linux-kernel - Re: [RFC PATCH 0/3] mca

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Date:	Wed, 10 Oct 2012 21:53:54 +0200
From:	Borislav Petkov <bp@...64.org>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Borislav Petkov <borislav.petkov@....com>,
	"jejb@...isc-linux.org" <jejb@...isc-linux.org>,
	"deller@....de" <deller@....de>
Subject: Re: [RFC PATCH 0/3] mca_config stuff

On Wed, Oct 10, 2012 at 03:35:56PM +0000, Luck, Tony wrote:
> > Therefore, I can toggle the bits in the mce code with mca_cfg.<bitname>.
> > When defining accessing them through the device attributes in sysfs, I
> > use a new macro DEVICE_BIT_ATTR which gets the corresponding bit number
> > of that same bit in the bitfield. This gives only one function which
> > operates on a bitfield instead of a single function per bit in the
> > bitfield.
> 
> Is this true across all architectures? I know that pa-risc instructions
> that operate on bitfields use "0" to operate on the high order bit
> rather than the low order one. I don't recall whether this spills over
> into the compiler. If it did, then you'd have to have different #defines
> for the bit numbers[1].  For this specific use case it wouldn't matter because
> you are just using it in x86 code. But device_store_bit() and device_show_bit()
> are in generic code - so they must be able to work across all architectures.
> 
> -Tony
> 
> [1] Or fix the store/show bit functions to transform the bit numbers from
> "little-bitian" to "big-bitian" on architectures that count the other way.

Ok, the question is whether those device_{show,store}_bit functions
should be really made available in generic code. If yes, then the store case
could be made to work on any arch like this:

       if (val)
               *bvec |= le64_to_cpu(BIT_64(bit));
       else
               *bvec &= le64_to_cpu(~BIT_64(bit));

by keeping the bitnumbers little endian.

IMHO of course.

Thanks.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
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