From: Daniel Axtens <dja@axtens•net>
To: Matt Brown <matthew.brown.dev@gmail•com>, linuxppc-dev@lists•ozlabs.org
Subject: Re: [v5 2/2] raid6/altivec: Add vpermxor implementation for raid6 Q syndrome
Date: Wed, 02 Aug 2017 09:01:26 +1000 [thread overview]
Message-ID: <878tj2sxix.fsf@linkitivity.dja.id.au> (raw)
In-Reply-To: <20170428055809.2675-2-matthew.brown.dev@gmail.com>
Hi Matt,
> The raid6 Q syndrome check has been optimised using the vpermxor
> instruction.
Very much a nit, but normally we'd write the change that the patch makes
as a command: "Optimise the raid6 Q syndrome generation using the
vpermxor instruction" - see
https://www.kernel.org/doc/html/v4.11/process/submitting-patches.html#describe-your-changes
> +static void noinline raid6_vpermxor$#_gen_syndrome_real(int disks, size_t bytes,
> + void **ptrs)
> +{
> + u8 **dptr = (u8 **)ptrs;
> + u8 *p, *q;
> + int d, z, z0;
> + unative_t wp$$, wq$$, wd$$;
> +
> + z0 = disks - 3; /* Highest data disk */
> + p = dptr[z0+1]; /* XOR parity */
> + q = dptr[z0+2]; /* RS syndrome */
> +
> + for (d = 0; d < bytes; d += NSIZE*$#) {
> + wp$$ = wq$$ = *(unative_t *)&dptr[z0][d+$$*NSIZE];
> +
> + for (z = z0-1; z>=0; z--) {
> + wd$$ = *(unative_t *)&dptr[z][d+$$*NSIZE];
> + /* P syndrome */
> + wp$$ = vec_xor(wp$$, wd$$);
> +
> + /*Q syndrome */
> + asm("vpermxor %0,%1,%2,%3":"=v"(wq$$):"v"(gf_high), "v"(gf_low), "v"(wq$$));
Initially I thought "why can't we break this over 2 lines?" and then I
remembered that the awk script can't handle that. A space between /* and
Q would be good though.
> + wq$$ = vec_xor(wq$$, wd$$);
I generated some of the unrolled code and inspected it. It's non-trivial
to follow but that's justifiable, it's due to:
- the complex maths
- the unrolling process
- consistency with the altivec code, which I think is worth keeping
I am not sure how you could make it any easier to read, so I don't think
that should block its acceptance into the kernel.
I am confident that this code works correctly and as described.
Reviewed-by: Daniel Axtens <dja@axtens•net>
Regards,
Daniel
> --
> 2.9.3
next prev parent reply other threads:[~2017-08-01 23:01 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-28 5:58 [v5 1/2] lib/raid6: Build proper files on corresponding arch Matt Brown
2017-04-28 5:58 ` [v5 2/2] raid6/altivec: Add vpermxor implementation for raid6 Q syndrome Matt Brown
2017-08-01 23:01 ` Daniel Axtens [this message]
2017-08-02 2:04 ` Michael Ellerman
2017-08-02 0:20 ` Daniel Axtens
2017-08-04 1:40 ` Matt Brown
2017-08-01 22:27 ` [v5 1/2] lib/raid6: Build proper files on corresponding arch Daniel Axtens
2017-08-02 2:00 ` Michael Ellerman
2017-08-04 1:33 ` Matt Brown
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