linux-kernel - Re: [PATCH V2 0/4] misc: xgene: Add support for APM X-Gene SoC Queue Manager/Traffic Manager

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Message-ID: <CAN1v_PtFz42crHW5=cUgw-fAnPn3rQht=o0t3nmjJRamYSLDMg@mail.gmail.com>
Date:	Sat, 4 Jan 2014 15:59:46 -0800
From:	Ravi Patel <rapatel@....com>
To:	Arnd Bergmann <arnd@...db.de>, Greg KH <gregkh@...uxfoundation.org>
Cc:	Loc Ho <lho@....com>, davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Jon Masters <jcm@...hat.com>,
	"patches@....com" <patches@....com>,
	Keyur Chudgar <kchudgar@....com>
Subject: Re: [PATCH V2 0/4] misc: xgene: Add support for APM X-Gene SoC Queue
 Manager/Traffic Manager

On Sat, Dec 21, 2013 at 11:03 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Saturday 21 December 2013 17:00:51 Loc Ho wrote:
>> On Sat, Dec 21, 2013 at 12:11 PM, Arnd Bergmann <arnd@...db.de> wrote:
>> > On Saturday 21 December 2013, Ravi Patel wrote:
>> >> This patch adds support for APM X-Gene SoC Queue Manager/Traffic Manager.
>> >>  QMTM is required by APM X-Gene SoC Ethernet, PktDMA (XOR Engine) and
>> >>  Security Engine subsystems. All subsystems communicate with QMTM using
>> >>  messages which include information about the work to be performed and
>> >>  the location of associated data buffers.
>> >
>> > Please describe here what the purpose of the qmtm is, as this is not
>> > entirely clear from the code.
>> >
>> > In particular, please describe how this differs from a dmaengine driver
>> > and why it is not possible to extend the dma slave API to describe qmtm
>> > as a dma engine.
>> >
>> [Loc Ho]
>> If the QM driver implements the DMA API, what about the actual DMA
>> engine driver which interfaces with this QM driver. We would have DMA
>> client interfaces with the X-Gene DMA driver (not available yet) via
>> DMA API which in turn interfaces with this QM driver via DMA API.
>> Won't this be kind of awkward? Also, the QM only manage messages (or
>> descriptors) which are 32-bytes or 64-bytes. It doesn't actually do
>> any data transfer of various sizes.
>
> Please describe here what the purpose of the qmtm is, as this is not
> entirely clear from the code or from your reply.
>
> Greg was guessing that it's a bus controller, my best guess is a DMA
> engine. If it's something completely different, you have to let
> us know what it is so we can do a proper review rather than guessing.
>
> Please provide a link to the data sheet if you are unable to explain.

Here is URL to a text document explaining role of QMTM device with CPU, Ethernet
subsystem.

https://drive.google.com/file/d/0B28TgQZ3JLoRRGNnbjJoUGNHWW8/edit?usp=sharing

For simplicity, I have shown only Ethernet.
PktDMA and Security subsystem interfaces with QMTM in the same way as Ethernet.
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