Re: [PATCH] 8250: Hypervisors always export working 16550A UARTs.

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Apr 29, 2016 at 05:02:57PM +0100, Richard W.M. Jones wrote:
> On Fri, Apr 29, 2016 at 08:54:13AM -0700, Greg KH wrote:
> > You are trying to take a generalized kernel and somehow "know" about the
> > hardware ahead of time it is going to run on.  That seems like two
> > conflicting requirements, don't you agree?
> 
> We would have the 8250 serial port in any kernel.  Even if Fedora
> kernel maintainers allowed us to have specialized kernels for each
> purpose, I would use the simple ISA serial port here because it allows
> us to capture debug messages very early in the boot.  Alternatives
> like virtio-console don't allow that.
> 
> The kernel does know what hardware it's running on - via the CPUID
> hypervisor leaf.  It's also possible for us to tell the kernel about
> the hardware using the command line, ACPI[*], DT, etc.  I'd really
> like to tell the kernel this is a 16550A, not broken, you don't need
> to spend time testing that.

Then properly describe the device to the kernel in a way that you know
you don't have to do the 20ms sleep from your system configuration file
(acpi, dt, etc.)

good luck,

greg k-h