At Fri, 25 Apr 2008 12:54:29 +0200,
Sebastian Siewior wrote:
>
> * Jaroslav Kysela | 2008-04-25 10:23:51 [+0200]:
>
> >On Fri, 25 Apr 2008, Takashi Iwai wrote:
> >
> >> At Fri, 25 Apr 2008 09:35:47 +0200 (CEST),
> >> Jaroslav Kysela wrote:
> >> >
> >> > On Fri, 25 Apr 2008, Takashi Iwai wrote:
> >> >
> >> > > > Sure. I applied the simple 'void *device_private_data' patch, because
> >> > > > current usage request is really trivial. We can implement complex code to
> >> > > > handle data for multiple "extra" devices on AC97 bus later.
> >> > >
> >> > > Actually, it's not "used" yet. The ucb1000 reads the data but no one
> >> > > stores yet. And, if its usage request is trivial, we should use "int
> >> >
> >> > Yes, I hope that the appropriate initialization code will be added to SoC
> >> > drivers, too.
> >> >
> >> > > irq" as in the original patch instead of void data and cast.
> >> >
> >> > But other SoC (or other) drivers might want to pass to extra devices on
> >> > AC97 bus something different or more complex. Mark Brown already noted
> >> > that. I would keep it as 'void *'.
> >>
> >> That's the very problem I've been trying to point out.
> >> The void pointer is good if the same driver assigns and casts. But,
> >> in this case, the allocator and the receiver are different. Thus,
> >> there is no guarantee that the data type is what you want. OTOH, if
> >> it's "int irq", this is crystal clear.
> >>
> >> So, in short:
> >>
> >> - if only one device needs such data, it should be a strong type like
> >> "int irq" anyway -- no extra need to cast to void pointer
> >> - if multiple devices need such a pass-away mechanism, then they can
> >> crash because you have no data type check. The void pointer is
> >> dangerous for multiple devices.
> >
> >I see. In this case, I would propose to add a 32-bit "magic" at the
> >start of 'void *' data. How about this modification:
> >
> >diff -r e2ff47e8771b include/ac97_codec.h
> >--- a/include/ac97_codec.h Fri Apr 25 08:29:05 2008 +0200
> >+++ b/include/ac97_codec.h Fri Apr 25 10:22:00 2008 +0200
> >@@ -407,6 +407,9 @@
> > #define AC97_RATES_MIC_ADC 4
> > #define AC97_RATES_SPDIF 5
> >
> >+/* device private data magic number */
> >+#define AC97_PDEVMAGIC_IRQ 0x20495251 /* in ASCII: <space>IRQ */
> >+
> > /*
> > *
> > */
> >@@ -545,6 +547,11 @@ static inline int ac97_can_spdif(struct
> > static inline int ac97_can_spdif(struct snd_ac97 * ac97)
> > {
> > return (ac97->ext_id & AC97_EI_SPDIF) != 0;
> >+}
> >+static inline int ac97_check_pdevdata_magic(struct snd_ac97 * ac97, unsigned int magic)
> >+{
> >+ return (ac97->device_private_data &&
> >+ *((unsigned int *)ac97->device_private_data) == magic);
> > }
> >
> > /* functions */
> >diff -r e2ff47e8771b kernel/drivers/input/touchscreen/ucb1400_ts.c
> >--- a/kernel/drivers/input/touchscreen/ucb1400_ts.c Fri Apr 25 08:29:05 2008 +0200
> >+++ b/kernel/drivers/input/touchscreen/ucb1400_ts.c Fri Apr 25 10:22:00 2008 +0200
> >@@ -492,14 +492,14 @@ static int ucb1400_ts_probe(struct devic
> > goto err_free_devs;
> > }
> >
> >- if (!ucb->ac97->device_private_data) {
> >+ if (!ac97_check_pdevdata_magic(usb->ac97, AC97_PDEVMAGIC_IRQ)) {
> > error = ucb1400_detect_irq(ucb);
> > if (error) {
> > printk(KERN_ERR "UCB1400: IRQ probe failed\n");
> > goto err_free_devs;
> > }
> > } else {
> >- ucb->irq = (int) ucb->ac97->device_private_data;
> >+ ucb->irq = ((int *) ucb->ac97->device_private_data)[1];
> > }
> >
> > error = request_irq(ucb->irq, ucb1400_hard_irq, IRQF_TRIGGER_RISING,
> >
> This is getting beyond what I planned. Now I have to allocate a struct
> and I don't like the void * to int casts. I just did it once to save an
> allocation of 4 bytes for my int *.
> Why is it a problem to keep an anonymous struct?
You describe in your text below :)
> If some one uses a
> wrong struct than it crashes immediatelly or bails out because
> 0x20495251 is way too large be an IRQ. Putting that magic and casting
> for every single possible data blows code for no good reason. Don't
> recover from errors which should not have happen, solve them at root
> level not where the leaves are.
The root level of the problem is that you pass the anonymous data.
It _IS_ unsafe and wrong unless handled properly.
> What I intended in first place is to allocate a private field in the bus
> struct so can pass informations to the lower driver.
As mentioned in my earlier mail, I'm fine with your first patch. The
problem occurs when we generalize it.
> If you need
> multiple arguments, create your own struct put it in the void * slot,
> your driver knows what to do.
Your driver does _not_ know what type it is because the data isn't
created by your driver but the controller driver. And, it's free to
attach your driver to any controller.
thanks,
Takashi
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