[alsa-devel] [RFC] ucb1400 touchscreen, irq auto probing and ac97 with its private field

[Jaroslav Kysela]

On Fri, 25 Apr 2008, Takashi Iwai wrote:

> > > > I see. In this case, I would propose to add a 32-bit "magic" at the 
> > > > start of 'void *' data. How about this modification:
> > > 
> > > The magic number sounds OK, but funky cast to integer pointer is bad.
> > > If you have a long or a pointer after int, you can have an alignment
> > > problem on 64bit archs, for example.
> > 
> > Not really. First 32-bit word has offset 0 and next 64-bit word has offset 
> > 8 on x86-64. So alignment is OK.
> 
> No, suppose the data like
> 
> 	int magic;
> 	void *ptr;
> 
> and the code to retrive the magic is
> 
> 	magic = *(int*)device_data;
> 
> and thus the next data pointer is (int*)device_data + 1.
> This doesn't work.

+ 1 does not work, but driver can define own private structure and it will 
work. The point is that 32-bit magic must be always at offset 0.

> > Anyway, I agree that with a struct, code is more readable without hidden 
> > things. Also, drivers can define own struct with magic member as first.
> > 
> > Here is corrected patch:
> 
> Hm, but this allows you pass only one value and has an ugly union.

As I mentioned, this structure can be redefined. I can imagine, that 
drivers won't use predefined structure and create own like:

struct mydata {
	unsigned int magic;	/* required */
	unsigned long value1;
	char *name;
	int somevalue;
};

The only required thing is to have 32-bit magic value at offset 0.

						Jaroslav

-----
Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.