linux-kernel - Re: [PATCH v5 15/15] misc: bcm-vk: add ttyVK support

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Message-ID: <140a5cde-67fd-f4b1-e4a9-9c68adf964c4@broadcom.com>
Date:   Thu, 1 Oct 2020 15:26:41 -0700
From:   Scott Branden <scott.branden@...adcom.com>
To:     Florian Fainelli <f.fainelli@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Olof Johansson <olof@...om.net>
Cc:     Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org,
        bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH v5 15/15] misc: bcm-vk: add ttyVK support

Hi Florian,

On 2020-09-30 7:36 p.m., Florian Fainelli wrote:
>
>
> On 9/30/2020 6:28 PM, Scott Branden wrote:
>> Add ttyVK support to driver to allow console access to VK card from host.
>>
>> Device node will be in the follow form /dev/bcm-vk.x_ttyVKy where:
>> x is the instance of the VK card
>> y is the tty device number on the VK card
>>
>> Signed-off-by: Scott Branden <scott.branden@...adcom.com>
>
> Was it not possible to:
>
> - emulate a standard 8250 UART and just expose an additional bar + MSI vector such that one could get a debug UART out of this device with little to no work on the host side?
I don't see how such emulation is simpler than what we have.  For a standard 8250 UART implementation how would we emulate a FIFO in memory accessible over PCIe.  I don't see how to do that with shared memory?  Also, we don't have an additional bar available on the PCIe device.  There is currently little to no work on the device side using a circular buffer with read and write pointers.
>
> - or use virtio console and implement virtio on the firmware that runs on VK?
Not that possible as the debug/console UARTs are used inside and outside of linux on the VK card.
>
> It's not a lot of code, but it just feels like this should not have to be done at all.
If there was a simpler way to do it, yes, I would like a "UART" running over PCIe shared memory via a circular buffer.
And that is what the code we developed does.


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