linux-kernel - RE: [PATCH] serial: Add OMAP high-speed UART driver.

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Message-ID: <E0D41E29EB0DAC4E9F3FF173962E9E94026ED963B1@dbde02.ent.ti.com>
Date:	Tue, 2 Mar 2010 20:33:40 +0530
From:	"G, Manjunath Kondaiah" <manjugk@...com>
To:	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	"S, Venkatraman" <svenkatr@...com>,
	Tony Lindgren <tony@...mide.com>,
	"Raja, Govindraj" <govindraj.raja@...com>,
	Greg KH <greg@...ah.com>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kevin Hilman <khilman@...prootsystems.com>,
	Olof Johansson <olof@...om.net>
Subject: RE: [PATCH] serial: Add OMAP high-speed UART driver.


Santosh,

> -----Original Message-----
> From: Shilimkar, Santosh 
> Sent: Tuesday, March 02, 2010 7:34 PM
> To: G, Manjunath Kondaiah; S, Venkatraman; Tony Lindgren; 
> Raja, Govindraj; Greg KH; linux-serial@...r.kernel.org; 
> linux-omap@...r.kernel.org; linux-kernel@...r.kernel.org; 
> Kevin Hilman; Olof Johansson
> Subject: RE: [PATCH] serial: Add OMAP high-speed UART driver.
> 
> <snip snip>
> > > > --
> > > CDAC is a shadow register used for monitoring the DMA channel.
> > >  I think it would be a lot
> > > simpler if omap_start_dma() always resets CDAC to 0, and the
> > > UART driver
> > > just not set it explicitly.
> > 
> > This seems to be better option than exposing CDAC read/write API
> > to other drivers since user need to write '0' before 
> starting any DMA
> > transfer which can be be done in omap_start_dma().
> > 
> > I am wondering how other drivers are using DMA transfer 
> API's without
> > resetting CDAC to zero.
> >
> It's needed only if some one is interested in that count. 
> UART seems to 
> using this counter where as other driver don't.
> 
> Why do you think drivers need to know about counter value ?

Reading of non zero value(after reset to zero and enabling dma channel) 
from CDAC register indicates that, DMA transfer has started and user can 
rely on DMA4_CCENi and DMA4_CCFNi element and frame counters.

If the CDAC value is zero even after starting DMA channel, indicates
error.

-Manjunath


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