[PATCH] serial: forbid 8250 on s390

[PATCH] serial: forbid 8250 on s390

[Christian Borntraeger]

Using "make kvmconfig" results in a potentially unusable linux image
on s390.  The reason is that both the (default on s390) sclp consoles
as well as the 8250 console register a ttyS<x> as console. Since there
will be no 8250 on s390 let's fence 8250. This will ensure that there
is always a working sclp console.

Reported-by: Alice Frosi <alice@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
---
 drivers/tty/serial/8250/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/tty/serial/8250/Kconfig b/drivers/tty/serial/8250/Kconfig
index a5c0ef1..16b1496 100644
--- a/drivers/tty/serial/8250/Kconfig
+++ b/drivers/tty/serial/8250/Kconfig
@@ -5,6 +5,7 @@
 
 config SERIAL_8250
 	tristate "8250/16550 and compatible serial support"
+	depends on !S390
 	select SERIAL_CORE
 	---help---
 	  This selects whether you want to include the driver for the standard
-- 
2.9.4

Re: [PATCH] serial: forbid 8250 on s390

[Greg Kroah-Hartman]

On Tue, Dec 12, 2017 at 09:08:35AM +0100, Christian Borntraeger wrote:
> Using "make kvmconfig" results in a potentially unusable linux image
> on s390.  The reason is that both the (default on s390) sclp consoles
> as well as the 8250 console register a ttyS<x> as console. Since there
> will be no 8250 on s390 let's fence 8250. This will ensure that there
> is always a working sclp console.
> 
> Reported-by: Alice Frosi <alice@linux.vnet.ibm.com>
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> Reviewed-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
> ---
>  drivers/tty/serial/8250/Kconfig | 1 +
>  1 file changed, 1 insertion(+)

Does this need to be backported to any stable kernels?  Or does anyone
who runs them on s390 already have this config option disabled?

thanks,

greg k-h

Re: [PATCH] serial: forbid 8250 on s390

[Christian Borntraeger]

On 12/12/2017 09:48 AM, Greg Kroah-Hartman wrote:
> On Tue, Dec 12, 2017 at 09:08:35AM +0100, Christian Borntraeger wrote:
>> Using "make kvmconfig" results in a potentially unusable linux image
>> on s390.  The reason is that both the (default on s390) sclp consoles
>> as well as the 8250 console register a ttyS<x> as console. Since there
>> will be no 8250 on s390 let's fence 8250. This will ensure that there
>> is always a working sclp console.
>>
>> Reported-by: Alice Frosi <alice@linux.vnet.ibm.com>
>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>> Reviewed-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
>> ---
>>  drivers/tty/serial/8250/Kconfig | 1 +
>>  1 file changed, 1 insertion(+)
> 
> Does this need to be backported to any stable kernels?  Or does anyone
> who runs them on s390 already have this config option disabled?

All shipped s390 kernels have 8250 disabled anyway. A backport of this
patch could be helpful to let users who want to use "make kvmconfig" give 
a working configuration but it is certainly not critical.

Christian

Re: [PATCH] serial: forbid 8250 on s390

[Alan Cox]

On Tue, 12 Dec 2017 09:08:35 +0100
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> Using "make kvmconfig" results in a potentially unusable linux image
> on s390.  The reason is that both the (default on s390) sclp consoles
> as well as the 8250 console register a ttyS<x> as console. Since there
> will be no 8250 on s390 let's fence 8250. This will ensure that there
> is always a working sclp console.

And there is no physical way to attach a PCI express UART to a 390 ?

Alan

Re: [PATCH] serial: forbid 8250 on s390

[Christian Borntraeger]

On 12/12/2017 04:50 PM, Alan Cox wrote:
> On Tue, 12 Dec 2017 09:08:35 +0100
> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> 
>> Using "make kvmconfig" results in a potentially unusable linux image
>> on s390.  The reason is that both the (default on s390) sclp consoles
>> as well as the 8250 console register a ttyS<x> as console. Since there
>> will be no 8250 on s390 let's fence 8250. This will ensure that there
>> is always a working sclp console.
> 
> And there is no physical way to attach a PCI express UART to a 390 ?

There is (some sort of) PCI but these cards are packaged in ibm specific I/O cages.
So someone would need to apply some some serious hardware and firmware hacking (since
only IBM-approved PCi cards are allowed) and you would need to have an MSI-capable
8250 as well as an MSI capable driver. (we only support MSI, no classic interrupts)
In addition to that that 8250 probably needs to support virtual functions since we always
run with LPARs.
And even if we can solve all these issues I would say it really does not make any sense at
all, while the current situation obviously breaks the use case "make kvmconfig"

So unless IBM sees some serious business case for 8250, I am inclined to answer you
question with "no" :-)