linux-kernel - RE: [PATCH 2/2] Add device_prep_dma

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Message-ID: <ABF87B0B6A38C0458E319AC973ED68AECE527E@zch01exm26.fsl.freescale.net>
Date:	Wed, 12 Mar 2008 10:28:49 +0800
From:	"Zhang Wei" <Wei.Zhang@...escale.com>
To:	"Dan Williams" <dan.j.williams@...el.com>
Cc:	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/2] Add device_prep_dma_interrupt support to fsldma.c

Hi, Dan, 

> -----Original Message-----
> From: dan.j.williams@...il.com 
> >  +
> >  +       /* Set End-of-link to the last link descriptor of 
> new list*/
> >  +       set_ld_eol(fsl_chan, new);
> 
> Question, is 'set_ld_eol' safe to call on descriptors that may not be
> the last in the list?  For example what about the following sequence:

set_ld_eol() is a safe function, which is only for preparing the tx descriptors
lists. When adding prepared tx descriptior list to DMA channel tx list,
the function append_ld_queue() should be called.

> 
> /* prepare two descriptors */
> tx1 = fsl_dma_prep_interrupt(chan);
> tx2 = fsl_dma_prep_memcpy(chan);
> 
> /* submit out of order */
> tx2->tx_submit(tx2);
> tx1->tx_submit(tx1);
> 
> This is only a concern if you plan to support channel switching at
> some point.  For example, switching from a memcpy channel to an xor
> channel.
> 
It's no problem. :) In fact, I've added out of order testing codes in fsl_dma_self_test()
 function.

But I have a question about device_prep_dma_interrupt(), which is no way to assign
 dest and src address. Is it a null tx action dma_async_tx_descriptor except to trigger
 an interrupt?

Thanks!
Wei.
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