linux-kernel - Re: One question about 8250 UART driver

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Message-ID: <20100817132427.GA17861@linux-mips.org>
Date:	Tue, 17 Aug 2010 14:24:27 +0100
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Alan Cox <alan@...ux.intel.com>
Cc:	Yong Wang <yong.y.wang@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Manuel Lauss <manuel.lauss@...glemail.com>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	Masayuki Ohtake <masa-korg@....okisemi.com>,
	"Khor, Andrew Chih Howe" <andrew.chih.howe.khor@...el.com>
Subject: Re: One question about 8250 UART driver

On Tue, Aug 17, 2010 at 09:16:22AM +0100, Alan Cox wrote:

> > We have a PCI based UART controller that is compatible with the
> > existing 8250 serial driver in most aspects. However, one exception
> > is that our UART controller can take advantage of the onboard DMA
> > contoller to achieve higher throughput. Could you please share your
> > insights about what is the proper way to add such DMA support into
> > existing 8250 driver?
> 
> I'd say the same as I said to the other folks who asked this - create a
> new driver. All the DMA based 8250 devices have differing DMA engines
> and it'll become unmanagable in the existing driver.
> 
> Once we can see which bits of the existing code are useful for the DMA
> 8250-style devices we can then create an 8250-lib.c which contains the
> bits that are useful to 8250 and to non standard 8250-like devices.

An existing example is drivers/serial/ioc3_serial.c.  IOC3 is an SGI
custom chip which is only being used in SGI systems.  It has a PCI
master and 486-style slave bus to which a SuperIO chip is connected.
The IOC3 can either be configured to use the chip in legacy mode; then
the standard 8250 driver will work fine.  It also support a DMA mode
with receive and transmit rings which make it similar to a network chip.

  Ralf
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