linux-kernel - Re: [PATCH v6] serial: rewrite pxa2xx-uart to use 8250

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Date:	Tue, 29 Dec 2015 17:50:53 +0100
From:	Robert Jarzmik <robert.jarzmik@...e.fr>
To:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	Sergei Ianovich <ynvich@...il.com>
Cc:	linux-kernel@...r.kernel.org, James Cameron <quozl@...top.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Russell King <linux@....linux.org.uk>,
	Jiri Slaby <jslaby@...e.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Arnd Bergmann <arnd@...db.de>,
	Joachim Eastwood <manabian@...il.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Scott Wood <scottwood@...escale.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Paul Burton <paul.burton@...tec.com>,
	Mans Rullgard <mans@...sr.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Masahiro Yamada <yamada.masahiro@...ionext.com>,
	Peter Hurley <peter@...leysoftware.com>,
	"open list\:SERIAL DRIVERS" <linux-serial@...r.kernel.org>
Subject: Re: [PATCH v6] serial: rewrite pxa2xx-uart to use 8250_core

Heikki Krogerus <heikki.krogerus@...ux.intel.com> writes:

> On Thu, Dec 24, 2015 at 06:15:14PM +0300, Sergei Ianovich wrote:
>> pxa2xx-uart was a separate uart platform driver. It was declaring
>> the same device names and numbers as 8250 driver. As a result,
>> it was impossible to use 8250 driver on PXA SoCs.
>> 
>> Upon closer examination pxa2xx-uart turned out to be a clone of
>> 8250_core driver.
>> 
>> Workaround for Erratum #19 according to Marvel(R) PXA270M Processor
>> Specification Update (April 19, 2010) is dropped. 8250_core reads
>> from FIFO immediately after checking DR bit in LSR.
>> 
>> The patch leaves the original SERIAL_PXA driver around. The original
>> driver is just marked DEPRECATED in Kconfig and C source. When
>> the original driver is considered safe to remove, no changes
>> to SERIAL_8250 will be necessary.
>> 
>> Compiling SERIAL_8250_CONSOLE and SERIAL_PXA_CONSOLE even without
>> SERIAL_8250_PXA breaks console for SERIAL_PXA. For this reasons, the new
>> and the original drivers are made mutually exclusive.
>> 
>> Signed-off-by: Sergei Ianovich <ynvich@...il.com>
>> CC: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
>
> Reviewed-by: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Tested-by: Robert Jarzmik <robert.jarzmik@...e.fr>

The tests were done on lubbock(pxa25x), mainstone(pxa27x) and zylonite(pxa3xx)
boards.

I don't expect any breakage in PXA architecture. There might be issues with :
 - arch/arm/mach-pxa/capc7117.c
 - arch/arm/mach-pxa/viper.c
  -arch/arm/mach-pxa/zeus.c (very small probability for this one)
The mitigation plan if any concern arises is to fallback on the
CONFIG_SERIAL_PXA configuration.

I find a bit misleading this chunk, but I'm not very good at KConfig :
 config SERIAL_PXA
-       bool "PXA serial port support"
+       bool "PXA serial port support (DEPRECATED)"
        depends on ARCH_PXA || ARCH_MMP
        select SERIAL_CORE
+       select SERIAL_8250_PXA if SERIAL_8250=y
+       select SERIAL_PXA_NON8250 if !SERIAL_8250=y

What bothers me a bit is that selecting this option has a different effect
whether SERIAL_8250 is set or not. Somebody selecting the deprecated option
obviously wants the old driver, but will be provided the new one if his config
has SERIAL_8250 set. Maybe someone else can comment on this ...

This apart, from pxa perspective, it's good to go.

Cheers.

-- 
Robert
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