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

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Message-Id: <AMUX2R.XLGW1EZOMU9B2@crapouillou.net>
Date:   Sun, 21 Nov 2021 20:08:34 +0000
From:   Paul Cercueil <paul@...pouillou.net>
To:     Lars-Peter Clausen <lars@...afoo.de>
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



Le dim., nov. 21 2021 at 19:49:03 +0100, Lars-Peter Clausen 
<lars@...afoo.de> a écrit :
> 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.
> 

That's still the case after the DMABUF changes of the series. Or can 
you point me exactly what you think is broken?

-Paul