linux-kernel - Re: [RFC v2 8/8] drm: tegra: Add gr2d device

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Date:	Mon, 3 Dec 2012 10:40:10 +0100
From:	Daniel Vetter <daniel@...ll.ch>
To:	Mark Zhang <nvmarkzhang@...il.com>
Cc:	Dave Airlie <airlied@...il.com>,
	Terje Bergstrom <tbergstrom@...dia.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Arto Merilainen <amerilainen@...dia.com>
Subject: Re: [RFC v2 8/8] drm: tegra: Add gr2d device

On Mon, Dec 3, 2012 at 10:30 AM, Mark Zhang <nvmarkzhang@...il.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.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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