linux-kernel - Re: [PATCH v2] ARM: PL011: Add support for Rx DMA buffer polling

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Date:	Fri, 25 Jan 2013 21:26:13 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Chanho Min <chanho.min@....com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Alan Cox <alan@...ux.intel.com>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org, Pawel Moll <pawel.moll@....com>
Subject: Re: [PATCH v2] ARM: PL011: Add support for Rx DMA buffer polling

On Fri, Jan 25, 2013 at 12:25:07PM -0800, Greg Kroah-Hartman wrote:
> On Wed, Jan 23, 2013 at 02:07:57PM +0900, Chanho Min wrote:
> > On Tue, Jan 22, 2013 at 10:18 PM, Linus Walleij
> > <linus.walleij@...aro.org> wrote:
> > >> - If poll timer is activated, We use consistent DMA mappings to avoid from
> > >>   the frequent cache operation of the timer function. sg->coherency is used
> > >>   to check if buffer is coherent.
> > >
> > > Hm. We can begin like this but maybe we should later patch it to
> > > make this the default unless it has performance impacts on our
> > > systems.
> > Good, I really hope to make this the default.
> > We expect it has no impact to system even if no polling use.
> > 
> > > Maybe this is a good opportunity to add some kerneldoc above
> > > this struct so as to help platform implementers. It will invariably
> > > be reused by Device Tree binding authors later on...
> > Agree, but, frankly I need help about what I didn't modify.
> > So We will leave this work for the next submit.
> 
> So, should this patch be applied?  Can I get an ack from someone who
> knows this driver/platform please?

Well, I thought I had explained that I'd prefer to see the poll rate
adjusted with the baud rate, but maybe I wasn't explicit enough.
Instead, what we seem to have ended up with are two new entries in
platform data (which we're trying to get away from with DT):

+       unsigned int dma_rx_poll_rate;                                        
+       unsigned int dma_rx_poll_timeout;                                     

If this were to be done, then receive DMA could be used on the Versatile
PB platforms without having it suck CPU usage unnecessarily at slower
baud rates.
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