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Date: Wed, 13 Jun 2012 08:21:18 -0700
Message-ID: <CAP6odji_JvRkJP1dSzGK5ww3y71h4k6pXyj4pOQBpqP-X7Nn-A@mail.gmail.com>
Subject: Re: [PATCH 2/5] drivers/net/ethernet/dec/tulip: Use standard
 __set_bit_le() function
From: Grant Grundler <grantgrundler@gmail.com>
To: Takuya Yoshikawa <takuya.yoshikawa@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
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On Wed, Jun 13, 2012 at 7:00 AM, Takuya Yoshikawa
<takuya.yoshikawa@gmail.com> wrote:
> On Wed, 13 Jun 2012 22:31:13 +0900
> Akinobu Mita <akinobu.mita@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.

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];
        }

thanks,
grant

>
> ? ? ? ?Takuya
>
>
> ===
> drivers/net/ethernet/dec/tulip/tulip_core.c:
>
> static void build_setup_frame_hash(u16 *setup_frm, struct net_device *dev)
> {
> ? ? ? ?struct tulip_private *tp = netdev_priv(dev);
> ? ? ? ?u16 hash_table[32];
> ? ? ? ?...
> }
>
> drivers/net/ethernet/dec/tulip/de2104x.c:
>
> static void build_setup_frame_hash(u16 *setup_frm, struct net_device *dev)
> {
> ? ? ? ?struct de_private *de = netdev_priv(dev);
> ? ? ? ?u16 hash_table[32];
> ? ? ? ?...
> }
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