linux-kernel - Re: [Bug fix]mips 64bits checksum error -- csum_tcpudp

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Date:   Thu, 26 Jan 2017 17:11:00 +0100
From:   Ralf Baechle <ralf@...ux-mips.org>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Mark Zhang <bomb.zhang@...il.com>,
        David Miller <davem@...emloft.net>,
        Alexander Duyck <aduyck@...antis.com>,
        linux-mips@...ux-mips.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Bug fix]mips 64bits checksum error -- csum_tcpudp_nofold

On Thu, Jan 26, 2017 at 07:57:49AM -0800, Alexander Duyck wrote:
> Date:   Thu, 26 Jan 2017 07:57:49 -0800
> From: Alexander Duyck <alexander.duyck@...il.com>
> To: Mark Zhang <bomb.zhang@...il.com>
> Cc: Ralf Baechle <ralf@...ux-mips.org>, David Miller <davem@...emloft.net>,
>  Alexander Duyck <aduyck@...antis.com>, linux-mips@...ux-mips.org,
>  "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
> Subject: Re: [Bug fix]mips 64bits checksum error -- csum_tcpudp_nofold
> Content-Type: text/plain; charset=UTF-8
> 
> On Wed, Jan 25, 2017 at 6:33 PM, Mark Zhang <bomb.zhang@...il.com> wrote:
> > Hi Alex,
> >
> >     Thanks for your reply.
> >     I tested your correction. The result is correct.
> >     The C language will cause this function(csum_tcpudp_nofold) become
> > 176 MIPS instructions. The assemble code is 150 MIPS instruction.
> >     If the MIPS CPU is running as 1GHz, C language cause more 60 nano
> > seconds during send/receive each tcp/udp packet. I'm not sure whether
> > it will cause any negative result if the frequency of CPU was lower.
> > MIPS arch is usually used in networking equipments.
> >     I think Ralf's correction is better.
> >
> >     - Mark
> >
> > Signed-off-by: Ralf Baechle <ralf@...ux-mips.org>
> >
> >  arch/mips/include/asm/checksum.h | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/arch/mips/include/asm/checksum.h b/arch/mips/include/asm/checksum.h
> > index 5c585c5..08b6ba3 100644
> > --- a/arch/mips/include/asm/checksum.h
> > +++ b/arch/mips/include/asm/checksum.h
> > @@ -186,7 +186,9 @@ static inline __wsum csum_tcpudp_nofold(__be32 saddr,
> >         "       daddu   %0, %4          \n"
> >         "       dsll32  $1, %0, 0       \n"
> >         "       daddu   %0, $1          \n"
> > +       "       sltu    $1, %0, $1      \n"
> >         "       dsra32  %0, %0, 0       \n"
> > +       "       daddu   %0, $1          \n"
> >  #endif
> >         "       .set    pop"
> >         : "=r" (sum)
> >
> 
> This looks good to me.
> 
> Acked-by: Alexander Duyck <alexander.h.duyck@...el.com>

I've actually checked in a slightly different version that this which
uses an ADDU rather than a DADDU for the second instruction added.  This
is because the DSRA32 ensures the 32 bit result in %0 is properly
signed extended to 64 bit as required by the MIPS architecture and the
ADDU then simply operates on that 32 bit %0.

  Ralf