linux-kernel - Re: Serial port initialization broken on Armada 370/XP due to "serial: 8250_dw: Don't use UPF_FIXED

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Message-ID: <512F428E.3080400@free-electrons.com>
Date:	Thu, 28 Feb 2013 12:42:06 +0100
From:	Gregory CLEMENT <gregory.clement@...e-electrons.com>
To:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
CC:	Jamie Iles <jamie@...ieiles.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jason Cooper <jason@...edaemon.net>,
	Andrew Lunn <andrew@...n.ch>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-serial@...r.kernel.org
Subject: Re: Serial port initialization broken on Armada 370/XP due to "serial:
 8250_dw: Don't use UPF_FIXED_TYPE"

Hi Heikki,

On 02/28/2013 10:26 AM, Heikki Krogerus wrote:
> Hi Gregory.
> 
> On Wed, Feb 27, 2013 at 05:08:04PM +0100, Gregory CLEMENT wrote:
>> I found the root of the problem in drivers/tty/serial/8250/8250.c
>>
>> in the autoconfig() function, when the IIR register is acceded, it is
>> done using serial_in(), this function return an int but is used as it
>> have returned a char. There is a lot of implicit cast to a char when
>> the returned value is put in a char variable, this seems to not be a
>> problem most of the time. The problematic line is the following:
>>
>> scratch = serial_in(up, UART_IIR) >> 6;
>>
>> the shift is done here before any cast or mask, and unfortunately my
>> hardware send 0xC1C1C1C1, that lead to get a '7' in the scratch
>> variable instead of a '3'.
> 
> OK, this is interesting. Why does it return that? dw_apb_uart_db.pdf I
> have says that bits 31:8 read as zero?

The UART paragraphs on the Armada 370/XP datasheet also says the same
thing.  Actually for all the register the bits 31:8 should be 0.I
suspect an hardware issue (or let's call it an optimization), as the
upper bits are not supposed to be used.

> 
>> Would you agree with this kind of patch to fix the issue?
>>
>> diff --git a/drivers/tty/serial/8250/8250.c b/drivers/tty/serial/8250/8250.c
>> index e2ac25a..0b284c6 100644
>> --- a/drivers/tty/serial/8250/8250.c
>> +++ b/drivers/tty/serial/8250/8250.c
>> @@ -1119,7 +1119,7 @@ static void autoconfig(struct uart_8250_port *up, unsigned int probeflags)
>>         serial_out(up, UART_LCR, 0);
>>
>>         serial_out(up, UART_FCR, UART_FCR_ENABLE_FIFO);
>> -       scratch = serial_in(up, UART_IIR) >> 6;
>> +       scratch = (serial_in(up, UART_IIR) & 0xFF) >> 6;
>>
>>         switch (scratch) {
>>         case 0:
> 
> Instead, can you test if it's enough for you to set the reg-io-width
> to 1 instead of 4:

Yes indeed it worked and it seems to be the correct description of my
hardware. So I will fix the dtsi file.

However isn't buggy to use a function as it returned a char whereas
it returns an int?

Regards,

> 
> diff --git a/arch/arm/boot/dts/armada-370-xp.dtsi b/arch/arm/boot/dts/armada-370-xp.dtsi
> index 4c0abe8..3a87a0e 100644
> --- a/arch/arm/boot/dts/armada-370-xp.dtsi
> +++ b/arch/arm/boot/dts/armada-370-xp.dtsi
> @@ -54,7 +54,7 @@
>                                 reg = <0xd0012000 0x100>;
>                                 reg-shift = <2>;
>                                 interrupts = <41>;
> -                               reg-io-width = <4>;
> +                               reg-io-width = <1>;
>                                 status = "disabled";
>                 };
>                 serial@...12100 {
> @@ -62,7 +62,7 @@
>                                 reg = <0xd0012100 0x100>;
>                                 reg-shift = <2>;
>                                 interrupts = <42>;
> -                               reg-io-width = <4>;
> +                               reg-io-width = <1>;
>                                 status = "disabled";
>                 };
>  
> Thanks,
> 


-- 
Gregory Clement, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com
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