Date:   Sun, 21 Nov 2021 17:52:48 +0000
From:   Paul Cercueil <paul@crapouillou.net>
Subject: Re: [PATCH 01/15] iio: buffer-dma: Get rid of incoming/outgoing
 queues
To:     Lars-Peter Clausen <lars@metafoo.de>
Cc:     Jonathan Cameron <jic23@kernel.org>,
        Alexandru Ardelean <ardeleanalex@gmail.com>,
        Michael Hennerich <Michael.Hennerich@analog.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Christian =?iso-8859-1?b?S/ZuaWc=?= <christian.koenig@amd.com>,
        linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linaro-mm-sig@lists.linaro.org
Message-Id: <0COX2R.BSNX3NW8N48T@crapouillou.net>
In-Reply-To: <e2689f0d-dc16-2519-57df-d98caadb07b0@metafoo.de>
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        <20211115141925.60164-2-paul@crapouillou.net>
        <e2689f0d-dc16-2519-57df-d98caadb07b0@metafoo.de>
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Hi Lars,

Le dim., nov. 21 2021 at 17:23:35 +0100, Lars-Peter Clausen 
<lars@metafoo.de> a ?crit :
> 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@crapouillou.net>
> The outgoing queue is going to be replaced by fences, but I think we 
> need to keep the incoming queue.

Blocks are always accessed in sequential order, so we now have a 
"queue->next_dequeue" that cycles between the buffers allocated for 
fileio.

>> [...]
>> @@ -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.

We do keep a reference to the buffers, in the queue->fileio.blocks 
array. When the buffer is enabled, all the blocks in that array that 
are in the "queued" state will be submitted to the DMA.

Cheers,
-Paul