RE: sw_ring.c poll problem

[Ge Gao <ggao@invensense.com>]

Thanks for the patch. There could be one problem with the implementation.
Consider the situation:
1. there are 10 packets in the kfifo.
2. Read 1 packet. Then poll again.
3. the poll will stuck at poll_wait(filp, &rb->pollq, wait) because it
needs the next store to wake it up, while in reality, it should return
immediately if there is data already in the kfifo.
4. It might be better to put poll inside ring buffer implementation itself
rather than industrial-buffer.c, such as providing a new method like poll
to do it.
What do you think?
Thanks.

Best Regards,

GE GAO



-----Original Message-----
From: Jonathan Cameron [mailto:jic23@cam.ac.uk]
Sent: Friday, May 18, 2012 10:31 AM
To: Ge Gao
Cc: Jonathan Cameron; linux-iio@vger.kernel.org
Subject: Re: sw_ring.c poll problem



On 05/18/2012 02:08 AM, Ge Gao wrote:
> Dear Jonathan,
> 	I check the kfifo buffer implementation under IIO directory.
However,
> I didn't find the "pollq" is released anywhere as sw_ring does.
> Without it, how can kfifo be polled if it is used by
industrial-buffer.c? Thanks.
Sorry for not getting back to you sooner.  Totally manic at the day job
today tracking two delightful bugs.

Anyhow, looks like I'd imagined we'd actually added poll support to kfifo
when it never got implemented.  Should be easy enough though.

diff --git a/drivers/iio/kfifo_buf.c b/drivers/iio/kfifo_buf.c index
6bf9d05..e69d6ce 100644
--- a/drivers/iio/kfifo_buf.c
+++ b/drivers/iio/kfifo_buf.c
@@ -6,6 +6,7 @@
 #include <linux/kfifo.h>
 #include <linux/mutex.h>
 #include <linux/iio/kfifo_buf.h>
+#include <linux/sched.h>

 struct iio_kfifo {
 	struct iio_buffer buffer;
@@ -35,6 +36,7 @@ static int iio_request_update_kfifo(struct iio_buffer
*r)
 	kfifo_free(&buf->kf);
 	ret = __iio_allocate_kfifo(buf, buf->buffer.bytes_per_datum,
 				   buf->buffer.length);
+	r->stufftoread = false;
 error_ret:
 	return ret;
 }
@@ -97,6 +99,10 @@ static int iio_store_to_kfifo(struct iio_buffer *r,
 	ret = kfifo_in(&kf->kf, data, r->bytes_per_datum);
 	if (ret != r->bytes_per_datum)
 		return -EBUSY;
+	
+	r->stufftoread = true;
+	wake_up_interruptible(&r->pollq);
+
 	return 0;
 }

@@ -112,6 +118,12 @@ static int iio_read_first_n_kfifo(struct iio_buffer
*r,
 	n = rounddown(n, r->bytes_per_datum);
 	ret = kfifo_to_user(&kf->kf, buf, n, &copied);

+	if (kfifo_is_empty(&kf->kf))
+		r->stufftoread = false;
+	/* verify it is still empty to avoid race */
+	if (!kfifo_is_empty(&kf->kf))
+		r->stufftoread = true;
+
 	return copied;
 }

.. is a totally untested patch to add poll support to it.
The little dance at the end is to avoid a write having occured between the
kfifo_is_empty and setting the flag false as that could wipe out the
existence of some data in the buffer. Given we only ever have one writer
and one reader that should work I think.

Jonathan

>
> Hi Ge,
>
> I realised after sending that message that I was being rather
> dismissive of your query.  Got up far too early this morning (as every
> morning ;)
>
> Anyhow, to give more details. sw_ring is probably never going to make
> it out of staging, hence the move to kfifo_buf. At somepoint we need
> to work out how to do equivalent functionality of sw_ring but I've not
> had time to more than start looking into this.
>
> As you saw, poll on sw_ring is a watershead signal indicating (in
> theory and last I checked it worked) that the ring is more than half
full.
> Any read that takes the fill level below half (test code just reads
> half the size of the buffer), should allow a new passing of the
> watershead to resignal poll. It's entirely possible there is a bug in
> there though I know it is been getting a fair bit of testing with some
> other drivers so could be todo with the precise way you are reading it
> hitting some corner case? (I'm
> stretching...)
>
> Right now I'd just move over to kfifo_buf if I were you. It's much
> more 'standard' in that it's a fifo and poll indicates if there is
> anything there at all.
>>> 	Thanks.
>>>
>>> Best regards,
>>>
>>> Ge GAO
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