linux-kernel - Re: [PATCH] serial/of-serial: Add 16654 chip to compatible string list

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Message-ID: <20120528163151.GB28290@n2100.arm.linux.org.uk>
Date:	Mon, 28 May 2012 17:31:51 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Roland Stigge <stigge@...com.de>
Cc:	alan@...ux.intel.com, gregkh@...uxfoundation.org,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	kevin.wells@....com, srinivas.bakki@....com,
	linux-arm-kernel@...ts.infradead.org, arnd@...db.de
Subject: Re: [PATCH] serial/of-serial: Add 16654 chip to compatible string
	list

On Mon, May 28, 2012 at 06:27:20PM +0200, Roland Stigge wrote:
> On 28/05/12 17:03, Russell King - ARM Linux wrote:
> >> Initially, with my RFC patch, there was an #ifdef for bigger FIFO in
> >> case of LPC32xx where we have a 16550A variant with 64 byte fifos.
> > 
> > What are all the differences?  Is it just a larger FIFO?
> 
> Yes, this is how it's summarized in the manual (LPC32xx SoC).
> 
> >> So maybe 16750 is the better choice for me, anyway. Already supported in
> >> of-serial. Works for now, but need more testing. Another hint is that
> >> 16750 is advertised as "IP core for Soc" which matches the case of LPC32xx.
> > 
> > 16750 also has automatic hardware flow control support, selectable through
> > bit 5 in the MCR register.  If your UART has that, then it's probably a
> > 16750 derivative rather than a 16550 or 16650 derivative.
> > 
> > 16650s have an EFR register at offset 2, selectable by writing 0xBF into
> > the LCR register, which the 16750 doesn't have.  16650 also has automatic
> > hardware flow control, bit this is selected through a couple of bits in
> > the EFR.
> 
> The 4 LPC32xx's "Standard" UARTs have neither of those.
> 
> Is it ok to use "ns16650", i.e. PORT_16650, or do I need to introduce a
> FIFO depth configuration?

I think you need a new type, because as I said above, 16650s have that
additional EFR, and we will attempt to access that register which
isn't present in yours.

It's not solely about fifo depth.
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