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

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Date:   Mon, 22 Nov 2021 16:17:59 +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 11/22/21 4:16 PM, Paul Cercueil wrote:
> Hi Lars,
>
> Le lun., nov. 22 2021 at 16:08:51 +0100, Lars-Peter Clausen 
> <lars@...afoo.de> a écrit :
>> On 11/21/21 9:08 PM, Paul Cercueil wrote:
>>>
>>>
>>> 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?
>>>
>> When you allocate a DMABUF with the allocate IOCTL and then submit it 
>> with the enqueue IOCTL before the buffer is enabled it will end up 
>> marked as queued, but not actually be queued anywhere.
>>
>
> Ok, it works for me because I never enqueue blocks before enabling the 
> buffer. I can add a requirement that blocks must be enqueued only 
> after the buffer is enabled.

I don't think that is a good idea. This way you are going to potentially 
drop data at the begining of your stream when the DMA isn't ready yet.