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

[Mikulas Patocka <mpatocka@redhat.com>]

On Sun, 10 May 2020, Maciej W. Rozycki wrote:

> On Thu, 7 May 2020, 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 :)
> 
> It would be good if PCI serial ports (on add-on cards) were unaffected.

After reading the Alpha specification, I am convinced that the issue is 
not timing, but reordering or merging of accesses to the MMIO space.

So, we need a barrier before a write (mandated by memory-barriers.txt), 
after a read (mandated by memory-barriers.txt) and between write and read 
(mandated by the alpha spec).

The performance of serial ports could be improved if we changed it to use 
read_relaxed and write_relaxed (the serial port never does DMA, so we 
don't have to deal with DMA ordering).

Mikulas