linux-kernel - Re: [PATCH] Input: ims-pcu - remove unneeded get_unaligned

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Message-ID: <20140104061650.GB20272@core.coreip.homeip.net>
Date:	Fri, 3 Jan 2014 22:16:50 -0800
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Andrey Smirnov <andrew.smirnov@...il.com>
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Input: ims-pcu - remove unneeded get_unaligned_xxx

On Fri, Jan 03, 2014 at 09:52:25PM -0800, Andrey Smirnov wrote:
> On Fri, Jan 3, 2014 at 9:28 PM, Dmitry Torokhov
> <dmitry.torokhov@...il.com> wrote:
> > pcu->cmd_buf[IMS_PCU_DATA_OFFSET] is word aligned so we do not need to use
> > get_unaligned_le16 to access it.
> >
> > Also let's add build time check to make sure it stays aligned.
> 
> - AFAIK, there's no guarantee the "pcu" itself is aligned

Yes. kmalloc returns aligned pointer, otherwise every access to members
other than u8 would risk unaligned exception.

> - This change assumes that aligning data on the 2-byte boundary is
> sufficient for all architectures that do not allow unaligned data
> access, which I don't think is a good assumption to make

What arches require word access be double-word aligned?

> - On x86 or any other architecture that allows unaligned access
> get_unaligned_le16() is actually results to call to le16_to_cpup(), so
> this change doesn't really save anything while imposing restrictions
> on the arrangement of the fields in struct ims_pcu and causing
> unnecessary build errors.

Unless somebody changes the layout there won't be any new build errors,
will there?

Thanks.

-- 
Dmitry
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