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

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Message-ID: <20171228152431.2zmikdjurx6356ng@gmail.com>
Date:   Thu, 28 Dec 2017 16:24:32 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     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"


* Julia Lawall <julia.lawall@...6.fr> wrote:

> 
> 
> 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.

That's OK as long as the cast is dominantly (long)->(pointer), because that 
doesn't really risk losing the underlying type.

It's the (pointer)->(pointer) and (pointer)->(long) conversions that are the most 
dangerous ones.

> [...]  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.

So since this data structure is not size critical, I'd really suggest using two or 
three fields:

	->driver_data.ptr
	->driver_data.const_ptr
	->driver_data.long

that way the fundamental types remains.

> 
> 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;

These could use driver_data.long or so?

Thanks,

	Ingo