linux-kernel - Re: [PATCH v7 15/18] NTB: Add support for EPF PCI-Express Non-Transparent Bridge

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Message-ID: <a590bff0-ba9e-8e19-4f09-4838d8afdbb6@ti.com>
Date:   Mon, 16 Nov 2020 21:15:53 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Arnd Bergmann <arnd@...nel.org>
CC:     Sherry Sun <sherry.sun@....com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "arnd@...db.de" <arnd@...db.de>,
        "jdmason@...zu.us" <jdmason@...zu.us>,
        "dave.jiang@...el.com" <dave.jiang@...el.com>,
        "allenbh@...il.com" <allenbh@...il.com>,
        "tjoseph@...ence.com" <tjoseph@...ence.com>,
        Rob Herring <robh@...nel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-ntb@...glegroups.com" <linux-ntb@...glegroups.com>
Subject: Re: [PATCH v7 15/18] NTB: Add support for EPF PCI-Express
 Non-Transparent Bridge

Hi Arnd,

On 16/11/20 9:07 pm, Arnd Bergmann wrote:
> On Mon, Nov 16, 2020 at 6:19 AM Kishon Vijay Abraham I <kishon@...com> wrote:
>> On 12/11/20 6:54 pm, Arnd Bergmann wrote:
>>>
>>> This looks very  promising indeed, I need to read up on the whole
>>> discussion there. I also see your slides at [1]  that help do explain some
>>> of it. I have one fundamental question that I can't figure out from
>>> the description, maybe you can help me here:
>>>
>>> How is the configuration managed, taking the EP case as an
>>> example? Your UseCase1 example sounds like the system that owns
>>> the EP hardware is the one that turns the EP into a vhost device,
>>> and creates a vhost-rpmsg device on top, while the RC side would
>>> probe the pci-vhost and then detect a virtio-rpmsg device to talk to.
>>
>> That's correct. Slide no 9 in [1] should give the layering details.
>>
>>> Can it also do the opposite, so you end up with e.g. a virtio-net
>>> device on the EP side and vhost-net on the RC?
>>
>> Unfortunately no. Again referring slide 9 in [1], we only have
>> vhost-pci-epf on the EP side which only creates a "vhost_dev" to deal
>> with vhost side of things. For doing the opposite, we'd need to create
>> virtio-pci-epf for EP side that interacts with core virtio (and also the
>> corresponding vhost back end on PCI host).
> 
> Ok, I see. So I think this is the opposite of drivers/misc/mic and
> the bluefield driver were using, so we would probably end up
> needing both.
> 
> Then again, I guess the NTB driver would give us the functionality
> for free, if it shows a symmetric link?

Right, NTB driver would need "pci_dev" on both sides of the link. But
that would also mean we cannot use pci EP framework which actually uses
"pci_epf".

Thanks
Kishon