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

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Date:   Mon, 22 Nov 2021 15:16:06 +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

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.

Cheers,
-Paul