netdev - Re: [PATCH] unaligned: delete 1-byte accessors

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Message-ID: <20190722060744.GA24253@avx2>
Date:   Mon, 22 Jul 2019 09:07:44 +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 02:48:46PM +0900, James Bottomley wrote:
> On Mon, 2019-07-22 at 08:22 +0300, Alexey Dobriyan wrote:
> > 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.
> 
> Not if this is an internal descriptor format, which is what this is
> mostly used for.

Maybe, but developer is supposed to look at all struct member usages
while changing types, right?

> > For that matter, drivers/scsi/ has exactly 2 get_unaligned() calls
> > one of which can be changed to get_unaligned_be32().
> 
> You haven't answered the "what is the benefit of this change" question.
>  I mean sure we can do it, but it won't make anything more efficient
> and it does help with the descriptor format to treat every structure
> field the same.

The benefit is less code, come on.

Another benefit is that typoing

	get_unaligned((u16*)p)

for
	get_unaligned((u8*)p)

will get detected.