linux-kernel - Re: [PATCH] unaligned: delete 1-byte accessors

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Message-ID: <20190722052244.GA4235@avx2>
Date:   Mon, 22 Jul 2019 08:22:44 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, axboe@...nel.dk, kvalo@...eaurora.org,
        john.johansen@...onical.com, linux-arch@...r.kernel.org
Subject: Re: [PATCH] unaligned: delete 1-byte accessors

On Mon, Jul 22, 2019 at 08:08:33AM +0900, James Bottomley wrote:
> On Mon, 2019-07-22 at 00:52 +0300, Alexey Dobriyan wrote:
> > Each and every 1-byte access is aligned!
> 
> The design idea of this is for parsing descriptors.  We simply chunk up
> the describing structure using get_unaligned for everything.  The
> reason is because a lot of these structures come with reserved areas
> which we may make use of later.  If we're using get_unaligned for
> everything we can simply change a u8 to a u16 in the structure
> absorbing the reserved padding.  With your change now I'd have to chase
> down every byte access and replace it with get_unaligned instead of
> simply changing the structure.
> 
> What's the significant advantage of this change that compensates for
> the problems the above causes?

HW descriptors have fixed endianness, you're supposed to use
get_unaligned_be32() and friends.

For that matter, drivers/scsi/ has exactly 2 get_unaligned() calls one of
which can be changed to get_unaligned_be32().