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

[Paul Cercueil]

Le lun., nov. 22 2021 at 16:17:59 +0100, Lars-Peter Clausen 
<lars@xxxxxxxxxx> a écrit :
> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > > > > > > 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.

You wouldn't drop data, but it could cause an underrun, yes. Is it such 
a big deal, knowing that the buffer was just enabled? I don't think you 
can disable then enable the buffer without causing a discontinuity in 
the stream.

-Paul