Re: [Xen-devel] [PATCH 26/27 v12] arm/xen: vpl011: Fix the slow early console SBSA UART output

[Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx>]

On Tue, Oct 17, 2017 at 02:44:29PM +0100, Dave Martin wrote:
> arch/arm/kernel/debug.S:
> 
> ENTRY(printascii)
>                 addruart_current r3, r1, r2
>                 b       2f
> 1:              waituart r2, r3
>                 senduart r1, r3
>                 busyuart r2, r3
>                 teq     r1, #'\n'
>                 moveq   r1, #'\r'
>                 beq     1b
> 2:              teq     r0, #0
>                 ldrneb  r1, [r0], #1
>                 teqne   r1, #0
>                 bne     1b
>                 ret     lr
> ENDPROC(printascii)
> 
> ENTRY(printch)
>                 addruart_current r3, r1, r2
>                 mov     r1, r0
>                 mov     r0, #0
>                 b       1b
> ENDPROC(printch)
> 
> 
> 
> Russell, do you know why we wait for the UART transmitter to go
> completely idle before queueing a new char?

It's the only way the /debug/ (and I stress /debug/) functions know
that the character has actually been sent before allowing control to
continue.  This is important for early-crashy-debug situations, but
probably less so for early_printk.

> For an individual printch this can makes sense, but it also introduces
> delay for every char in printascii.
> 
> This seems to interact interestingly with virtualised UARTs, because we
> may thrash between the guest and hypervisor per-char, though there
> may be a way to reduce the impact of this on the emulation side.

Well, I guess re-using the early /debugging/ code for early printk is
showing more and more issues - and reusing this code in this way is
something that I've never really been a fan of.

I'd personally like to see the coupling between the two things gone -
I never really wanted that coupling in the first place.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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