linux-kernel - Re: [PATCH] rtc: jz4740: Add support for the JZ4725B, JZ4760, JZ4770

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Date:   Mon, 23 Jul 2018 08:18:55 -0600
From:   Rob Herring <robh@...nel.org>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Mark Rutland <mark.rutland@....com>,
        "open list:REAL TIME CLOCK (RTC) SUBSYSTEM" 
        <linux-rtc@...r.kernel.org>, devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rtc: jz4740: Add support for the JZ4725B, JZ4760, JZ4770

On Fri, Jul 20, 2018 at 1:04 PM Paul Cercueil <paul@...pouillou.net> wrote:
>
> Hi,
>
> Le ven. 20 juil. 2018 à 17:39, Rob Herring <robh@...nel.org> a écrit :
> > On Sat, Jul 14, 2018 at 03:50:08PM +0200, Paul Cercueil wrote:
> >>
> >>
> >>  Le sam. 14 juil. 2018 à 15:32, Alexandre Belloni
> >>  <alexandre.belloni@...tlin.com> a écrit :
> >>  > On 14/07/2018 15:25:33+0200, Paul Cercueil wrote:
> >>  > >  Hi Alexandre,
> >>  > >
> >>  > >  Le sam. 14 juil. 2018 à 15:19, Alexandre Belloni
> >>  > >  <alexandre.belloni@...tlin.com> a écrit :
> >>  > >  > Hello,
> >>  > >  >
> >>  > >  > On 13/07/2018 17:14:24+0200, Paul Cercueil wrote:
> >>  > >  > >  The RTC in the JZ4725B works just like the one in the
> >> JZ4740.
> >>  > >  > >
> >>  > >  > >  The RTC in the JZ4760 and JZ4770 work just like the one
> >> in the
> >>  > >  > > JZ4780.
> >>  > >  > >
> >>  > >  > >  Signed-off-by: Paul Cercueil <paul@...pouillou.net>
> >>  > >  > >  ---
> >>  > >  > >
> >> Documentation/devicetree/bindings/rtc/ingenic,jz4740-rtc.txt
> >>  > > |  3
> >>  > >  > > +++
> >>  > >  > >   drivers/rtc/rtc-jz4740.c
> >>  > > | 11
> >>  > >  > > ++++++++++-
> >>  > >  > >   2 files changed, 13 insertions(+), 1 deletion(-)
> >>  > >  > >
> >>  > >  > >  diff --git
> >>  > >  > >
> >> a/Documentation/devicetree/bindings/rtc/ingenic,jz4740-rtc.txt
> >>  > >  > >
> >> b/Documentation/devicetree/bindings/rtc/ingenic,jz4740-rtc.txt
> >>  > >  > >  index 41c7ae18fd7b..a9e821de84f2 100644
> >>  > >  > >  ---
> >>  > > a/Documentation/devicetree/bindings/rtc/ingenic,jz4740-rtc.txt
> >>  > >  > >  +++
> >>  > > b/Documentation/devicetree/bindings/rtc/ingenic,jz4740-rtc.txt
> >>  > >  > >  @@ -4,6 +4,9 @@ Required properties:
> >>  > >  > >
> >>  > >  > >   - compatible: One of:
> >>  > >  > >     - "ingenic,jz4740-rtc" - for use with the JZ4740 SoC
> >>  > >  > >  +  - "ingenic,jz4725b-rtc" - for use with the JZ4725B SoC
> >>  > >  > >  +  - "ingenic,jz4760-rtc" - for use with the JZ4760 SoC
> >>  > >  > >  +  - "ingenic,jz4770-rtc" - for use with the JZ4770 SoC
> >>  > >  > >     - "ingenic,jz4780-rtc" - for use with the JZ4780 SoC
> >>  > >  > >   - reg: Address range of rtc register set
> >>  > >  > >   - interrupts: IRQ number for the alarm interrupt
> >>  > >  > >  diff --git a/drivers/rtc/rtc-jz4740.c
> >>  > > b/drivers/rtc/rtc-jz4740.c
> >>  > >  > >  index d0a891777f44..1c867e3a0ea5 100644
> >>  > >  > >  --- a/drivers/rtc/rtc-jz4740.c
> >>  > >  > >  +++ b/drivers/rtc/rtc-jz4740.c
> >>  > >  > >  @@ -54,6 +54,9 @@
> >>  > >  > >
> >>  > >  > >   enum jz4740_rtc_type {
> >>  > >  > >    ID_JZ4740,
