Re: [PATCH 05/10] ALSA: axd: add buffers manipulation files

From: Qais Yousef
Date: Thu Aug 27 2015 - 10:21:25 EST

On 08/26/2015 07:43 PM, Mark Brown wrote:
On Mon, Aug 24, 2015 at 01:39:14PM +0100, Qais Yousef wrote:

+ /*
+ * must ensure we have one access at a time to the queue and rd_idx
+ * to be preemption and SMP safe
+ * Sempahores will ensure that we will only read after a complete write
+ * has finished, so we will never read and write from the same location.
+ */
In what way will sempahores ensure that we will only read after a
complete write?

This comment needs fixing. What it is trying to say is that if we reached this point of the code then we're certainly allowed to modify the buffer queue and {rd, wr}_idx because the semaphore would have gone to sleep otherwise if the queue is full/empty.

Should I just remove the reference to Semaphores from the comment or worth rephrasing it?

Would it be better to rename {rd, wr}_{idx, sem} to {take, put}_{idx, sem}?

+ buf = bufferq->queue[bufferq->rd_idx];
So buffers are always retired in the same order that they are acquired?

I don't think I get you here. axd_bufferq_take() and axd_bufferq_put() could be called in any order.

What this code is trying to do is make a contiguous memory area behave as a ring buffer. Then this ring buffer behave as a queue. We use semaphore counts to control how many are available to take/put. rd_idx and wr_idx should always point at the next location to take/put from/to.

Does this help answering your question?

+int axd_bufferq_put(struct axd_bufferq *bufferq, char *buf, int buf_size)
+ int ret;
+ if (!bufferq->queue)
+ return 0;
+ if (buf_size < 0)
+ buf_size = bufferq->stride;
We've got strides as well? What is that?

We break the contiguous buffer area allocated for us into smaller buffers separated by (or of size) stride.

+void axd_bufferq_abort_take(struct axd_bufferq *bufferq)
+ if (axd_bufferq_is_empty(bufferq)) {
+ bufferq->abort_take = 1;
+ up(&bufferq->rd_sem);
+ }
+void axd_bufferq_abort_put(struct axd_bufferq *bufferq)
+ if (axd_bufferq_is_full(bufferq)) {
+ bufferq->abort_put = 1;
+ up(&bufferq->wr_sem);
+ }
These look *incredibly* racy. Why are they here and why are they safe?

If we want to restart the firmware we will need to abort any blocking reads or writes for the user space to react. I also needed that to implement nonblocking access in user space when this was a sysfs based driver. It was important then to implement omx IL component correctly.

Do I need to support nonblock reads and writes in ALSA? If I use SIGKILL as you suggested in the other email when restarting and nonblock is not important then I can remove this.

I just looked at the code history and I was in the past sending SIGBUS to the user if we needed to restart then I opted to the abort approach as it will allow the application to terminate gracefully as it should get EOF instead then and hide the need to restart the firmware in a better way. What do you think?

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