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

On Fri, 25 Apr 2008, Takashi Iwai wrote:

At Fri, 25 Apr 2008 10:23:51 +0200 (CEST),
Jaroslav Kysela wrote:

quoted

On Fri, 25 Apr 2008, Takashi Iwai wrote:

quoted

At Fri, 25 Apr 2008 09:35:47 +0200 (CEST),
Jaroslav Kysela wrote:

quoted

On Fri, 25 Apr 2008, Takashi Iwai wrote:

quoted

quoted

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.

quoted

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:

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.

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:

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 11:43:11 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_UIRQ	0x55495251	/* in ASCII: UIRQ, for UCB1400 */
+
 /*
  *
  */

@@ -470,6 +473,15 @@ struct snd_ac97_template {
 	const struct snd_ac97_res_table *res_table;	/* static resolution */
 };
 
+struct ac97_pdevdata {
+	unsigned int magic;	/* see AC97_PDEVMAGIC */
+	union {
+		int ivalue;
+		long lvalue;
+		void *pvalue;
+	} u;
+};
+
 struct snd_ac97 {
 	/* -- lowlevel (hardware) driver specific -- */
 	struct snd_ac97_build_ops * build_ops;

@@ -478,7 +490,7 @@ struct snd_ac97 {
 	/* This field is used by device drivers which serve devices which are
 	 * attached to the AC97 bus.
 	 */
-	void *device_private_data;
+	struct ac97_pdevdata *device_private_data;
 	/* --- */
 	struct snd_ac97_bus *bus;
 	struct pci_dev *pci;	/* assigned PCI device - used for quirks */

@@ -545,6 +557,11 @@ 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 &&
+	       ac97->device_private_data->magic == 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 11:43:11 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_UIRQ)) {
 		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 = ucb->ac97->device_private_data->u.ivalue;
 	}
 
 	error = request_irq(ucb->irq, ucb1400_hard_irq, IRQF_TRIGGER_RISING,

-----

Jaroslav Kysela [off-list ref]
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.