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

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Date:   Mon, 10 Oct 2022 13:58:00 -0700
From:   Elliot Berman <quic_eberman@...cinc.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Jiri Slaby <jirislaby@...nel.org>,
        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 10/10/2022 1:23 PM, Greg Kroah-Hartman wrote:
> On Sun, Oct 09, 2022 at 01:59:21PM -0700, Elliot Berman wrote:
>>
>>
>> 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:
> 
> I'm not asking about the framework (although that is a good question,
> you need to document why this has to be new.)  I'm asking why pick a new
> name?  You will not have a name conflict in your system with this device
> with any other tty name right?
> 

That's correct, I didn't see any other instances of "ttyGH" in kernel.

>>   - 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.
> 
> I'm sorry, but I don't understand this.
> 
> Why is this all a new api at all?  What about the virtio api?  Why not
> just use that instead?

We want to support virtual machines and Virtual Machine Managers (the 
userspace component) that don't support virtio. Qualcomm has some 
lightweight VMs that have a Gunyah driver stack but no virtio stack. 
Further, implementing a simple userspace VMM to launch a Linux kernel is 
much easier with the Gunyah console as no device paravirtualization is 
needed to get some output from Linux. I don't anticipate these VMs to be 
commonplace, but they do exist.

Thanks,
Elliot