linux-kernel - Re: [PATCH char-misc-next 11/19] dma: Add support to program MIC x100 status descriptiors

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Date:	Fri, 21 Aug 2015 13:59:39 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Sudeep Dutt <sudeep.dutt@...el.com>
Cc:	Ashutosh Dixit <ashutosh.dixit@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-kernel@...r.kernel.org, Nikhil Rao <nikhil.rao@...el.com>,
	Siva Yerramreddy <yshivakrishna@...il.com>
Subject: Re: [PATCH char-misc-next 11/19] dma: Add support to program MIC
 x100 status descriptiors

On Thu, Aug 20, 2015 at 10:30:50AM -0700, Sudeep Dutt wrote:
> On Thu, 2015-08-20 at 10:50 +0530, Vinod Koul wrote:
> > On Mon, Jul 27, 2015 at 04:58:17PM -0700, Ashutosh Dixit wrote:

> > > +		/*
> > > +		 * This is a hack to program status descriptor since
> > > +		 * DMA engine API doesn't have support for this.
> > > +		 */
> > what do you mean by programming status descriptor, what do you need to
> > program?
> > 
> 
> The MIC X100 DMA engine has a special status descriptor which writes an
> 8 byte immediate data value to a destination address. It is used to
> signal completion of all DMA descriptors prior to the status descriptor.
> The DMA engine API does not allow drivers to pass a 8 byte value. We are
> allowed to pass a source physical address but we cannot determine the
> source value within the DMA driver using that information specifically
> with the IOMMU enabled. We have added this workaround so that we can
> program this special status update descriptor without making any changes
> to the DMA engine API.
I think this is a special case so you should export an API and let client
program using that API.

-- 
~Vinod
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