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From: Lada Trimasova <>
To: Vineet Gupta <>
Cc: "" <>,
	"" <>,
	"" <>,
	Alexey Brodkin <>,
Subject: Re: [PATCH] arc: use little endian accesses
Date: Thu, 10 Mar 2016 18:57:48 +0000
Message-ID: <> (raw)
Message-ID: <20160310185748.5f4btGTGMi0YAvH2xc7mHRpVUuCK7cpK8FBbvl1_FEk@z> (raw)
In-Reply-To: <>

Hi Vineet, Alexey, Arnd,
On Thu, 2016-03-10 at 09:55 +0000, Vineet Gupta wrote:

Can you or Lada provide more details as to exactly what is not working - what
driver to be precise 8250 or dw-8250.
And where exactly the failure shows up. I want to understand this more
Noam told me off list that he has no issues with both big endian ARC + 8250 in
systemc model or silicon.


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.


  reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-09 17:21 Lada Trimasova
2016-03-10  5:05 ` Vineet Gupta
2016-03-10  5:19   ` Vineet Gupta
2016-03-10  5:19     ` Vineet Gupta
2016-03-10  7:44   ` Alexey Brodkin
2016-03-10  9:55     ` Vineet Gupta
2016-03-10 18:57       ` Lada Trimasova [this message]
2016-03-10 18:57         ` Lada Trimasova
2016-03-10 19:23         ` Arnd Bergmann
2016-03-11 12:44           ` Vineet Gupta
2016-03-12  4:20             ` Vineet Gupta
2016-03-12  4:20               ` Vineet Gupta
2016-03-11  5:07         ` Noam Camus
2016-03-11  5:07           ` Noam Camus
2016-03-10  7:45   ` Arnd Bergmann
2016-03-10  7:45     ` Arnd Bergmann
2016-03-11  5:18     ` Vineet Gupta

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