Re: [PATCH 2/2 v4] alpha: add a delay before serial port read

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, May 07, 2020 at 04:18:49AM -0400, Mikulas Patocka wrote:
> 
> 
> On Wed, 6 May 2020, Greg Kroah-Hartman wrote:
> 
> > On Wed, May 06, 2020 at 01:04:38PM -0400, Mikulas Patocka wrote:
> > > 
> > > I've created this patch that adds a global macro/variable 
> > > serial_port_needs_delay. I've also deleted UPQ_DELAY_BEFORE_READ and test 
> > > serial_port_needs_delay directly in io_serial_in, so that the compiler 
> > > will optimize it out on non-alpha architectures.
> > 
> > That's not good, what about systems with hundreds of serial ports?
> 
> I doubt that someone will conect hundreds of serial ports to such an old 
> alpha machine :)



> 
> > > > But, there is no other way to detect this based on hardware
> > > > signatures/types instead?  That is usually the best way to do it, right?
> > > 
> > > It's hard to detect Alpha without using '#ifdef CONFIG_ALPHA' :) The ISA 
> > > serial port hardware is simple, so I think that you can't distinguish it 
> > > just based on its behavior.
> > 
> > The ISA serial port hardware does not have a unique vendor/product id
> > somewhere?  Some other sort of definition that we can use to determine
> > exactly what type of system we are running on?
> 
> AFAIK it doesn't. You can only distinguish 8250, 16550 and 16550A - but 
> not the vendor.
> 
> > > Index: linux-stable/drivers/tty/serial/8250/8250_port.c
> > > ===================================================================
> > > --- linux-stable.orig/drivers/tty/serial/8250/8250_port.c	2020-05-06 18:54:24.000000000 +0200
> > > +++ linux-stable/drivers/tty/serial/8250/8250_port.c	2020-05-06 18:54:24.000000000 +0200
> > > @@ -30,6 +30,7 @@
> > >  #include <linux/uaccess.h>
> > >  #include <linux/pm_runtime.h>
> > >  #include <linux/ktime.h>
> > > +#include <linux/pci.h>
> > >  
> > >  #include <asm/io.h>
> > >  #include <asm/irq.h>
> > > @@ -442,6 +443,9 @@ static unsigned int mem32be_serial_in(st
> > >  
> > >  static unsigned int io_serial_in(struct uart_port *p, int offset)
> > >  {
> > > +	if (serial_port_needs_delay)
> > > +		ndelay(300);
> > 
> > Again, this should be a per-port thing, not all ports in the system are
> > this broken, right?
> > 
> > thanks,
> > 
> > greg k-h
> 
> Here is the patch that uses per-port flag UPQ_DELAY_BEFORE_READ. The flag 
> is activated if we have the specific PCI-ISA bridge and if the serial port 
> is an ISA port.


Better, care to submit this in a format that it can be applied in?

thanks,

greg k-h