>
> > (...)
> > > +struct kinetis_sim_regs {
> > > +       u32 sopt1;      /* System Options Register 1 */
> > > +       u32 rsv0[1024];
> > > +       u32 sopt2;      /* System Options Register 2 */
> > > +       u32 rsv1;
> > > +       u32 sopt4;      /* System Options Register 4 */
> > > +       u32 sopt5;      /* System Options Register 5 */
> > > +       u32 sopt6;      /* System Options Register 6 */
> > > +       u32 sopt7;      /* System Options Register 7 */
> > > +       u32 rsv2[2];
> > > +       u32 sdid;       /* System Device Identification Register */
> > > +       u32 scgc[KINETIS_SIM_CG_NUMREGS];       /* Clock Gating Regs 1...7 */
> > > +       u32 clkdiv1;    /* System Clock Divider Register 1 */
> > > +       u32 clkdiv2;    /* System Clock Divider Register 2 */
> > > +       u32 fcfg1;      /* Flash Configuration Register 1 */
> > > +       u32 fcfg2;      /* Flash Configuration Register 2 */
> > > +       u32 uidh;       /* Unique Identification Register High */
> > > +       u32 uidmh;      /* Unique Identification Register Mid-High */
> > > +       u32 uidml;      /* Unique Identification Register Mid Low */
> > > +       u32 uidl;       /* Unique Identification Register Low */
> > > +       u32 clkdiv3;    /* System Clock Divider Register 3 */
> > > +       u32 clkdiv4;    /* System Clock Divider Register 4 */
> > > +       u32 mcr;        /* Misc Control Register */
> > > +};
> > 
> > Now there is this design pattern where you copy the datasheet
> > register map to a struct again.
> > 
> > This is not good if there is a second revision of the hardware and some
> > registers are shuffled around. IMO it is better to just use #defines
> > for register
> > offsets, so you can do exceptions later. Else a new hardware revision
> > leads to a new struct with new accessor functions etc etc.
> > 
> 
> I don't see how replacing this structure with bunch of defines could make 
> anyones life easier. As registers are shuffled around due to updated 
> hardware revision they could be shuffled in this structure too. Doing 
> this with buch of defines would require eager and careful adaptation of 
> all the defines. I don't see how this could be easier.
> 
> Note that I'm not making any instances of the structure (it is used only 
> for casting), so shuffling its fields around should not affect the code 
> that follows.
> 
> After recent purge it is only used within this macro:
> 
> #define KINETIS_SIM_PTR(base, reg) \
>         (&(((struct kinetis_sim_regs *)(base))->reg))
> 
> ...and used like this:
> 
>         ioread32(KINETIS_SIM_PTR(sim, clkdiv1));
> 
> IMHO changing the struct internals does not require subsequent changes in 
> any of those.
> 
> Thanks,
> Paul
> 
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