linux-kernel - Re: [PATCH 09/17] m68k: Implement xor_unlock_is_negative

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Message-ID: <ZQm67lGOBBdC2Dl9@casper.infradead.org>
Date:   Tue, 19 Sep 2023 16:14:54 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Greg Ungerer <gregungerer@...tnet.com.au>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH 09/17] m68k: Implement xor_unlock_is_negative_byte

On Tue, Sep 19, 2023 at 02:35:25PM +0000, David Laight wrote:
> From: Matthew Wilcox <willy@...radead.org>
> > Sent: 19 September 2023 15:26
> > 
> > On Tue, Sep 19, 2023 at 01:23:08PM +0000, David Laight wrote:
> > > > Well, that sucks.  What do you suggest for Coldfire?
> > >
> > > Can you just do a 32bit xor ?
> > > Unless you've got smp m68k I'd presume it is ok?
> > > (And assuming you aren't falling off a page.)
> > 
> > Patch welcome.
> 
> My 68020 book seems to be at work and I'm at home.
> (The 286, 386 and cy7c600 (sparc 32) books don't help).
> 
> But if the code is trying to do *ptr ^= 0x80 and check the
> sign flag then you just need to use eor.l with 0x80000000
> on the same address.

I have a 68020 book; what I don't have is a Coldfire manual.

Anyway, that's not the brief.  We're looking to (eg) clear bit 0
and test whether bit 7 was set.  So it's the sign bit of the byte,
not the sign bit of the int.