linux-kernel - Re: [PATCH/RFC v4 1/2] reset: Add support for dedicated reset controls

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Date:   Wed, 19 Sep 2018 15:16:02 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Alex Williamson <alex.williamson@...hat.com>,
        KVM list <kvm@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC v4 1/2] reset: Add support for dedicated reset controls

Hi Eric,

On Wed, Sep 19, 2018 at 2:09 PM Auger Eric <eric.auger@...hat.com> wrote:
> On 9/17/18 6:39 PM, Geert Uytterhoeven wrote:
> > In some SoCs multiple hardware blocks may share a reset control.
> > The existing reset control API for shared resets will only assert such a
> > reset when the drivers for all hardware blocks agree.
> > The existing exclusive reset control API still allows to assert such a
> > reset, but that impacts all other hardware blocks sharing the reset.
> >
> > Sometimes a driver needs to reset a specific hardware block, and be 100%
> > sure it has no impact on other hardware blocks.  This is e.g. the case
> > for virtualization with device pass-through, where the host wants to
> > reset any exported device before and after exporting it for use by the
> > guest, for isolation.
> >
> > Hence a new flag for dedicated resets is added to the internal methods,
> > with a new public reset_control_get_dedicated() method, to obtain an
> > exclusive handle to a reset that is dedicated to one specific hardware
> > block.
> >
> > This supports both DT-based and lookup-based reset controls.
> >
> > Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> > ---
> > v4:
> >   - New.
> >
> > Notes:
> >   - Dedicated lookup-based reset controls were not tested,
> >   - Several internal functions now take 3 boolean flags, and should
> >     probably be converted to take a bitmask instead,
> >   - I think __device_reset() should call __reset_control_get() with
> >     dedicated=true.  However, that will impact existing users,
>
> why should it?

device_reset{,_optional}() are supposed to reset the passed device.
If the reset is not dedicated, doing so will reset other devices, too.

> > --- a/drivers/reset/core.c
> > +++ b/drivers/reset/core.c
> > @@ -459,9 +459,38 @@ static void __reset_control_put_internal(struct reset_control *rstc)
> >       kref_put(&rstc->refcnt, __reset_control_release);
> >  }
> >
> > +static bool __of_reset_is_dedicated(const struct device_node *node,
> > +                                 const struct of_phandle_args args)
> > +{
> > +     struct of_phandle_args args2;
> > +     struct device_node *node2;
> > +     int index, ret;
> > +
> > +     for_each_node_with_property(node2, "resets") {
> > +             if (node == node2)
> > +                     continue;
> > +
> > +             for (index = 0; ; index++) {
> > +                     ret = of_parse_phandle_with_args(node2, "resets",
> > +                                                      "#reset-cells", index,
> > +                                                      &args2);
> > +                     if (ret)
> > +                             break;
> > +
> > +                     if (args2.np == args.np &&
> > +                         args2.args_count == args.args_count &&
> > +                         !memcmp(args2.args, args.args,
> > +                                 args.args_count * sizeof(args.args[0])))
> > +                             return false;
> You need to call of_node_put(args2.np) (see of_parse_phandle_with_args
> kernel doc)

Thanks, nice catch!

> Isn't it sufficient to check device_node handles are equal?

That would make it work with #reset-cells == 0 only.
If #reset-cells > 0, the reset line specifier includes extra arguments.

On the Renesas SoCs I'm using, there's a single reset controller, so
args.np is always the same.  The actual reset line is specified by
args.args[0].  See the "resets" properties in e.g.
https://git.kernel.org/pub/scm/linux/kernel/git/geert/renesas-drivers.git/tree/arch/arm64/boot/dts/renesas/r8a7795.dtsi

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds