On Wed, Aug 22, 2007 at 08:09:18PM +0200, Matthias Hopf wrote: > On Aug 22, 07 17:21:09 +0300, Daniel Stone wrote: > > > But that exactly makes version control difficult (lots of cases, > > > especially if the exact requirements aren't clear from day one). > > > > I honestly don't understand the problem. Your ioctl number is _that_ > > request. It's not referring to a vague concept, but a single structure > > you've already specified. > > Well, you already know what parameters you need for selecting > crtcs/outputs/modes/etc.? I would assume that the needed parameters will > change over time, as new features are thought of etc. And I could > imagine that this happens frequently.
Yes. So, just as with requests, when you change the parameters, you no longer have the same ioctl. So you renumber. > > If you have ioctl number 32, your next free one is 43, and you want to > > change the semantics of UPDATE_WINDOW, then you rename 32 to > > UPDATE_WINDOW_OLD, create UPDATE_WINDOW as 43 with the new structure, > > and handle both cases. > > I consider this giving headaches. > I never had any similar issues with a tag-based approach. The problem there comes with the added complexity of parsing all the data. > > > Without a version field, this is made impossible, something that must be > > > avoided at all costs. > > > > Except that your ioctl then becomes variably-sized unless you add loads > > of padding and hope for the best. > > If every tag is int32 and its parameter int32 as well, this isn't too > problematic. Question is whether everything needed can be represented in > 32 bit values, strings are a completely different concept and don't > match well here, as pointers are probably difficult to handle over the > user land / kernel barrier... Easy to use, hard to use right. Cheer,s Daniel
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