Message-ID: <CA+mB=1Jd3T9BrqdF_KmFORNABaq_y548tspO5vzTfbsH=duDSQ@mail.gmail.com>
Date:	Thu, 27 Feb 2014 23:34:34 +0530
From:	Srikanth Thokala <sthokal@...inx.com>
To:	Jassi Brar <jaswinder.singh@...aro.org>
Cc:	Srikanth Thokala <sthokal@...inx.com>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	"Koul, Vinod" <vinod.koul@...el.com>,
	Michal Simek <michal.simek@...inx.com>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
	Levente Kurusa <levex@...ux.com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	lkml <linux-kernel@...r.kernel.org>, dmaengine@...r.kernel.org,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 1/3] dma: Support multiple interleaved frames with
 non-contiguous memory

On Wed, Feb 26, 2014 at 8:32 PM, Jassi Brar <jaswinder.singh@...aro.org> wrote:
> On 26 February 2014 23:21, Srikanth Thokala <sthokal@...inx.com> wrote:
>> On Mon, Feb 24, 2014 at 7:39 AM, Jassi Brar <jaswinder.singh@...aro.org> wrote:
>>> On 21 February 2014 23:37, Srikanth Thokala <sthokal@...inx.com> wrote:
>>>> On Thu, Feb 20, 2014 at 3:23 PM, Jassi Brar <jaswinder.singh@...aro.org>
>>>> wrote:
>>>>> On 20 February 2014 14:54, Srikanth Thokala <sthokal@...inx.com> wrote:
>>>>>> On Wed, Feb 19, 2014 at 12:33 AM, Jassi Brar <jaswinder.singh@...aro.org>
>>>>>> wrote:
>>>>>>> On 18 February 2014 23:16, Srikanth Thokala <sthokal@...inx.com> wrote:
>>>>>>>> On Tue, Feb 18, 2014 at 10:20 PM, Jassi Brar
>>>>>>>> <jaswinder.singh@...aro.org> wrote:
>>>>>>>>> On 18 February 2014 16:58, Srikanth Thokala <sthokal@...inx.com>
>>>>>>>>> wrote:
>>>>>>>>>> On Mon, Feb 17, 2014 at 3:27 PM, Jassi Brar
>>>>>>>>>> <jaswinder.singh@...aro.org> wrote:
>>>>>>>>>>> On 15 February 2014 17:30, Srikanth Thokala <sthokal@...inx.com>
>>>>>>>>>>> wrote:
>>>>>>>>>>>> The current implementation of interleaved DMA API support multiple
>>>>>>>>>>>> frames only when the memory is contiguous by incrementing
>>>>>>>>>>>> src_start/
>>>>>>>>>>>> dst_start members of interleaved template.
>>>>>>>>>>>>
>>>>>>>>>>>> But, when the memory is non-contiguous it will restrict slave
>>>>>>>>>>>> device
>>>>>>>>>>>> to not submit multiple frames in a batch.  This patch handles this
>>>>>>>>>>>> issue by allowing the slave device to send array of interleaved dma
>>>>>>>>>>>> templates each having a different memory location.
>>>>>>>>>>>>
>>>>>>>>>>> How fragmented could be memory in your case? Is it inefficient to
>>>>>>>>>>> submit separate transfers for each segment/frame?
>>>>>>>>>>> It will help if you could give a typical example (chunk size and gap
>>>>>>>>>>> in bytes) of what you worry about.
>>>>>>>>>>
>>>>>>>>>> With scatter-gather engine feature in the hardware, submitting
>>>>>>>>>> separate
>>>>>>>>>> transfers for each frame look inefficient. As an example, our DMA
>>>>>>>>>> engine
>>>>>>>>>> supports up to 16 video frames, with each frame (a typical video
>>>>>>>>>> frame
>>>>>>>>>> size) being contiguous in memory but frames are scattered into
>>>>>>>>>> different
>>>>>>>>>> locations. We could not definitely submit frame by frame as it would
>>>>>>>>>> be
>>>>>>>>>> software overhead (HW interrupting for each frame) resulting in video
>>>>>>>>>> lags.
>>>>>>>>>>
>>>>>>>>> IIUIC, it is 30fps and one dma interrupt per frame ... it doesn't seem
>>>>>>>>> inefficient at all. Even poor-latency audio would generate a higher
>>>>>>>>> interrupt-rate. So the "inefficiency concern" doesn't seem valid to
>>>>>>>>> me.
>>>>>>>>>
>>>>>>>>> Not to mean we shouldn't strive to reduce the interrupt-rate further.
>>>>>>>>> Another option is to emulate the ring-buffer scheme of ALSA.... which
>>>>>>>>> should be possible since for a session of video playback the frame
>>>>>>>>> buffers' locations wouldn't change.
>>>>>>>>>
>>>>>>>>> Yet another option is to use the full potential of the
>>>>>>>>> interleaved-xfer api as such. It seems you confuse a 'video frame'
>>>>>>>>> with the interleaved-xfer api's 'frame'. They are different.
>>>>>>>>>
