linux-kernel - Re: [PATCH v3] Input: Add EVIOC mechanism for MT slots

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Date:	Sun, 5 Feb 2012 20:40:36 +0100
From:	"Henrik Rydberg" <rydberg@...omail.se>
To:	Chase Douglas <chasedouglas@...il.com>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] Input: Add EVIOC mechanism for MT slots

> > Besides leaving a possible giant stack crash in your code, it assumes
> > memory is somehow magically allocated. Not good practise in low-level
> > programming. You wouldn't use a template this way, would you?
> 
> No, which is why I think this interface is bad. I should be able to
> dynamically set the size of the array, but it's not possible with this
> interface.

It is possible (using num_slots == 0 or creating your own struct), but
not ideal, granted. The patch serves the purpose of definining the
binary interface, the rest is up to userland.

> I think the implementation is fine in terms of how the plumbing works. I
> just don't think this macro should be included. Make the user create the
> struct themselves:
> 
> In linux/input.h:
> 
> struct input_mt_request {
> 	__u32 code;
> 	__s32 values[];
> };

The above (the first) version is not ideal either, since it cannot be
used as it is.

> It could be argued that we should leave the macro around for people who
> want to statically define the size of the request, but I think that is
> leading them down the wrong path. It's easier, but it will lead to
> broken code if you pick the wrong size.

Rather than creating both a suboptimal static and a suboptimal dynamic
version, removing the struct altogether is tempting. All that is
really needed is a clear definition of the binary interface. The rest
can easily be set up in userland, using whatever method is preferred.

Thanks.
Henrik
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