Re: [PATCH v6 03/17] media: v4l2: async: Add v4l2_async_notifier_add_subdev

From: Steve Longerbeam
Date: Sat Aug 04 2018 - 12:39:43 EST


Hi Jacopo,


On 08/03/2018 08:13 AM, jacopo mondi wrote:
Hi Steven,
I've a small remark, which is probably not only related to your
patches but was there alreay... Anyway, please read below..


On Mon, Jul 09, 2018 at 03:39:03PM -0700, Steve Longerbeam wrote:
<snip>
+static int __v4l2_async_notifier_register(struct v4l2_async_notifier *notifier)
+{
struct v4l2_async_subdev *asd;
int ret;
int i;
@@ -399,29 +445,25 @@ static int __v4l2_async_notifier_register(struct v4l2_async_notifier *notifier)

mutex_lock(&list_lock);

- for (i = 0; i < notifier->num_subdevs; i++) {
- asd = notifier->subdevs[i];
+ if (notifier->subdevs) {
+ for (i = 0; i < notifier->num_subdevs; i++) {
+ asd = notifier->subdevs[i];

- switch (asd->match_type) {
- case V4L2_ASYNC_MATCH_CUSTOM:
- case V4L2_ASYNC_MATCH_DEVNAME:
- case V4L2_ASYNC_MATCH_I2C:
- case V4L2_ASYNC_MATCH_FWNODE:
- if (v4l2_async_notifier_has_async_subdev(
- notifier, asd, i)) {
- dev_err(dev,
- "asd has already been registered or in notifier's subdev list\n");
- ret = -EEXIST;
+ ret = v4l2_async_notifier_asd_valid(notifier, asd, i);
+ if (ret)
goto err_unlock;
- }
- break;
- default:
- dev_err(dev, "Invalid match type %u on %p\n",
- asd->match_type, asd);
- ret = -EINVAL;
- goto err_unlock;
+
+ list_add_tail(&asd->list, &notifier->waiting);
+ }
+ } else {
+ i = 0;
+ list_for_each_entry(asd, &notifier->asd_list, asd_list) {
+ ret = v4l2_async_notifier_asd_valid(notifier, asd, i++);
Here the call stack looks like this, if I'm not mistaken:

list_for_each_entry(asd, notifier->asd_list, i) {
v4l2_async_notifier_asd_valid(notifier, asd, i):
v4l2_async_notifier_has_async_subdev(notifier, asd, i):
list_for_each_entry(asd_y, notifier->asd_list, j) {
if (j >= i) break;
if (asd == asd_y) return true;
}
}

Which is an optimization of O(n^2), but still bad.

This was there already there, it was:

Agreed, it should be safe to remove the check for duplicate
asd's at notifier registration, since this check is done now
in v4l2_async_notifier_add_subdev().

Steve

for (i = 0; i < notifier->num_subdevs; i++) {
v4l2_async_notifier_has_async_subdev(notifier, notifier->subdevs[i], i):
for (j = 0; j < i; j++) {
if (notifier->subdevs[i] == notifier->subdevs[j])
return true;
}
}
}

We're not talking high performances here, but I see no reason to go through
the list twice, as after switching to use your here introduced
v4l2_async_notifier_add_subdev() async subdevices are tested at endpoint
parsing time in v4l2_async_notifier_fwnode_parse_endpoint(), which
guarantees we can't have doubles later, at notifier registration time.

If I'm not wrong, this can be anyway optimized later.

Thanks
j