From: Russell King - ARM Linux <linux@armlinux.org.uk>
To: Dave Martin <Dave.Martin@arm.com>
Cc: Rob Herring <robh@kernel.org>,
Stefano Stabellini <sstabellini@kernel.org>,
Andre Przywara <andre.przywara@arm.com>,
Julien Grall <julien.grall@arm.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Bhupinder Thakur <bhupinder.thakur@linaro.org>,
xen-devel@lists.xenproject.org
Subject: Re: [PATCH 26/27 v12] arm/xen: vpl011: Fix the slow early console SBSA UART output
Date: Tue, 17 Oct 2017 15:03:02 +0100 [thread overview]
Message-ID: <20171017140302.GN20805@n2100.armlinux.org.uk> (raw)
In-Reply-To: <20171017134429.GV19485@e103592.cambridge.arm.com>
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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next prev parent reply other threads:[~2017-10-17 14:03 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-13 10:40 [PATCH 26/27 v12] arm/xen: vpl011: Fix the slow early console SBSA UART output Bhupinder Thakur
2017-10-13 10:40 ` [PATCH 27/27 v12] arm/xen: vpl011: Correct the logic for asserting/de-asserting SBSA UART TX interrupt Bhupinder Thakur
2017-10-13 13:59 ` Dave Martin
2017-10-18 10:26 ` Andre Przywara
2017-10-18 10:47 ` Bhupinder Thakur
2017-10-18 13:41 ` [PATCH RFC] ARM: vPL011: use receive timeout interrupt Andre Przywara
2017-10-18 16:32 ` Bhupinder Thakur
2017-10-23 16:01 ` Andre Przywara
2017-10-24 11:00 ` Julien Grall
2017-10-24 11:27 ` Andre Przywara
2017-10-24 12:56 ` Bhupinder Thakur
2017-10-24 12:56 ` Bhupinder Thakur
2017-10-17 9:51 ` [PATCH 26/27 v12] arm/xen: vpl011: Fix the slow early console SBSA UART output Andre Przywara
2017-10-17 11:19 ` Dave Martin
2017-10-17 12:53 ` Rob Herring
2017-10-17 13:44 ` Dave Martin
2017-10-17 14:03 ` Russell King - ARM Linux [this message]
2017-10-17 14:49 ` Dave P Martin
2017-10-18 10:17 ` Bhupinder Thakur
2017-10-18 10:31 ` Andre Przywara
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