RE: [PATCH] serial: Add OMAP high-speed UART driver.
From: Shilimkar, Santosh
Date: Tue Mar 02 2010 - 10:13:40 EST
> -----Original Message-----
> From: G, Manjunath Kondaiah
> Sent: Tuesday, March 02, 2010 8:34 PM
> To: Shilimkar, Santosh; S, Venkatraman; Tony Lindgren; Raja, Govindraj; Greg KH; linux-
> serial@xxxxxxxxxxxxxxx; linux-omap@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Kevin Hilman; Olof
> Subject: RE: [PATCH] serial: Add OMAP high-speed UART driver.
> > -----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@xxxxxxxxxxxxxxx;
> > linux-omap@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > 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
Not necessary. The DMA can still wait for the hw sync signal and the CDAC
can remain 0 if the hw sync in not received. This will be any way returned
by DMA error ( SYNC lost)
This register was mainly suppose to be used for debug purposes.
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