Hi Aleksey,

On 12/06/2016 12:58 PM, Aleksey Makarov wrote:

It turns out that this approach does not work for all the existing hardware.
There are two problems with AppliedMicro X-Gene based boards
([discussion], [v1]):

1. Their console is a 16550 port that requires 32-bit access.  Now SPCR does not
   have any method to specify this.

2. Some of the boards don't use the "standard" 16550 clock rate so supplying
   a baud rate makes it change to a random baud rate.

Patch 1/4 uses 'Register Bit Width' field of the ACPI Generic Address
Structure that specifies the address of the UART registers to
decide if the driver should use "mmio32" access instead of "mmio".
This fixes problem 1 for existing hardware/firmware.

To fix problem 2, I suggest to introduce a new value '0' for the "Baud Rate"
field of SPCR (now this value is reserved).  I would like to discuss if this
could be added to SPCR spec and will fix the problem.

Part 1 could well be solved in this way, provided it can be made part
of the mainstream specification. It seems fairly reasonable, however.

I'm not sure I subscribe to part 2 because there could be all manner
of havoc changing the interpretation of existing zero values. That is
why I personally favor an AppliedMicro SPCR subtype specific to them.

That all said, I did discuss exactly part 2 with the folks responsible
for the SPCR specification and they are thinking about their preferred
solution, which will likely either be a new subtype for Applied, or
perhaps allow for what you describe. I will followup when I know.

Jon.

-- 
Computer Architect | Sent from my Fedora powered laptop