Message-ID: <9b06ce8f-887f-b977-d277-6d0a8e04a2dc@gmail.com>
Date:   Wed, 5 Dec 2018 14:40:24 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Guenter Roeck <linux@...ck-us.net>
Cc:     arnd@...db.de, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, benh@...nel.crashing.org,
        paulus@...ba.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] serial: 8250: Default SERIAL_OF_PLATFORM to SERIAL_8250

On 12/4/18 9:47 PM, Michael Ellerman wrote:
> Hi Florian,
> 
> Florian Fainelli <f.fainelli@...il.com> writes:
>> +PPC folks
>>
>> On 11/23/18 10:20 AM, Guenter Roeck wrote:
>>> On Mon, Nov 19, 2018 at 12:50:50PM -0800, Guenter Roeck wrote:
>>>> On Mon, Nov 19, 2018 at 10:44:30AM -0800, Florian Fainelli wrote:
>>>>> On 11/15/18 5:16 PM, Guenter Roeck wrote:
>>>>>> On Thu, Nov 15, 2018 at 11:48:20AM -0800, Florian Fainelli wrote:
>>>>>>>
>>>>>>> OK, would you mind testing this below? It seems to me that 8250_of.c is
>>>>>>> incompatible with arch/powerpc/kernel/legacy_serial.c and that is what
>>>>>>> is causing the issue here.
>>>>>>>
>>>>>>> diff --git a/drivers/tty/serial/8250/Kconfig
>>>>>>> b/drivers/tty/serial/8250/Kconfig
>>>>>>> index d7737dca0e48..21cb14cbd34a 100644
>>>>>>> --- a/drivers/tty/serial/8250/Kconfig
>>>>>>> +++ b/drivers/tty/serial/8250/Kconfig
>>>>>>> @@ -483,7 +483,7 @@ config SERIAL_8250_PXA
>>>>>>>
>>>>>>>  config SERIAL_OF_PLATFORM
>>>>>>>         tristate "Devicetree based probing for 8250 ports"
>>>>>>> -       depends on SERIAL_8250 && OF
>>>>>>> +       depends on SERIAL_8250 && OF && !(PPC && PPC_UDBG_16550)
>>>>>>>         default SERIAL_8250
>>>>>>>         help
>>>>>>>           This option is used for all 8250 compatible serial ports that
>>>>>>
>>>>>> 44x/virtex5_defconfig has both PPC_UDBG_16550 and SERIAL_OF_PLATFORM enabled
>>>>>> and fails to boot (or display anything on the console) with this patch applied.
>>>>>
>>>>> Thanks for trying, can you either share or provide a link to the mpc85xx
>>>>> and ml507 qemu command lines that you use? I spent a good chunk of my
>>>>> time trying to get a kernel to boot but has failed so far.
>>>>>
>>>>
>>>
>>> Any update ? I still see the boot failures in next-20181123.
>>
>> Yes, took most of last week's off sorry for the delay. I have finally
>> been able to boot a kernel using your instructions, thanks. The problem
>> is the following call chain:
>>
>> of_platform_serial_probe()
>>  -> serial8250_register_8250_port()
>>     -> up->port.uartclk == 0, return -EINVAL
>> 	-> irq_dispose_mapping()
>>
>> and then we basically unwind what we just did with
>> of_platform_serial_probe() including disabling the UART's IRQ, comment
>> out the irq_dispose_mapping() and you will have a functional console
>> again. 8250_of.c is clearly not a full replacement for legacy_serial.c
>> (that was a first attempt), so we need to keep that code around. Making
>> the depends even more conditional also does not sound too appealing,
>> because while we have identified mpc85xx as being problematic, who knows
>> about other platforms. So the best I have that does not involve a revert
>> is this below.
>>
>> Ben, Michael, would that sound reasonable to you? It seems to me that
>> there is a million ways to shoot the legacy_serial 8250 registration in
>> the foot in any cases, and having CONFIG_SERIAL_OF_PLATFORM just made it
>> painfully obvious.
> 
> We generally try to avoid changing the device tree from inside Linux,
> it's meant to describe the hardware as we're given it, not the state of
> Linux drivers etc. Perhaps this is an exceptional case but it still
> seems fishy.

Technically this is changing the Linux internal representation of a
Device Tree node reference (device_node), which is a bit away from
actual HW representation.

> 
> I also worry that marking the device node disabled might break something
> else, but that's maybe me being paranoid.

The code as it is today only makes use of device_node pointers/Device
Tree to find out about the UART base address and a bunch of properties,
but it registers the UART as platform_device without making use of the
existing OF infrastructure to do that (historical reasons really).

> 
> I guess I don't really understand how things are supposed to work
> though. We have the 8250 driver and also the OF platform driver, the
> former has already setup the device and then the OF driver just comes
> along and takes over?

