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Date: Tue, 04 Dec 2012 09:49:41 +0800 From: Mark Zhang <nvmarkzhang@...il.com> To: Daniel Vetter <daniel@...ll.ch> 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 12/03/2012 05:40 PM, Daniel Vetter wrote: > 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. Ah, I see. Thanks. Yes, u64 still works as 32 bit pointer. Mark > -Daniel > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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