On Thu, 25 Oct 2012 14:46:09 +0100, Russell King - ARM Linux wrote:

On Thu, Oct 25, 2012 at 03:42:02PM +0200, Jean Delvare wrote:

quoted

On Thu, 25 Oct 2012 14:14:59 +0100, Russell King - ARM Linux wrote:

quoted

You also miss one very very very big point.  This will break every I2C
using userspace program out there unless it is rebuilt - this change will
require the exact right version of those userspace programs for the
kernel that they're being used on.

How that? The extra field is added in a hole, so we don't change the
struct size nor the relative positions of existing fields. Why would
user-space care?

You know the layout of that struct for certain across all Linux supported
architectures, including some of the more obscure ones which may not
require pointers to be aligned?

No I don't, I naively supposed it would be fine. I would expect gcc to
always align pointers even when the architecture doesn't need them to
be, for performance reasons, even when it doesn't have to, as long as
attribute packed isn't used. After all, integers don't _have_ to be
aligned on x86, but gcc aligns them.

The original idea of using the hole in the i2c_msg structure is from
David Brownell, who was apparently familiar with such practice, so I
assumed it was OK. Actually I still assume it is, until someone comes
with an supported architecture where it is not.

-- 
Jean Delvare