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

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Message-ID: <20160429173202.GA9134@kroah.com>
Date:	Fri, 29 Apr 2016 10:32:02 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	"Richard W.M. Jones" <rjones@...hat.com>
Cc:	jslaby@...e.com, peter@...leysoftware.com,
	andriy.shevchenko@...ux.intel.com, phillip.raffeck@....de,
	anton.wuerfel@....de, yamada.masahiro@...ionext.com,
	matwey@....msu.ru, valentinrothberg@...il.com,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	ddutile@...hat.com
Subject: Re: [PATCH] 8250: Hypervisors always export working 16550A UARTs.

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