>>>>>>>>> Assuming your one video frame is F bytes long and Gk is the gap in
>>>>>>>>> bytes between end of frame [k] and start of frame [k+1] and  Gi != Gj
>>>>>>>>> for i!=j
>>>>>>>>> In the context of interleaved-xfer api, you have just 1 Frame of 16
>>>>>>>>> chunks. Each chunk is Fbytes and the inter-chunk-gap(ICG) is Gk  where
>>>>>>>>> 0<=k<15
>>>>>>>>> So for your use-case .....
>>>>>>>>>   dma_interleaved_template.numf = 1   /* just 1 frame */
>>>>>>>>>   dma_interleaved_template.frame_size = 16  /* containing 16 chunks */
>>>>>>>>>    ...... //other parameters
>>>>>>>>>
>>>>>>>>> You have 3 options to choose from and all should work just as fine.
>>>>>>>>> Otherwise please state your problem in real numbers (video-frames'
>>>>>>>>> size, count & gap in bytes).
>>>>>>>>
>>>>>>>> Initially I interpreted interleaved template the same.  But, Lars
>>>>>>>> corrected me
>>>>>>>> in the subsequent discussion and let me put it here briefly,
>>>>>>>>
>>>>>>>> In the interleaved template, each frame represents a line of size
>>>>>>>> denoted by
>>>>>>>> chunk.size and the stride by icg.  'numf' represent number of frames
>>>>>>>> i.e.
>>>>>>>> number of lines.
>>>>>>>>
>>>>>>>> In video frame context,
>>>>>>>> chunk.size -> hsize
>>>>>>>> chunk.icg -> stride
>>>>>>>> numf -> vsize
>>>>>>>> and frame_size is always 1 as it will have only one chunk in a line.
>>>>>>>>
>>>>>>> But you said in your last post
>>>>>>>   "with each frame (a typical video frame size) being contiguous in
>>>>>>> memory"
>>>>>>>  ... which is not true from what you write above. Anyways, my first 2
>>>>>>> suggestions still hold.
>>>>>>
>>>>>> Yes, each video frame is contiguous and they can be scattered.
>>>>>>
>>>>> I assume by contiguous frame you mean as in framebuffer?  Which is an
>>>>> array of bytes.
>>>>> If yes, then you should do as I suggest first,  frame_size=16  and numf=1.
>>>>
>>>> I think am confusing you.  I would like to explain with an example.  Lets
>>>> say
>>>> each video frame is 4k size starting at address 0x10004000 (-0x10005000) and
>>>> other frame at 0x20002000 (-0x20003000), and so on.
>>>>
>>> As I said plz dont confuse video frame with DMA frame.... in video
>>> frame the stride is constant(zero or not) whereas in DMA context the
>>> stride must be zero for the frame to be called contiguous.
>>>
>>>> So, the frames are
>>>> scattered in memory and as the template doesnt allow multiple src_start/
>>>> dst_start we could not use single template to fill the HW descriptors (of
>>>> frames).  So, I feel your suggestion might not work if the frames are
>>>> scattered.
>>>> Also, how could we get 'vsize' value in your approach?
>>>>
>>> The client driver(video driver) should know the frame parameters.
>>> Practically you'll have to populate 16 transfer templates (frame
>>> attributes and locations won't change for a session) and submit to be
>>> transferred _cyclically_.
>>>
>>>>  More importantly,
>>>> we are overriding the semantics of interleaved template members.
>>>>
>>> Not at all. Interleaved-dma isn't meant for only constant stride/icg
>>> transfers. Rather it's for identical frames with random strides.
>>>
>>>>
>>>>>
>>>>> If no, then it seems you are already doing the right thing.... the
>>>>> ring-buffer scheme. Please share some stats how the current api is
>>>>> causing you overhead because that is a very common case (many
>>>>> controllers support LLI) and you have 467ms (@30fps with 16-frames
>>>>> ring-buffer) to queue in before you see any frame drop.
>>>>
>>>> As I mentioned earlier in the thread, our hardware has a SG engine where by
>>>> we could send multiple frames in a batch.  Using the original implementation
>>>> of interleaved API, we have three options to transfer.
>>>>
>>>> One is to send frame by frame to the hardware.  We get a async_tx desc for
>>>> each frame and then we submit it to hardware triggering it to transfer this
>>>> BD.
>>>> We queue the next descriptor on the pending queue and whenever there is
