linux-kernel - Re: [PATCH 01/15] iio: buffer-dma: Get rid of incoming/outgoing queues

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Date:   Sun, 21 Nov 2021 17:23:35 +0100
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Paul Cercueil <paul@...pouillou.net>,
        Jonathan Cameron <jic23@...nel.org>
Cc:     Alexandru Ardelean <ardeleanalex@...il.com>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH 01/15] iio: buffer-dma: Get rid of incoming/outgoing
 queues

On 11/15/21 3:19 PM, Paul Cercueil wrote:
> The buffer-dma code was using two queues, incoming and outgoing, to
> manage the state of the blocks in use.
>
> While this totally works, it adds some complexity to the code,
> especially since the code only manages 2 blocks. It is much easier to
> just check each block's state manually, and keep a counter for the next
> block to dequeue.
>
> Since the new DMABUF based API wouldn't use these incoming and outgoing
> queues anyway, getting rid of them now makes the upcoming changes
> simpler.
>
> Signed-off-by: Paul Cercueil <paul@...pouillou.net>
The outgoing queue is going to be replaced by fences, but I think we 
need to keep the incoming queue.
> [...]
> @@ -442,28 +435,33 @@ EXPORT_SYMBOL_GPL(iio_dma_buffer_disable);
>   static void iio_dma_buffer_enqueue(struct iio_dma_buffer_queue *queue,
>   	struct iio_dma_buffer_block *block)
>   {
> -	if (block->state == IIO_BLOCK_STATE_DEAD) {
> +	if (block->state == IIO_BLOCK_STATE_DEAD)
>   		iio_buffer_block_put(block);
> -	} else if (queue->active) {
> +	else if (queue->active)
>   		iio_dma_buffer_submit_block(queue, block);
> -	} else {
> +	else
>   		block->state = IIO_BLOCK_STATE_QUEUED;
> -		list_add_tail(&block->head, &queue->incoming);
If iio_dma_buffer_enqueue() is called with a dmabuf and the buffer is 
not active, it will be marked as queued, but we don't actually keep a 
reference to it anywhere. It will never be submitted to the DMA, and it 
will never be signaled as completed.