On Fri, Apr 29, 2016 at 04:37:57PM +0100, Richard W.M. Jones wrote:
> On Fri, Apr 29, 2016 at 08:16:35AM -0700, Greg KH wrote:
> > On Fri, Apr 29, 2016 at 09:10:06AM +0100, Richard W.M. Jones wrote:
> > > On Thu, Apr 28, 2016 at 03:56:33PM -0700, Greg KH wrote:
> > > > On Thu, Apr 28, 2016 at 11:18:33PM +0100, Richard W.M. Jones wrote:
> > > > > Currently autoconf spends 25ms (on my laptop) testing if the UART
> > > > > exported to it by KVM is an 8250 without FIFO and/or with strange
> > > > > quirks, which it obviously isn't. Assume it is exported to us by a
> > > > > hypervisor, it's a normal, working 16550A.
> > > > >
> > > > > Signed-off-by: Richard W.M. Jones <rjones@xxxxxxxxxx>
> > > > > ---
> > > > > drivers/tty/serial/8250/8250_port.c | 7 +++++++
> > > > > 1 file changed, 7 insertions(+)
> > > > >
> > > > > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> > > > > index 00ad2637..de19924 100644
> > > > > --- a/drivers/tty/serial/8250/8250_port.c
> > > > > +++ b/drivers/tty/serial/8250/8250_port.c
> > > > > @@ -1171,6 +1171,13 @@ static void autoconfig(struct uart_8250_port *up)
> > > > > if (!port->iobase && !port->mapbase && !port->membase)
> > > > > return;
> > > > >
> > > > > + /* Hypervisors always export working 16550A devices. */
> > > > > + if (cpu_has_hypervisor) {
> > > > > + up->port.type = PORT_16550A;
> > > > > + up->capabilities |= UART_CAP_FIFO;
> > > > > + return;
> > > > > + }
> > > >
> > > > Have you audited vmware, virtualbox, and everyone else that provides a
> > > > virtual uart device that it will work properly here?
> > > >
> > > > qemu isn't all the world :)
> > >
> > > Attached below is a slightly different approach. If the user passes a
> > > special flag on the kernel command line then we force 16550A and avoid
> > > the 25ms delay. Since the user chooses the flag, any concerns about
> > > the behaviour of the hypervisor or use of VFIO should be moot.
> >
> > No, no more module parameters, that's crazy, what happens when you have
> > 64 serial ports in a system, which one is this option for?
>
> In this (very special) case, the domain is running under qemu and
> I know it only has one serial port that doesn't need probing.
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?
good luck,
greg k-h
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