linux-kernel - RE: [PATCHv4 3/8] asm-generic: introduce be48 unaligned accessors

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Message-ID: <2d8895d24fff46738f98c2dae74fa440@AcuMS.aculab.com>
Date:   Fri, 4 Mar 2022 01:31:26 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Keith Busch' <kbusch@...nel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "axboe@...nel.dk" <axboe@...nel.dk>, "hch@....de" <hch@....de>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        Hannes Reinecke <hare@...e.de>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: RE: [PATCHv4 3/8] asm-generic: introduce be48 unaligned accessors

From: Keith Busch
> Sent: 03 March 2022 20:13
> 
> The NVMe protocol extended the data integrity fields with unaligned
> 48-bit reference tags.

If they are reference tags, are they only interpreted by the
sending system?
In which case they don't need to be big-endian since the
actual value doesn't really matter.

> Provide some helper accessors in preparation for these.
> 
...
> diff --git a/include/asm-generic/unaligned.h b/include/asm-generic/unaligned.h
> index 1c4242416c9f..8fc637379899 100644
> --- a/include/asm-generic/unaligned.h
> +++ b/include/asm-generic/unaligned.h
> @@ -126,4 +126,30 @@ static inline void put_unaligned_le24(const u32 val, void *p)
>  	__put_unaligned_le24(val, p);
>  }
> 
> +static inline void __put_unaligned_be48(const u64 val, __u8 *p)
> +{
> +	*p++ = val >> 40;
> +	*p++ = val >> 32;
> +	*p++ = val >> 24;
> +	*p++ = val >> 16;
> +	*p++ = val >> 8;
> +	*p++ = val;
> +}

Although that matches __put_unaligned_be24() I think I'd use
array indexing not pointer increments.
The compiler will probably generate the same code anyway.

However it is probably better to do:
	put_unaligned_be16(val >> 32, p);
	put_unaligned_be32(val, p + 2);
so you get 2 memory accesses on x86 (etc) instead of 6.

Similarly for __get_unaligned_be48() where it is likely
so make a bigger difference.

	David

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