Re: serial8250_init and platform_device

[Kumar Gala <kumar.gala@freescale.com>]

Russell,

I think this all makes sense to me.  I'm just wondering why we would 
have a platform device register in a system for 'legacy ISA' when we 
know the system doesnt have any ports that will fit the category.

As you show in example #2 you have

.../devices/platform/serial82500
.../devices/platform/serial8250

why have the 'serial8250' if you know your system doesnt have any ports 
that will exist there?

- kumar

On Jan 20, 2005, at 9:44 AM, Russell King wrote:

> On Thu, Jan 20, 2005 at 09:23:56AM -0600, Kumar Gala wrote:
>  > No problem, let me try to clarify.  I'm trying to figure out in the 
> ARM
>  > case if there are 2 platform_devices that are registered and if 
> this is
> > the desired behavior (and if so why?).
>
> Yes.  The first (by serial8250_init) is the ISA compatibility platform
>  device, which registers the "old" static list of serial devices found
>  in include/asm-*/serial.h.
>
> The second registers a platform device which contains the 8250 serial
>  devices for the particular platform.
>
> > In serial8250_init() we call platform_device_register_simple(), this
> > will be one registration of a serial8250 device.  In my example of
> > vr1000, arch/arm/mach-s3c2410/cpu.c:s3c_arch_init() calls
> > platform_device_register, the 2nd time a serial8250 device is
> > registered.
>
> Correct.
>
> > > We then register the device driver, which allows us to pick up on 
> the
>  > >  platform devices.  This causes the placeholder registrations to 
> be
>  > >  reassigned to the platform devices on a first come first served 
> basis
>  > >  via the standard registration call serial8250_register_port().
> >
> > I'm not following you here, its not clear if you mean we have 2
> > platform devices registered in the system, but only one actually has
> > serial ports that are registered.
>
> We have two platform devices registered - one representing the
>  compatibility devices, and one (or more) representing the platform's
>  own devices.
>
> To illustrate this, let's assume your architecture always has ports at
>  a fixed set of addresses, and then has a set of platform specific 
> ports.
>  You may wish to have one platform device for the platform common 
> serial
>  devices which always gets registered.  Your platform specific
>  initialisation code could then register another platform device which
>  contains details of the platform specific serial devices.
>
> > If you are using SERIAL_PORT_DFNS,
>  > it will be the platform_device created in serial8250_init(), if you 
> are
> > not it will be the platform_device created elsewhere?
>
> Initially, all serial devices are attached to the platform device
>  created in serial8250_init(), whether or not they are listed in
>  SERIAL_PORT_DFNS or not.  Serial devices not listed in 
> SERIAL_PORT_DFNS
> remain in an unconfigured state.
>
> When the 8250 platform device driver is registered, serial devices
>  are "stolen" from this "private" platform device, and are owned by
>  the platform device which registered them.
>
> Lets take some examples:
>
> 1. SERIAL_PORT_DFNS contains port at 0x3f8, irq4, and we have space for
>     4 serial ports.  No other platform devices are registered.
>
> # cat /proc/tty/driver/serial
> serinfo:1.0 driver revision:
>  0: uart:16550A port:000003F8 irq:4 tx:0 rx:0
>  1: uart:unknown port:00000000 irq:0
>  2: uart:unknown port:00000000 irq:0
>  3: uart:unknown port:00000000 irq:0
>  # tree /sys/class/tty/ttyS*
> /sys/class/tty/ttyS0
> |-- dev
>  `-- device -> ../../../devices/platform/serial8250
> /sys/class/tty/ttyS1
> |-- dev
>  `-- device -> ../../../devices/platform/serial8250
> /sys/class/tty/ttyS2
> |-- dev
>  `-- device -> ../../../devices/platform/serial8250
> /sys/class/tty/ttyS3
> |-- dev
>  `-- device -> ../../../devices/platform/serial8250
>
> 2. SERIAL_PORT_DFNS contains port at 0x3f8, irq4, and we have space for
>     4 serial ports.  A platform device also describing 0x3f8, irq 4 is
>     registered.
>
> # cat /proc/tty/driver/serial
> serinfo:1.0 driver revision:
>  0: uart:16550A port:000003F8 irq:4 tx:0 rx:0
>  1: uart:unknown port:00000000 irq:0
>  2: uart:unknown port:00000000 irq:0
>  3: uart:unknown port:00000000 irq:0
>  # tree /sys/class/tty/ttyS*
> /sys/class/tty/ttyS0
> |-- dev
>  |-- device -> ../../../devices/platform/serial82500
> `-- driver -> ../../../bus/platform/drivers/serial8250
> /sys/class/tty/ttyS1
> |-- dev
>  `-- device -> ../../../devices/platform/serial8250
> /sys/class/tty/ttyS2
> |-- dev
>  `-- device -> ../../../devices/platform/serial8250
> /sys/class/tty/ttyS3
> |-- dev
>  `-- device -> ../../../devices/platform/serial8250
>
> 3. SERIAL_PORT_DFNS is empty, otherwise the same as case (2).  Results
>     are identical to case (2).
>
> HTH.
>
> -- 
> Russell King
>   Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
>  maintainer of:  2.6 PCMCIA      - http://pcmcia.arm.linux.org.uk/
>                  2.6 Serial core