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Message-ID: <fd6b84ae605c652f652c25e3b8f3e234b82edd8e.camel@linux.ibm.com>
Subject: Re: [PATCH] powerpc/8xx: Fix clearing of bits 20-23 in ITLB miss
From:   Leonardo Bras <leonardo@linux.ibm.com>
To:     christophe.leroy@c-s.fr, benh@kernel.crashing.org,
        paulus@samba.org, mpe@ellerman.id.au
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Date:   Thu, 20 Feb 2020 18:53:04 -0300
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References: <1cd9c970771ba9f08621ae8357340c93f386bc24.camel@linux.ibm.com>
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On Tue, 2020-02-11 at 01:28 -0300, Leonardo Bras wrote:
> Looks a valid change.
> rlwimi  r10, r10, 0, 0x0f00 means:=20
> r10 =3D ((r10 << 0) & 0x0f00) | (r10 & ~0x0f00) which ends up being
> r10 =3D r10=20
>=20
> On ISA, rlwinm is recommended for clearing high order bits.
> rlwinm  r10, r10, 0, ~0x0f00 means:
> r10 =3D (r10 << 0) & ~0x0f00
>=20
> Which does exactly what the comments suggests.
>=20
> FWIW:
> Reviwed-by: Leonardo Bras <leonardo@linux.ibm.com>


Sorry, I just realized the above was not very clear on my part.

What I meant to say was:
I think your change is correct, as it correctly fixes this line.

I would suggest adding the text bellow to your commit message, making
it easier to understand why rlwimi is not the right instruction clear
bytes 20-23, and why rlwinm is.

The current instruction can be translated to C as:
rlwimi  r10, r10, 0, 0x0f00
r10 =3D ((r10 << 0) & 0x0f00) | (r10 & ~0x0f00) 	->
r10 =3D (r10 & 0x0f00) | (r10 & ~0x0f00)		->
r10 =3D r10

The new proposed instruction can be translated to C as:
rlwinm  r10, r10, 0, ~0x0f00 	->
r10 =3D (r10 << 0) & ~0x0f00

Which clears bits 20-23 as comment on code states.

Best regards,

Leonardo Bras


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