linux-kernel - Re: [PATCH v1] x86/platform/intel-mid: Revert "Make 'bt_sfi

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Message-ID: <alpine.DEB.2.20.1712281453270.8594@hadrien>
Date:   Thu, 28 Dec 2017 15:00:22 +0100 (CET)
From:   Julia Lawall <julia.lawall@...6.fr>
To:     Ingo Molnar <mingo@...nel.org>
cc:     Julia Lawall <julia.lawall@...6.fr>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
        Darren Hart <dvhart@...radead.org>,
        platform-driver-x86@...r.kernel.org,
        Bhumika Goyal <bhumirks@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v1] x86/platform/intel-mid: Revert "Make 'bt_sfi_data'
 const"



On Thu, 28 Dec 2017, Ingo Molnar wrote:

>
> * Julia Lawall <julia.lawall@...6.fr> wrote:
>
> > > > [...] There does seem to be a few cases where the field actually does hold an
> > > > integer.  I guess this is not a problem?
> > >
> > > Could you point to such an example?
> >
> > drivers/thermal/intel_soc_dts_thermal.c:#define BYT_SOC_DTS_APIC_IRQ	86
> >
> > and then:
> >
> > static const struct x86_cpu_id soc_thermal_ids[] = {
> >         { X86_VENDOR_INTEL, 6, INTEL_FAM6_ATOM_SILVERMONT1, 0,
> >                 BYT_SOC_DTS_APIC_IRQ},
> >         {}
> > };
> >
> > and finally:
> >
> >  soc_dts_thres_irq = (int)match_cpu->driver_data;
> >
> > Also:
> >
> > arch/x86/kernel/apic/apic.c
> >
> > #define DEADLINE_MODEL_MATCH_REV(model, rev)    \
> > 	{ X86_VENDOR_INTEL, 6, model, X86_FEATURE_ANY, (unsigned long)rev
> > }
> >
> > DEADLINE_MODEL_MATCH_REV ( INTEL_FAM6_BROADWELL_X,      0x0b000020),
> > DEADLINE_MODEL_MATCH_REV ( INTEL_FAM6_HASWELL_CORE,     0x22),
> > etc. (all 2-digit numbers in the remaining case).
>
> Ok - I think in these cases the resulting long->pointer type conversion is a _lot_
> less dangerous than the pointer->long conversion which caused the regression.
>
> So unless the resulting code is excessively ugly, this feels like the right
> approach to me.

The problem is that this case will inevitably have a cast somewhere.  Many
of the values put into the driver_data field really are const, so the type
has to be const void *.  When the value is extracted from the structure,
there will thus need to be a cast on it, and the current cast

ddata = (struct bt_sfi_data *)id->driver_data;

works fine, whether the original structure is const or not.

I also got a couple of complaints about non-pointer types:

arch/x86/kernel/apic/apic.c:621:9: warning: cast from pointer to integer
of different size [-Wpointer-to-int-cast]
   rev = (u32)m->driver_data;

drivers/thermal/intel_soc_dts_thermal.c:68:22: warning: cast from pointer
to integer of different size [-Wpointer-to-int-cast]
  soc_dts_thres_irq = (int)match_cpu->driver_data;

julia