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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Sat, 21 Nov 2020 10:18:26 +0100
Message-ID: <CAMuHMdVRXxEU_R_Sdi7tSR7y7FoU+fFScsfUCVS+JbXU9BWt8A@mail.gmail.com>
Subject: Re: [PATCH] m68k: Fix WARNING splat in pmac_zilog driver
To:     Finn Thain <fthain@telegraphics.com.au>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Joshua Thompson <funaho@jurai.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        stable <stable@vger.kernel.org>,
        linux-m68k <linux-m68k@lists.linux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
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Hi Finn,

On Sat, Nov 21, 2020 at 12:47 AM Finn Thain <fthain@telegraphics.com.au> wrote:
> On Fri, 20 Nov 2020, Geert Uytterhoeven wrote:
> > On Fri, Nov 20, 2020 at 5:51 AM Finn Thain <fthain@telegraphics.com.au> wrote:
> > > Don't add platform resources that won't be used. This avoids a
> > > recently-added warning from the driver core, that can show up on a
> > > multi-platform kernel when !MACH_IS_MAC.
> > >
> > > ------------[ cut here ]------------
> > > WARNING: CPU: 0 PID: 0 at drivers/base/platform.c:224 platform_get_irq_optional+0x8e/0xce
> > > 0 is an invalid IRQ number
> > > Modules linked in:
> > > CPU: 0 PID: 0 Comm: swapper Not tainted 5.9.0-multi #1
> > > Stack from 004b3f04:
> > >         004b3f04 00462c2f 00462c2f 004b3f20 0002e128 004754db 004b6ad4 004b3f4c
> > >         0002e19c 004754f7 000000e0 00285ba0 00000009 00000000 004b3f44 ffffffff
> > >         004754db 004b3f64 004b3f74 00285ba0 004754f7 000000e0 00000009 004754db
> > >         004fdf0c 005269e2 004fdf0c 00000000 004b3f88 00285cae 004b6964 00000000
> > >         004fdf0c 004b3fac 0051cc68 004b6964 00000000 004b6964 00000200 00000000
> > >         0051cc3e 0023c18a 004b3fc0 0051cd8a 004fdf0c 00000002 0052b43c 004b3fc8
> > > Call Trace: [<0002e128>] __warn+0xa6/0xd6
> > >  [<0002e19c>] warn_slowpath_fmt+0x44/0x76
> > >  [<00285ba0>] platform_get_irq_optional+0x8e/0xce
> > >  [<00285ba0>] platform_get_irq_optional+0x8e/0xce
> > >  [<00285cae>] platform_get_irq+0x12/0x4c
> > >  [<0051cc68>] pmz_init_port+0x2a/0xa6
> > >  [<0051cc3e>] pmz_init_port+0x0/0xa6
> > >  [<0023c18a>] strlen+0x0/0x22
> > >  [<0051cd8a>] pmz_probe+0x34/0x88
> > >  [<0051cde6>] pmz_console_init+0x8/0x28
> > >  [<00511776>] console_init+0x1e/0x28
> > >  [<0005a3bc>] printk+0x0/0x16
> > >  [<0050a8a6>] start_kernel+0x368/0x4ce
> > >  [<005094f8>] _sinittext+0x4f8/0xc48
> > > random: get_random_bytes called from print_oops_end_marker+0x56/0x80 with crng_init=0
> > > ---[ end trace 392d8e82eed68d6c ]---
> > >
> > > Cc: Michael Ellerman <mpe@ellerman.id.au>
> > > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > > Cc: Paul Mackerras <paulus@samba.org>
> > > Cc: Joshua Thompson <funaho@jurai.org>
> > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > Cc: Jiri Slaby <jirislaby@kernel.org>
> > > Cc: stable@vger.kernel.org # v5.8+
> > > References: commit a85a6c86c25b ("driver core: platform: Clarify that IRQ 0 is invalid")
> > > Reported-by: Laurent Vivier <laurent@vivier.eu>
> > > Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
> > > ---
> > > The global platform_device structs provide the equivalent of a direct
> > > search of the OpenFirmware tree, for platforms that don't have OF.
> > > The purpose of that search is discussed in the comments in pmac_zilog.c:
> > >
> > >          * First, we need to do a direct OF-based probe pass. We
> > >          * do that because we want serial console up before the
> > >          * macio stuffs calls us back
> > >
> > > The actual platform bus matching takes place later, with a module_initcall,
> > > following the usual pattern.
> >
> > I think it would be good for this explanation to be part of the
> > actual patch description above.
> >
>
> Thanks for your review.
>
> I take that explanation as read because it was fundamental to the changes
> I made to pmac_zilog.c back in 2009 with commit ec9cbe09899e ("pmac-zilog:
> add platform driver").

That's a long time ago ;-)
I asked because to the casual reader, it's far from obvious why the platform
device use-time is different from the platform device's resources use-time.

> IMO, being that it isn't news, it doesn't belong in the changelog.
> However, I agree that it needs to be documented. How about I add a comment
> to pmac_zilog.c?

Fine for me.

> > > --- a/drivers/tty/serial/pmac_zilog.c
> > > +++ b/drivers/tty/serial/pmac_zilog.c
> > > @@ -1697,18 +1697,17 @@ extern struct platform_device scc_a_pdev, scc_b_pdev;
> > >
> > >  static int __init pmz_init_port(struct uart_pmac_port *uap)
> > >  {
> > > -       struct resource *r_ports;
> > > -       int irq;
> > > +       struct resource *r_ports, *r_irq;
> > >
> > >         r_ports = platform_get_resource(uap->pdev, IORESOURCE_MEM, 0);
> > > -       irq = platform_get_irq(uap->pdev, 0);
> > > -       if (!r_ports || irq <= 0)
> > > +       r_irq = platform_get_resource(uap->pdev, IORESOURCE_IRQ, 0);
> > > +       if (!r_ports || !r_irq)
> > >                 return -ENODEV;
> > >
> > >         uap->port.mapbase  = r_ports->start;
> > >         uap->port.membase  = (unsigned char __iomem *) r_ports->start;
> > >         uap->port.iotype   = UPIO_MEM;
> > > -       uap->port.irq      = irq;
> > > +       uap->port.irq      = r_irq->start;
> > >         uap->port.uartclk  = ZS_CLOCK;
> > >         uap->port.fifosize = 1;
> > >         uap->port.ops      = &pmz_pops;
> >
> > Given the resources are no longer present on !MAC, just doing
> >
> >             r_ports = platform_get_resource(uap->pdev, IORESOURCE_MEM, 0);
> >     +       if (!r_ports)
> >     +               return -ENODEV;
> >             irq = platform_get_irq(uap->pdev, 0);
> >
> > should be sufficient?
>
> I think your suggestion is shorter but not better. Commit a85a6c86c25b
> (which introduced the WARNING) suggests that testing for irq == 0 is
> undesirable. My patch resolves that.
>
> As a bonus, by simply testing for the existence of both resources, I've
> addressed the mistake I made when I originally added the slick
> platform_get_irq() call instead of consistently using
> platform_get_resource().
>
> platform_get_irq() hides a bunch of architecture-specific logic that is
> not appropriate here. The WARNING itself is a good example of that kind of
> logic.
>
> Do you agree? If so, I will add this explanation to the commit log.

OK, your main motivation is to get rid of the zero-check.
Leaving it could indeed trigger some janitorial changes by people who
don't understand the code at all, so it's good to avoid that ;-)

Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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