>>>> a completion interrupt we submit the next BD on this queue to hardware. In
>>>> this implementation we are not efficiently using the SG engine in the
>>>> hardware
>>>> as we transferring frame by frame even HW allows us to transfer multiple
>>>> frames.
>>>>
>>> Sending one frame at a time will likely cause jitters. That shouldn't be done.
>>>
>>>> The second option is to queue all the BDs until the maximum frames that HW
>>>> is
>>>> capable to transfer in a batch and then submit to SG engine in the HW.  With
>>>> this approach I feel there will be additional software overhead to track the
>>>> number
>>>> of maximum transfers and few additional cycles to release the cookie of each
>>>> desc.
>>>> Here, each desc represents a frame.
>>>>
>>> APIs are written for the gcd of h/w. We should optimize only for
>>> majority and here isn't even anything gained after changing the api.
>>> What you want to 'optimize' has been there since ever. Nobody
>>> considers that overhead. BTW you don't even want to spend a few 'extra
>>> cycles' but what about every other platform that doesn't support this
>>> and will have to scan for such transfer requests?
>>>
>>>> The last option is the current implementation of the driver along with this
>>>> change in
>>>> API.  It will allow us to send array of interleaved templates wherein we
>>>> could allocate
>>>> a single async desc which will handle multiple frames (or segments) and just
>>>> submit
>>>> this desc to HW.  Then we program the current to first frame, tail to last
>>>> frame and
>>>> HW will complete this transfer.  Here, each desc represents multiple frames.
>>>>
>>>> My point here is the driver should use hardware resources efficiently and I
>>>> feel the
>>>> driver will be inefficient if we dont use them.
>>>>
>>> I am afraid you are getting carried away by the 'awesomeness' of your
>>> hardware. RingBuffers/Cyclic transfers  are meant for cases just like
>>> yours.
>>> Consider the following ... if you queue 16 frames and don't care to
>>> track before all are transmitted, you'll have very high latency. Video
>>> seeks will take unacceptably long and give the impression of a slow
>>> system. Whereas if you get callbacks for each frame rendered, you
>>> could updates frames from the next one thereby having very quick
>>> response time.
>>
>> Not at all, at least in our hardware, when we submit transfers it is most
>> likely to be completed. There are few errors, but mostly are recoverable
>> and it doesnt stop the transfer unless there is a critical error which needs
>> a reset to the whole system.  So, at least in my use case there will be
>> no latency.  I am not saying hardware is great, but it is the IP implementation.
>>
> I am not talking about the error cases.
> Apply your patch locally so that you queue 16frames and not get
> notified upon each frame 'rendered'...  now click on 'seek bar' of the
> video player. See how slow it is to jump to play from the new
> location.  Or if the frames are to be encoded after dma transfer...
> see the 'padding' that would need to be done at the end.  These
> concerns are common with audio subsystem using ring-buffers.
> Anyways that is just FYI, I don't care how you implement your platform.
>
>> Regarding this API change, I had earlier explained my use case in my
>> v2 thread.
>> Lars and Vinod came up with this resolution to allow array of
>> interleaved templates.
>> I also feel this is reasonable change for the subsystem.
>>
> I don't see you getting any better performance from your hardware and
> I certainly don't find your usecase anything new (many dma controllers
> support LLI and they work just fine as such). And I have already
> explained how you could do, whatever you want, without this change.
> So a polite NAK from me.

Ok.  I would go with your suggestion of having frame_size = 16 and will
send v4 dropping this change.

Thanks for the valuable suggestions.

Srikanth

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