linux-kernel - Re: [PATCH v2 0/3] arm: aspeed: Add UART routing support

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Date:   Wed, 8 Sep 2021 12:42:49 +0300
From:   Paul Fertser <fercerpav@...il.com>
To:     Chia-Wei Wang <chiawei_wang@...eedtech.com>
Cc:     robh+dt@...nel.org, joel@....id.au, andrew@...id.au,
        lee.jones@...aro.org, osk@...gle.com, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-aspeed@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        openbmc@...ts.ozlabs.org,
        Konstantin Klubnichkin <kitsok@...dex-team.ru>
Subject: Re: [PATCH v2 0/3] arm: aspeed: Add UART routing support

Hello,

On Thu, Sep 02, 2021 at 10:18:13AM +0800, Chia-Wei Wang wrote:
> Add UART routing driver and the device tree nodes.

Thank you for working on exposing this functionality in upstreamable
way, that's so much better than all the register-level hacks in U-Boot
and similar approaches!

One (somewhat) related question that I hope you do not mind answering:
is there anything special regarding the routing or other configuration
that needs to be done for VUART to work with IRQs?

The reason I ask is that I have tried hard (and I know several other
developers who have too) to use VUART functionality but somehow as
soon as Linux was booting on host and starting to use the IRQ-based
16550 driver the communication was halted both ways. Basically, the
BMC firmware was enabling VUART in DTS, then setting LPC address to
0x3F8 and LPC IRQ to 4 and reading/writing using the corresponding
/dev/ttyS* node. The datasheet is not clearly telling what other
actions need to be performed for this to work. Not using VUART and
instead routing UART1 lines with exactly the same pinctrl/pinmux
worked just fine. One detail is that with VUART the host wasn't seeing
new interrupts but when they were simulated by exporting the LPC
interrupt pin via /sys/class/gpio and toggling it manually the data
was getting through.

Does UART1 need some explicit disabling for VUART IRQs to work? It
looks like setting LPC address and IRQ number in VUART is enough to
override the register part but probably not for the interrupt?

-- 
Be free, use free (http://www.gnu.org/philosophy/free-sw.html) software!
mailto:fercerpav@...il.com