linux-kernel - Re: [PATCH] arc: use little endian accesses

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Message-Id: <201603102023.59425.arnd@arndb.de>
Date:	Thu, 10 Mar 2016 20:23:59 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Lada Trimasova <Lada.Trimasova@...opsys.com>
Cc:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"noamc@...hip.com" <noamc@...hip.com>,
	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	"linux-snps-arc@...ts.infradead.org" 
	<linux-snps-arc@...ts.infradead.org>
Subject: Re: [PATCH] arc: use little endian accesses

On Thursday 10 March 2016, Lada Trimasova wrote:
> Driver is 8250, kernel is built for BE arc, nsim option in model "nsim_isa_big_endian = 1".
> 
> With current "readl" and "writel" implementation for ARC we read word from memory without any endianess manipulation. So in case of little endian architecture we get what we want: the first memory byte is the low byte in the word. But in case of big endian architecture the word endianess is swapped: the first memory byte is the high word byte.
> 
> And for example, let's see what happens when we use "readl" in  function "serial8250_early_in" in driver/tty/serial/8250.
> 
> Take a look to one line from memory dump:
> 0xf0000010: 0x0b    0x00    0x00    0x00    0x60    0x00    0x00    0x00
> 
> When kernel is built for little endian architecture, we read this data in status register in function "serial_putc" using "readl" function in driver/tty/serial/8250 as:
> r0: 0x0000006
> The low byte is 0x0b, so the condition "if ((status & BOTH_EMPTY) == BOTH_EMPTY)"  is true, as BOTH_EMPTY is some mask with low bytes set.
> 
> When kernel is built with "CPU_BIG_ENDIAN" and model nsim option is "nsim_isa_big_endian=1", we read this data as:
> r0: 0x6000000
> So as you can see the low byte is 0x00 and above mentioned condition never becomes true, we can't continue initialization.
> 

Ok, this sounds like a completely normal architecture implementation then,
and your patch looks correct.

If some other driver breaks because of this change, you should investigate
what went wrong there, and treat it as a driver specific problem.

	Arnd