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Subject[PATCH][RFC] kernel.h: provide array iterator
Date
Simplify array iteration with a helper to iterate each entry in an array.
Utilise the existing ARRAY_SIZE macro to identify the length of the array
and pointer arithmetic to process each item as a for loop.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
---
include/linux/kernel.h | 10 ++++++++++
1 file changed, 10 insertions(+)

The use of static arrays to store data is a common use case throughout the
kernel. Along with that is the obvious need to iterate that data.

In fact there are just shy of 5000 instances of iterating a static array:
git grep "for .*ARRAY_SIZE" | wc -l
4943

When working on the UVC driver - I found that I needed to split one such
iteration into two parts, and at the same time felt that this could be
refactored to be cleaner / easier to read.

I do however worry that this simple short patch might not be desired or could
also be heavily bikeshedded due to it's potential wide spread use (though
perhaps that would be a good thing to have more users) ... but here it is,
along with an example usage below which is part of a separate series.

The aim is to simplify iteration on static arrays, in the same way that we have
iterators for lists. The use of the ARRAY_SIZE macro, provides all the
protections given by "__must_be_array(arr)" to this macro too.

Regards

Kieran

=============================================================================
Example Usage from a pending UVC development:

+#define for_each_uvc_urb(uvc_urb, uvc_streaming) \
+ for_each_array_element(uvc_urb, uvc_streaming->uvc_urb)

/*
* Uninitialize isochronous/bulk URBs and free transfer buffers.
*/
static void uvc_uninit_video(struct uvc_streaming *stream, int free_buffers)
{
- struct urb *urb;
- unsigned int i;
+ struct uvc_urb *uvc_urb;

uvc_video_stats_stop(stream);

- for (i = 0; i < UVC_URBS; ++i) {
- struct uvc_urb *uvc_urb = &stream->uvc_urb[i];
+ for_each_uvc_urb(uvc_urb, stream)
+ usb_kill_urb(uvc_urb->urb);

- urb = uvc_urb->urb;
- if (urb == NULL)
- continue;
+ flush_workqueue(stream->async_wq);

- usb_kill_urb(urb);
- usb_free_urb(urb);
+ for_each_uvc_urb(uvc_urb, stream) {
+ usb_free_urb(uvc_urb->urb);
uvc_urb->urb = NULL;
}

if (free_buffers)
uvc_free_urb_buffers(stream);
}
=============================================================================




diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index ce51455e2adf..95d7dae248b7 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -70,6 +70,16 @@
*/
#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))

+/**
+ * for_each_array_element - Iterate all items in an array
+ * @elem: pointer of array type for iteration cursor
+ * @array: array to be iterated
+ */
+#define for_each_array_element(elem, array) \
+ for (elem = &(array)[0]; \
+ elem < &(array)[ARRAY_SIZE(array)]; \
+ ++elem)
+
#define u64_to_user_ptr(x) ( \
{ \
typecheck(u64, x); \
--
2.7.4
\
 
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