> >>  > >  > >  + ID_JZ4725B,
> >>  > >  > >  + ID_JZ4760,
> >>  > >  > >  + ID_JZ4770,
> >>  > >  >
> >>  > >  > I wouldn't introduce those ids unless there are handling
> >>  > > differences at
> >>  > >  > some point.
> >>  > >
> >>  > >  Well there are handling differences, see below.
> >>  > >
> >>  > >  > >    ID_JZ4780,
> >>  > >  > >   };
> >>  > >  > >
> >>  > >  > >  @@ -114,7 +117,7 @@ static inline int
> >>  > > jz4740_rtc_reg_write(struct
> >>  > >  > > jz4740_rtc *rtc, size_t reg,
> >>  > >  > >   {
> >>  > >  > >    int ret = 0;
> >>  > >  > >
> >>  > >  > >  - if (rtc->type >= ID_JZ4780)
> >>  > >  > >  + if (rtc->type >= ID_JZ4760)
> >>  > >  >
> >>  > >  > This would avoid that change (and the test would preferably
> >> be
> >>  > >  > (rtc->type == ID_JZ4780))
> >>  > >
> >>  > >  That branch should be taken if the SoC is JZ4760, JZ4770 or
> >> JZ4780.
> >>  > >  It should not be taken if the SoC is JZ4740 or JZ4725B.
> >>  >
> >>  > Sure but you can achieve that with only 2 ids...
> >>  >
> >>  > >
> >>  > >  > >            ret = jz4780_rtc_enable_write(rtc);
> >>  > >  > >    if (ret == 0)
> >>  > >  > >            ret = jz4740_rtc_wait_write_ready(rtc);
> >>  > >  > >  @@ -300,6 +303,9 @@ static void jz4740_rtc_power_off(void)
> >>  > >  > >
> >>  > >  > >   static const struct of_device_id jz4740_rtc_of_match[] =
> >> {
> >>  > >  > >    { .compatible = "ingenic,jz4740-rtc", .data = (void
> >>  > > *)ID_JZ4740 },
> >>  > >  > >  + { .compatible = "ingenic,jz4725b-rtc", .data = (void
> >>  > > *)ID_JZ4725B
> >>  > >  > > },
> >>  > >  > >  + { .compatible = "ingenic,jz4760-rtc", .data = (void
> >>  > > *)ID_JZ4760 },
> >>  > >  > >  + { .compatible = "ingenic,jz4770-rtc", .data = (void
> >>  > > *)ID_JZ4770 },
> >>  >
> >>  > By doing the correct mapping here e.g:
> >>  >
> >>  > { .compatible = "ingenic,jz4725b-rtc", .data = (void *)ID_JZ4740
> >> },
> >>
> >>  Not very pretty and future-proof if you ask me...
> >
> > Looks to me like this can be handled entirely in DT without driver
> > changes like the other patches. Correct usage of compatible strings is
> > what gives you future-proofing. And no driver change is better than
> > needless changing.
>
> If I make e.g. the jz4760 and jz4770 use the jz4780 compatible string,
> but
> then I want to implement a new feature that only exists on the jz4780,
> how
> can I do it without breaking everything?

You specify both:

compatible = "ingenic,jz4760-rtc", "ingenic,jz4780-rtc";

You match on the less specific compatible until you have features or
bugs to handle for the more specific compatible. In your case, you
would have to start matching on the 4760 and 4770 and not support the
feature for those. A bit backwards from normal, but would still work.

> > It may look a bit wierd if 4780 is the fallback for 4770, but if the
> > 4780 is older, then that actually makes sense.
>
> The 4770 is older than the 4780.

As I said, it's weird, but it will all still work. You could fix this
if you want. It would only matter if you had a old dtb with a new
kernel and care about that case working.

Rob