linux-kernel - Re: RFC: usb: musb: Adding CPPI4.1 DMA driver under drivers/dma

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Date:	Fri, 13 Jan 2012 23:41:13 +0300
From:	Sergei Shtylyov <sshtylyov@...sta.com>
To:	"Gupta, Ajay Kumar" <ajay.gupta@...com>
CC:	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: RFC: usb: musb: Adding CPPI4.1 DMA driver under drivers/dma

Hello.

On 01/13/2012 01:40 PM, Gupta, Ajay Kumar wrote:

> CPPI4.1 (Communication Port Programming Interface) is a TI specific DMA
> controller used in multiple TI platform such as AM33x, DA8x, AM35x, TI81x.
> The DMA engine is mainly used by musb controller on above platform.

    There are (at least out of tree) platforms using CPPI 4.1 for Ethernet.

> Earlier version of the driver was submitted by Sergei Shtylyov at [1] but
> was not merged due to disagreement on the location of driver in kernel.

    To be precise, TI requested pulling out already queued (to arch/arm/common/) 
CPPI 4.1 driver from RMK's patch system without even notifying me. At least that 
was RMK's version...

> Refer the discussions at [1] for more details.

> [1] http://marc.info/?l=linux-usb&m=125087318308323

    Unfortunately, that's not all of the discussion of interest.

> The new implementation of the CPPI4.1 DMA slave driver will be in the
> drivers/dma folder, complying to dmaengine framework.

    Has there been any work in this direction done yet?

> It would involve changes in existing musb driver also for which current
> plan is to maintain the compatibility of non-CPPI4.1 DMA in musb driver.

    I didn't quite understand this part...

> The task is planned to be spitted into below subtasks.

> (1) Post RFC on the API details, changes envisaged in musb driver and other
> challenges

    First of all, I foresee changes in drivers/dma/ to cope with the entity in 
CPPI 4.1 called the queue manager (there's also buffer manager but it wasn't 
implemented on DA8xx, so I didn't design any API for it).

> (2) Implement and post RFC for the CPPI4.1 DMA driver and changes needed in
> musb driver.

> Let me know if you have any thoughts/comments on this.

> Regards,
> Ajay

WBR, Sergei

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