linux-kernel - Re: [PATCH 2/6] ARM: DTS: da850: Use the new DT bindings for the eDMA3

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Message-ID: <56702FC3.5050705@ti.com>
Date:	Tue, 15 Dec 2015 20:50:35 +0530
From:	Sekhar Nori <nsekhar@...com>
To:	Peter Ujfalusi <peter.ujfalusi@...com>
CC:	<linux-arm-kernel@...ts.infradead.org>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<khilman@...prootsystems.com>
Subject: Re: [PATCH 2/6] ARM: DTS: da850: Use the new DT bindings for the
 eDMA3

On Tuesday 15 December 2015 07:35 PM, Peter Ujfalusi wrote:
> On 12/15/2015 03:48 PM, Sekhar Nori wrote:

>>>> In future, if/when we gain QDMA support, the QDMA channels could be used
>>>> for memcopy.
>>>
>>> Well, in short there is no way to get the qDMA working in a different way
>>> either. qDMA channel is in essence using 'normal' eDMA channel. This means
>>> that we still need to reserve the eDMA channel to be used for memcpy, but
>>> instead of SW triggering it (as we do it right now), we would need to use the
>>> channel as qDMA and set things up accordingly. I don't really see the benefit
>>> for qDMA mode to be honest.
>>
>> I guess the only advantage is that they will not clash with peripheral
>> mode usage. But even then, some sort of reservation is needed. So I
>> guess QDMA is no better than the EDMA reserved channels?
> 
> It will clash with the peripheral mode use since it needs to take one of the
> eDMA channels. qDMA mode is basically differs from the mode we are using by
> how the channel is triggered. Currently we start the memcpy with SW trigger.

Alright, so I was missing this detail. Thanks for clarifying it on IRC.
The EDMA specification makes it look like 8 additional QDMA channels can
be used over 32 DMA channels, but in reality they are not additional
channels since TCC is only valid between 0-31.

So, I agree. QDMA does not buy us any parallel transfers.

Thanks,
Sekhar
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