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

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Message-ID: <332d001d-8b5a-bba2-c490-ed2e5efd0b1d@metafoo.de>
Date:   Sun, 21 Nov 2021 19:49:03 +0100
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        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 11/21/21 6:52 PM, Paul Cercueil wrote:
> Hi Lars,
>
> Le dim., nov. 21 2021 at 17:23:35 +0100, Lars-Peter Clausen 
> <lars@...afoo.de> a écrit :
>> 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.
>
> 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.
>
But not when used in combination with the DMA buf changes later in this 
series.