Re: [PATCH v2 02/15] serial: core: add uart_iotype_mmio/legacy_io helper functions

[Maciej W. Rozycki]

On Mon, 4 May 2026, Andy Shevchenko wrote:

> > > Why do we use 'legacy'? Still in use in modern CPUs...
> > 
> >  Deprecated in PCIe and not available in numerous systems.  Also actually 
> > called "legacy" in some serial port datasheets aged ~20 years now.  While 
> > some contemporary CPUs indeed retain the port I/O address space, it's for 
> > legacy use anyway, you don't want to rely on it in new designs.
> 
> For the holder of the new (modern) CPU which supports the IO ports, this is
> definitely not a legacy interface despite on whatever PCIe or other datasheets
> call it.

 I appreciate your point of view, however I disagree that the presence of 
an interface in a contemporary chip makes the interface modern.

 I see the port I/O space so much legacy as say the PC/AT DMA controller 
(8237 pair), which is similarly present in current x86 chipsets.  If this 
stuff was not present, such as say the PC/AT interrupt controller (8259A 
pair), which I believe has been removed from some x86 system designs, then 
it would be obsolete/removed rather than legacy.

 It is analogous to PCI/e systems that lack a southbridge and are called 
"legacy-free", as the whole southbridge stuff, the main consumer of the 
port I/O space still remaining in use, is legacy nowadays (the other one I 
know of being the 8255-based PC parallel port, which has been considered a 
legacy interface as well, even though you can still buy and plug one into 
a modern PCIe system).

 NB I have a couple of modern x86 CPUs around too that support the port 
I/O space, but it doesn't change my view as to the nomenclature.

 FWIW,

  Maciej