From: Peter Ujfalusi <peter.ujfalusi@ti.com>
To: Vinod Koul <vkoul@kernel.org>,
Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: <dmaengine@vger.kernel.org>,
Michal Simek <michal.simek@xilinx.com>,
Hyun Kwon <hyun.kwon@xilinx.com>,
Tejas Upadhyay <tejasu@xilinx.com>,
Satish Kumar Nagireddy <SATISHNA@xilinx.com>
Subject: Re: [PATCH v3 2/6] dmaengine: Add interleaved cyclic transaction type
Date: Thu, 13 Feb 2020 16:15:38 +0200 [thread overview]
Message-ID: <736038ef-e8b2-5542-5cda-d8923e3a4826@ti.com> (raw)
In-Reply-To: <20200213140709.GH2618@vkoul-mobl>
Hi Vinod, Laurent,
On 13/02/2020 16.07, Vinod Koul wrote:
> On 13-02-20, 15:48, Laurent Pinchart wrote:
>> Hi Vinod,
>>
>> On Thu, Feb 13, 2020 at 06:59:38PM +0530, Vinod Koul wrote:
>>> On 10-02-20, 16:06, Laurent Pinchart wrote:
>>>
>>>>>>> The use case here is not to switch to a new configuration, but to switch
>>>>>>> to a new buffer. If the transfer had to be terminated manually first,
>>>>>>> the DMA engine would potentially miss a frame, which is not acceptable.
>>>>>>> We need an atomic way to switch to the next transfer.
>>>>>>
>>>>>> So in this case you have, let's say a cyclic descriptor with N buffers
>>>>>> and they are cyclically capturing data and providing to client/user..
>>>>>
>>>>> For the display case it's cyclic over a single buffer that is repeatedly
>>>>> displayed over and over again until a new one replaces it, when
>>>>> userspace wants to change the content on the screen. Userspace only has
>>>>> to provide a new buffer when content changes, otherwise the display has
>>>>> to keep displaying the same one.
>>>>
>>>> Is the use case clear enough, or do you need more information ? Are you
>>>> fine with the API for this kind of use case ?
>>>
>>> So we *know* when a new buffer is being used?
>>
>> The user of the DMA engine (the DRM DPSUB driver in this case) knows
>> when a new buffer needs to be used, as it receives it from userspace. In
>> response, it prepares a new interleaved cyclic transaction and queues
>> it. At the next IRQ, the DMA engine driver switches to the new
>> transaction (the implementation is slightly more complex to handle race
>> conditions, but that's the idea).
>>
>>> IOW would it be possible for display (rather a dmaengine facing
>>> display wrapper) to detect that we are reusing an old buffer and keep
>>> the cyclic and once detected prepare a new descriptor, submit a new
>>> one and then terminate old one which should trigger next transaction
>>> to be submitted
>>
>> I'm not sure to follow you. Do you mean that the display driver should
>> submit a non-cyclic transaction for every frame, reusing the same buffer
>> for every transaction, until a new buffer is available ? The issue with
>> this is that if the CPU load gets high, we may miss a frame, and the
>> display will break. The DPDMA hardware implements cyclic support for
>> this reason, and we want to use that feature to comply with the real
>> time requirements.
>
> Sorry to cause confusion :) I mean cyclic
>
> So, DRM DPSUB get first buffer
> A.1 Prepare cyclic interleave txn
> A.2 Submit the txn (it doesn't start here)
> A.3 Invoke issue_pending (that starts the txn)
>
> DRM DPSUB gets next buffer:
> B.1 Prepare cyclic interleave txn
> B.2 Submit the txn
> B.3 Call terminate for current cyclic txn (we need an updated terminate
> which terminates the current txn, right now we have terminate_all which
> is a sledge hammer approach)
> B.4 Next txn would start once current one is started
>
> Does this help and make sense in your case
That would be a clean way to handle it. We were missing this API for a
long time to be able to cancel the ongoing transfer (whether it is
cyclic or slave_sg, or memcpy) and move to the next one if there is one
pending.
+1 from me if it counts ;)
>
> Thanks
>
- Péter
Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki
next prev parent reply other threads:[~2020-02-13 14:15 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-23 2:29 [PATCH v3 0/6] dma: Add Xilinx ZynqMP DPDMA driver Laurent Pinchart
2020-01-23 2:29 ` [PATCH v3 1/6] dt: bindings: dma: xilinx: dpdma: DT bindings for Xilinx DPDMA Laurent Pinchart
2020-01-23 2:29 ` [PATCH v3 2/6] dmaengine: Add interleaved cyclic transaction type Laurent Pinchart
2020-01-23 8:03 ` Peter Ujfalusi
2020-01-23 8:43 ` Vinod Koul
2020-01-23 8:51 ` Peter Ujfalusi
2020-01-23 12:23 ` Laurent Pinchart
2020-01-24 6:10 ` Vinod Koul
2020-01-24 8:50 ` Laurent Pinchart
2020-02-10 14:06 ` Laurent Pinchart
2020-02-13 13:29 ` Vinod Koul
2020-02-13 13:48 ` Laurent Pinchart
2020-02-13 14:07 ` Vinod Koul
2020-02-13 14:15 ` Peter Ujfalusi [this message]
2020-02-13 16:52 ` Laurent Pinchart
2020-02-14 4:23 ` Vinod Koul
2020-02-14 16:22 ` Laurent Pinchart
2020-02-17 10:00 ` Peter Ujfalusi
2020-02-19 9:25 ` Vinod Koul
2020-02-26 16:30 ` Laurent Pinchart
2020-03-02 3:47 ` Vinod Koul
2020-03-02 7:37 ` Laurent Pinchart
2020-03-03 4:32 ` Vinod Koul
2020-03-03 19:22 ` Laurent Pinchart
2020-03-04 5:13 ` Vinod Koul
2020-03-04 8:01 ` Laurent Pinchart
2020-03-04 15:37 ` Vinod Koul
2020-03-04 16:00 ` Laurent Pinchart
2020-03-04 16:24 ` Vinod Koul
[not found] ` <20200311155248.GA4772@pendragon.ideasonboard.com>
2020-03-18 15:14 ` Laurent Pinchart
2020-03-25 16:00 ` Laurent Pinchart
2020-03-26 7:02 ` Vinod Koul
2020-04-08 17:00 ` Laurent Pinchart
2020-04-15 15:12 ` Laurent Pinchart
2020-03-06 14:49 ` Peter Ujfalusi
2020-03-11 23:15 ` Laurent Pinchart
2020-02-26 16:24 ` Laurent Pinchart
2020-03-02 3:42 ` Vinod Koul
2020-01-24 7:20 ` Peter Ujfalusi
2020-01-24 7:38 ` Peter Ujfalusi
2020-01-24 8:58 ` Laurent Pinchart
2020-01-24 8:56 ` Laurent Pinchart
2020-01-23 2:29 ` [PATCH v3 3/6] dmaengine: virt-dma: Use lockdep to check locking requirements Laurent Pinchart
2020-01-23 2:29 ` [PATCH v3 4/6] dmaengine: xilinx: dpdma: Add the Xilinx DisplayPort DMA engine driver Laurent Pinchart
2020-01-23 2:29 ` [PATCH v3 5/6] dmaengine: xilinx: dpdma: Add debugfs support Laurent Pinchart
2020-01-23 2:29 ` [PATCH v3 6/6] arm64: dts: zynqmp: Add DPDMA node Laurent Pinchart
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