linux-kernel - Re: [RFT PATCH v3 12/27] of/address: Add infrastructure to declare MMIO as non-posted

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Date:   Fri, 5 Mar 2021 10:05:08 -0600
From:   Rob Herring <robh@...nel.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Hector Martin <marcan@...can.st>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        Marc Zyngier <maz@...nel.org>, Arnd Bergmann <arnd@...nel.org>,
        Olof Johansson <olof@...om.net>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Mark Kettenis <mark.kettenis@...all.nl>,
        Tony Lindgren <tony@...mide.com>,
        Mohamed Mediouni <mohamed.mediouni@...amail.com>,
        Stan Skowronek <stan@...ellium.com>,
        Alexander Graf <graf@...zon.com>,
        Will Deacon <will@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Corbet <corbet@....net>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Hellwig <hch@...radead.org>,
        "David S. Miller" <davem@...emloft.net>,
        devicetree <devicetree@...r.kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Linux Documentation List <linux-doc@...r.kernel.org>,
        Linux Samsung SOC <linux-samsung-soc@...r.kernel.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFT PATCH v3 12/27] of/address: Add infrastructure to declare
 MMIO as non-posted

On Fri, Mar 5, 2021 at 9:13 AM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
>
> On Thu, Mar 4, 2021 at 11:40 PM Hector Martin <marcan@...can.st> wrote:
> >
> > This implements the 'nonposted-mmio' and 'posted-mmio' boolean
> > properties. Placing these properties in a bus marks all child devices as
> > requiring non-posted or posted MMIO mappings. If no such properties are
> > found, the default is posted MMIO.
> >
> > of_mmio_is_nonposted() performs the tree walking to determine if a given
> > device has requested non-posted MMIO.
> >
> > of_address_to_resource() uses this to set the IORESOURCE_MEM_NONPOSTED
> > flag on resources that require non-posted MMIO.
> >
> > of_iomap() and of_io_request_and_map() then use this flag to pick the
> > correct ioremap() variant.
> >
> > This mechanism is currently restricted to Apple ARM platforms, as an
> > optimization.
>
> ...
>
> > @@ -896,7 +899,10 @@ void __iomem *of_iomap(struct device_node *np, int index)
> >         if (of_address_to_resource(np, index, &res))
> >                 return NULL;
> >
> > -       return ioremap(res.start, resource_size(&res));
> > +       if (res.flags & IORESOURCE_MEM_NONPOSTED)
> > +               return ioremap_np(res.start, resource_size(&res));
> > +       else
> > +               return ioremap(res.start, resource_size(&res));
>
> This doesn't sound right. Why _np is so exceptional? Why don't we have
> other flavours (it also rings a bell to my previous comment that the
> flag in ioresource is not in the right place)?
>
> ...
>
> > +       if (res.flags & IORESOURCE_MEM_NONPOSTED)
> > +               mem = ioremap_np(res.start, resource_size(&res));
> > +       else
> > +               mem = ioremap(res.start, resource_size(&res));
> > +
>
> Ditto.
>
> ...
>
> > +       while (node) {
> > +               if (!of_property_read_bool(node, "ranges")) {
> > +                       break;
> > +               } else if (of_property_read_bool(node, "nonposted-mmio")) {
> > +                       of_node_put(node);
> > +                       return true;
> > +               } else if (of_property_read_bool(node, "posted-mmio")) {
> > +                       break;
> > +               }
> > +               parent = of_get_parent(node);
> > +               of_node_put(node);
> > +               node = parent;
> > +       }
>
> I believe above can be slightly optimized. Don't we have helpers to
> traverse to all parents?

We don't. I only found a handful of cases mostly in arch/powerpc.
Given that and this series is big enough already, I don't think we
need a helper as part of it. But patches welcome.

Rob