Slow performance of VUART on AST2500 with 5.15.5

[Zev Weiss]

On Wed, Dec 15, 2021 at 10:54:19PM CST, Oskar Senft wrote:
>Hi Zev
>
>> >Is anyone aware of AST2500 VUART (or something else that would affect
>> >performance on an AST2500) having gotten broken somewhere between
>> >5.2.11 and 5.15.5?
>>
>> Yes, this is a known issue.
>
>And I felt that I was going crazy! Thanks for confirming that this is
>a known issue.
>
>> Prior to commit 54da3e381c2 ("serial: 8250_aspeed_vuart: use UPF_IOREMAP
>> to set up register mapping"), a long-standing bug in the aspeed-vuart
>> driver had a side-effect of unintentionally leaving the VUART's FIFOs
>> disabled.  With that patch applied to fix the bug, the FIFOs get enabled
>> as they were originally intended to be, but that in turn seems to expose
>> another bug with the host-side THRE bit not getting set when it should,
>> so the host-side driver's write routine ends up waiting for a timeout to
>> expire on every character (when it would otherwise continue on to the
>> next character upon seeing THRE asserted).  I think this may be a VUART
>> hardware problem, though that hasn't been officially confirmed thus far.
>
>Did you reach out to Aspeed yet? I've had some good experiences when
>talking with them directly.
>

I've discussed it a little with Troy on Discord, though I haven't yet
heard any conclusive statements about the recommended course of action.

>
>> I'm hoping we can land on a better solution, but as a temporary
>> band-aid, hacking the driver to treat the device as an 8250 instead of a
>> 16550A (effectively just re-disabling the FIFOs) should speed things
>> back up:
>
>I can confirm that this fixes things, thank you! Is it worth dropping
>that into upstream for the time being, or do you think a "better
>solution" is imminent?
>

I don't really know what the timeline on a better workaround might be
(or even if one is likely to exist); hopefully someone from Aspeed might
be more likely to have some insight on that...

I wouldn't be opposed to dropping that hack into the mainline driver at
least for the time being though; seems like it shouldn't leave things
any worse off than they were before the accidental bugfix side-effect of
the UPF_IOREMAP patch, anyway.  Joel or Andrew, any particular opinions
here? 


Zev