linux-kernel - Re: [PATCH v4 14/14] tty: gunyah: Add tty console driver for RM Console Services

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Date:   Sun, 9 Oct 2022 13:59:21 -0700
From:   Elliot Berman <quic_eberman@...cinc.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
CC:     Bjorn Andersson <quic_bjorande@...cinc.com>,
        Murali Nalajala <quic_mnalajal@...cinc.com>,
        Trilok Soni <quic_tsoni@...cinc.com>,
        "Srivatsa Vaddagiri" <quic_svaddagi@...cinc.com>,
        Carl van Schaik <quic_cvanscha@...cinc.com>,
        Prakruthi Deepak Heragu <quic_pheragu@...cinc.com>,
        Andy Gross <agross@...nel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Jassi Brar <jassisinghbrar@...il.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        Mark Rutland <mark.rutland@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Marc Zyngier <maz@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        "Will Deacon" <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "Arnd Bergmann" <arnd@...db.de>, <devicetree@...r.kernel.org>,
        <linux-doc@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 14/14] tty: gunyah: Add tty console driver for RM
 Console Services



On 10/7/2022 12:40 AM, Greg Kroah-Hartman wrote:
> On Thu, Oct 06, 2022 at 10:59:51PM -0700, Elliot Berman wrote:
>>
>> "GH" is the shorthand we've been using for "Gunyah". I didn't find
>> documentation for dynamically assigned char devices, but if it exists, I can
>> add entry for ttyGH.
> 
> Why use a new name at all?  Why not stick with the existing tty names
> and device numbers?
> 

I can use hvc framework, although driver-level buffering is needed on
both the get_chars/put_chars paths because:

  - get_chars wants to poll for characters, but Gunyah will push
    characters to Linux
  - put_chars can be called in atomic context in the printk console path.
    Gunyah RM calls can sleep, so we add to buffer and queue work to
    write the characters.

I also chose to use new tty driver because the Gunyah hypervisor call to 
open the console (gh_rm_console_open) can only be done after starting 
the VM. Gunyah will only forward characters sent from the other VM to 
Linux after the gh_rm_console_open call is made. When launching a VM, 
users would want to open console before VM starts so they can get 
startup messages from the VM. I planned to use the carrier_raised() to 
hold tty_port_block_until_ready until the VM is started and the 
gh_rm_console_open() happens.

Thanks,
Elliot