linux-kernel - Re: [patch 21/32] NTB/msi: Convert to msi_on_each

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Date:   Wed, 1 Dec 2021 11:46:27 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Marc Zygnier <maz@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Megha Dey <megha.dey@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-pci@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jon Mason <jdmason@...zu.us>,
        Dave Jiang <dave.jiang@...el.com>,
        Allen Hubbe <allenbh@...il.com>, linux-ntb@...glegroups.com,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>, x86@...nel.org,
        Joerg Roedel <jroedel@...e.de>,
        iommu@...ts.linux-foundation.org
Subject: Re: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()



On 2021-12-01 11:14 a.m., 'Jason Gunthorpe' via linux-ntb wrote:
> On Wed, Dec 01, 2021 at 06:35:35PM +0100, Thomas Gleixner wrote:
>> On Wed, Dec 01 2021 at 09:00, Jason Gunthorpe wrote:
>>> On Wed, Dec 01, 2021 at 11:16:47AM +0100, Thomas Gleixner wrote:
>>>> Looking at the device slices as subdevices with their own struct device
>>>> makes a lot of sense from the conceptual level.
>>>
>>> Except IMS is not just for subdevices, it should be usable for any
>>> driver in any case as a general interrupt mechiansm, as you alluded to
>>> below about ethernet queues. ntb seems to be the current example of
>>> this need..
>>
>> But NTB is operating through an abstraction layer and is not a direct
>> PCIe device driver.
> 
> I'm not sure exactly how NTB seems to be split between switchtec and
> the ntb code, but since the ntbd code seems to be doing MMIO touches,
> it feels like part of a PCIe driver?

Eh, sort of. NTB has lots of layers. At the top you'll find ntb_netdev
which is an network interface. Below that is ntb_transport() which is a
generic queue pair. Below that is the hardware driver itself (ie
switchtec) through the abstraction layer. The switchtec driver is split
in two: the main driver which just allows for information and
administration of the switch itself and switchtec_ntb which is the
module that provides the NTB abstractions to twiddle its registers.

ntb_transport() doesn't directly do MMIO touches (as it doesn't know
what the underlying hardware registers are). Except for the memory
windows which are usually setup to be a specific BAR (or parts of a
BAR). ntb_transport will do MMIO writes to those specific BAR address
which correspond to writing into buffers on the peer.

Logan