Re: [PATCH v4 14/14] tty: gunyah: Add tty console driver for RM Console Services - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Elliot Berman <quic_eberman@quicinc.com>
Cc: Jiri Slaby <jirislaby@kernel.org>,
	Bjorn Andersson <quic_bjorande@quicinc.com>,
	Murali Nalajala <quic_mnalajal@quicinc.com>,
	Trilok Soni <quic_tsoni@quicinc.com>,
	Srivatsa Vaddagiri <quic_svaddagi@quicinc.com>,
	Carl van Schaik <quic_cvanscha@quicinc.com>,
	Prakruthi Deepak Heragu <quic_pheragu@quicinc.com>,
	Andy Gross <agross@kernel.org>,
	Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
	Jassi Brar <jassisinghbrar@gmail.com>,
	linux-arm-kernel@lists.infradead.org,
	Mark Rutland <mark.rutland@arm.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Marc Zyngier <maz@kernel.org>, Rob Herring <robh+dt@kernel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	Jonathan Corbet <corbet@lwn.net>, Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	devicetree@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 14/14] tty: gunyah: Add tty console driver for RM Console Services
Date: Mon, 10 Oct 2022 22:23:29 +0200	[thread overview]
Message-ID: <Y0R/QbysXa6ebNd8@kroah.com> (raw)
In-Reply-To: <615493a8-449d-b105-709e-0642dfb5359b@quicinc.com>

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?

>  - 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?

thanks,

greg k-h