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Date: Tue, 04 Dec 2012 09:49:41 +0800
From: Mark Zhang <nvmarkzhang@gmail.com>
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To: Daniel Vetter <daniel@ffwll.ch>
CC: Dave Airlie <airlied@gmail.com>, Terje Bergstrom <tbergstrom@nvidia.com>,
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        "linux-tegra@vger.kernel.org" <linux-tegra@vger.kernel.org>,
        Arto Merilainen <amerilainen@nvidia.com>
Subject: Re: [RFC v2 8/8] drm: tegra: Add gr2d device
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On 12/03/2012 05:40 PM, Daniel Vetter wrote:
> On Mon, Dec 3, 2012 at 10:30 AM, Mark Zhang <nvmarkzhang@gmail.com> wrote:
>> I'm new in kernel development. Could you tell me or give me some
>> materials to read that why we need to align the size of IOCTL structures
>> to 64bit? I can understand if we're working in a 64bit kernel but why we
>> need to do this if we're in a 32bit arm kernel? Besides, why the
>> pointers in IOCTL structure should be declared as u64?
> 
> Because in a few years/months you'll have arm64, but still the same
> driver with the same ioctls ... and if the ioctls are not _exactly_
> the same you get to write compat ioctl code which copies the old 32bit
> struct into the 64bit struct the kernel understands. Hence your ioctl
> must be laid out exactly the same for both 32bit and 64bit, which
> happens if you naturally align/pad everything to 64bits and only use
> fixed-sized integers and no pointers.

Ah, I see. Thanks. Yes, u64 still works as 32 bit pointer.

Mark
> -Daniel
>