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. > > > > 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. You might be able to handle it as a platform device. It's an onboard peripheral after all and wired permanently subject to further run-time configuration. Otherwise it's a generic off-the-shelf pre-LPC-bus PC Super I/O chip. Even if we can detect it it'll be there on some x86 machine. And the issue is a problem that may well be anywhere between the CPU, the northbridge, the southbridge and the Super I/O, and the weak MMIO ordering of the Alpha CPU does not help narrowing it down. Let me see... It's an NS PC87332 piece. For Avanti technical spec see: <https://manx-docs.org/collections/mds-199909/cd1/alpha/pcdsatia.pdf> and for the National Semiconductor piece search for "PC87332.pdf" (no direct link, but you can download it indirectly): 2.5.8 SuperI/O Identification Register (SID, Index = 08h) The SID Register is accessed, like the other configuration registers, through the Index Register. This read-only register is used to identify the PC87332 device. 7 6 5 4 3 2 1 0 0 0 0 1 X X X X Super I/O Identification Reg. (SID) Index = 08h I'm not sure how reliable the uniqueness of the four bits in the SID register is across various PC Super I/O chips. I doubt that the chip has any observable issues with our serial driver on x86 systems though. I'm not sure if the situation is fully understood here, but we have a regression and a working solution now is better than a perfect one in the unspecified future. We can always improve once we get to the bottom of the issue. I'm in lockdown away from my Alpha machine, but I can try verifying the solution, also with PCI/e serial ports once I am out of lockdown and back the right home sometime. Maciej
