Date: Thu, 14 Jun 2012 07:28:31 +0900
From: Takuya Yoshikawa <takuya.yoshikawa@gmail.com>
To: Grant Grundler <grantgrundler@gmail.com>
Cc: Akinobu Mita <akinobu.mita@gmail.com>,
        Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>,
        akpm@linux-foundation.org, bhutchings@solarflare.com,
        grundler@parisc-linux.org, arnd@arndb.de, benh@kernel.crashing.org,
        avi@redhat.com, mtosatti@redhat.com, linux-net-drivers@solarflare.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH 2/5] drivers/net/ethernet/dec/tulip: Use standard
 __set_bit_le() function
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On Wed, 13 Jun 2012 08:21:18 -0700
Grant Grundler <grantgrundler@gmail.com> wrote:

> >> >> Should this hash_table be converted from u16 hash_table[32] to
> >> >> DECLARE_BITMAP(hash_table, 16 * 32) to ensure that it is aligned
> >> >> on long-word boundary?
> >> >
> >> > I think hash_table is already long-word aligned because it is placed
> >> > right after a pointer.
> >>
> >> I recommend converting to proper bitmap. ?Because such an implicit
> >> assumption is easily broken by someone touching this function.
> >
> > Do you mean something like:
> > ? ? ? ?DECLARE_BITMAP(__hash_table, 16 * 32);
> > ? ? ? ?u16 *hash_table = (u16 *)__hash_table;
> > ?
> >
> > Grant, what do you think about this?
> 
> Hi Takuya,
> two thoughts:
> 1) while I agree with Akinobu and thank him for pointing out a
> _potential_ alignment problem, this is a separate issue and your
> existing patch should go in anyway. There are probably other drivers
> with _potential_ alignment issues. Akinobu could get credit for
> finding them by submitting patches after reviewing calls to set_bit
> and set_bit_le() - similar to what you are doing now.

I prefer approach 1.

hash_table is local in build_setup_frame_hash(), so if further
improvement is also required, we can do that locally there later.

Thanks,
	Takuya


> 2) I generally do not like declaring one type and then using an alias
> of a different type to reference the same memory address. We have a
> simple alternative since hash_table[] is indexed directly only in one
> hunk of code:
>         for (i = 0; i < 32; i++) {
>                 *setup_frm++ = ((u16 *)hash_table)[i];
>                 *setup_frm++ = ((u16 *)hash_table)[i];
>         }
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