dw8250_set_termios() questions

From: Alex Elder
Date: Fri Jul 11 2014 - 08:57:10 EST

Heikki, I have not been a subscriber of the linux-serial
mailing list and didn't see this patch go by:
serial: 8250_dw: clock rate handling for all ACPI platforms

I had been working on doing something very similar for some
Broadcom device tree based devices and it might have been
helpful for me to have seen it. What I ended up with was
*very* similar to what you did. Here is the last version
of the patch I posted:

There *are* some differences, and I'd like to inquire about
them before I simply use the code you have for my purpose.

These first two relate to whether I can use your
code as-is:
- Why do you skip setting the clock if a null "old"
pointer is supplied?
- I don't believe it's necessary to surround the clock
rate change with clk_disable_unprepare() and
clk_prepare_enable(). Do you believe otherwise?

This one is addressed to how your code is used now:
- Alan Cox had this question about my patch, and
it seems to apply to your code as well:
"This assumes an arbitarily configurable clock,
which is not I think the usual case."
His point is that the clock, if adjustable, may
not support a rate that produces an acceptable
signal rate. Put another way, there may be a
better frequency than what the clock framework
selects that (in combination with the UART
divisor latch registers) produces the best--or
even a good--signal. Is there any chance any
ACPI platforms will suffer this problem?


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