When the OF platform driver comes along it tries to register the same
device instance, fails to do so because the port->clk is 0, which the
8250 core thinks is an error, and then it starts unwinding the 8250 port
structure, which in turns steps on the instance that was registered
manually by legacy_serial.c

> 
> eg. in the boot log from the mpc8544ds you see:
> 
>   serial8250.0: ttyS0 at MMIO 0xe0004500 (irq = 42, base_baud = 115200) is a 16550A
>   printk: console [ttyS0] enabled
>   printk: console [ttyS0] enabled
>   printk: bootconsole [udbg0] disabled
>   printk: bootconsole [udbg0] disabled
>   of_serial: probe of e0004500.serial failed with error -22
> 
> ie. the 8250 core has already setup the device, so there should really
> be nothing to do?
> 
> If the 8250 code could detect that it already has this port registered
> then maybe things would work.
> 
> The patch below works for me with mpc8544ds. Whether it's worth changing
> the 8250 code this much to accomodate legacy_serial I'm not sure.

Not sure either, that was the route I initially took, but I was worried
about the impact and the inability to test that across a variety of
systems to make sure we would not be breaking 8250, which is really
ubiquitous. Your patch looks very reasonable though, so this might be an
appropriate course of action to take. It's not like we are supposed to
do what is happening here :)

The change in legacy_serial.c was self contained an while a hack, which
quite frankly, that whole file is as well, did the job ;)



> 
> cheers
> 
> 
> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> index 94f3e1c64490..c7d7858fdb3d 100644
> --- a/drivers/tty/serial/8250/8250_core.c
> +++ b/drivers/tty/serial/8250/8250_core.c
> @@ -905,7 +905,7 @@ static struct platform_device *serial8250_isa_devs;
>   */
>  static DEFINE_MUTEX(serial_mutex);
>  
> -static struct uart_8250_port *serial8250_find_match_or_unused(struct uart_port *port)
> +static struct uart_8250_port *serial8250_find_existing(struct uart_port *port)
>  {
>  	int i;
>  
> @@ -921,10 +921,17 @@ static struct uart_8250_port *serial8250_find_match_or_unused(struct uart_port *
>  	if (i < nr_uarts && serial8250_ports[i].port.type == PORT_UNKNOWN &&
>  			serial8250_ports[i].port.iobase == 0)
>  		return &serial8250_ports[i];
> +
> +	return NULL;
> +}
> +
> +static struct uart_8250_port *serial8250_find_unused(struct uart_port *port)
> +{
> +	int i;
> +
>  	/*
> -	 * We didn't find a matching entry, so look for the first
> -	 * free entry.  We look for one which hasn't been previously
> -	 * used (indicated by zero iobase).
> +	 * Look for the first free entry. We look for one which hasn't been
> +	 * previously used (indicated by zero iobase).
>  	 */
>  	for (i = 0; i < nr_uarts; i++)
>  		if (serial8250_ports[i].port.type == PORT_UNKNOWN &&
> @@ -960,12 +967,18 @@ int serial8250_register_8250_port(struct uart_8250_port *up)
>  	struct uart_8250_port *uart;
>  	int ret = -ENOSPC;
>  
> -	if (up->port.uartclk == 0)
> -		return -EINVAL;
> -
>  	mutex_lock(&serial_mutex);
>  
> -	uart = serial8250_find_match_or_unused(&up->port);
> +	uart = serial8250_find_existing(&up->port);
> +	if (!uart) {
> +		if (up->port.uartclk == 0) {
> +			ret = -EINVAL;
> +			goto out;
> +		}
> +
> +		uart = serial8250_find_unused(&up->port);
> +	}
> +
>  	if (uart && uart->port.type != PORT_8250_CIR) {
>  		if (uart->port.dev)
>  			uart_remove_one_port(&serial8250_reg, &uart->port);
> @@ -974,7 +987,10 @@ int serial8250_register_8250_port(struct uart_8250_port *up)
>  		uart->port.membase      = up->port.membase;
>  		uart->port.irq          = up->port.irq;
>  		uart->port.irqflags     = up->port.irqflags;
> -		uart->port.uartclk      = up->port.uartclk;
> +
> +		if (up->port.uartclk != 0)
> +			uart->port.uartclk      = up->port.uartclk;
> +
>  		uart->port.fifosize     = up->port.fifosize;
>  		uart->port.regshift     = up->port.regshift;
>  		uart->port.iotype       = up->port.iotype;
> @@ -1056,6 +1072,7 @@ int serial8250_register_8250_port(struct uart_8250_port *up)
>  			ret = 0;
>  		}
>  	}
> +out:
>  	mutex_unlock(&serial_mutex);
>  
>  	return ret;
> 


-- 
Florian

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