Re: [PATCH RFC 07/15] Documentation: Interrupt Message store

[Dey, Megha]

Hi Jason,

On 4/23/2020 1:04 PM, Jason Gunthorpe wrote:
> On Tue, Apr 21, 2020 at 04:34:30PM -0700, Dave Jiang wrote:
>
> > diff --git a/Documentation/ims-howto.rst b/Documentation/ims-howto.rst
> > new file mode 100644
> > index 000000000000..a18de152b393
> > +++ b/Documentation/ims-howto.rst
> > @@ -0,0 +1,210 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +.. include:: <isonum.txt>
> > +
> > +==========================
> > +The IMS Driver Guide HOWTO
> > +==========================
> > +
> > +:Authors: Megha Dey
> > +
> > +:Copyright: 2020 Intel Corporation
> > +
> > +About this guide
> > +================
> > +
> > +This guide describes the basics of Interrupt Message Store (IMS), the
> > +need to introduce a new interrupt mechanism, implementation details of
> > +IMS in the kernel, driver changes required to support IMS and the general
> > +misconceptions and FAQs associated with IMS.
>
> I'm not sure why we need to call this IMS in kernel documentat? I know
> Intel is using this term, but this document is really only talking
> about extending the existing platform_msi stuff, which looks pretty
> good actually.

hmmm, so maybe we call it something else or just say dynamic platform-msi?

> A lot of this is good for the cover letter..

Well, I got a lot of comments internally and externally about how the 
cover page needs to have just the basics and all the ugly details can go 
in the Documentation. So well, I am confused here.
> > +Implementation of IMS in the kernel
> > +===================================
> > +
> > +The Linux kernel today already provides a generic mechanism to support
> > +non-PCI compliant MSI interrupts for platform devices (platform-msi.c).
> > +To support IMS interrupts, we create a new IMS IRQ domain and extend the
> > +existing infrastructure. Dynamic allocation of IMS vectors is a requirement
> > +for devices which support Scalable I/O Virtualization. A driver can allocate
> > +and free vectors not just once during probe (as was the case with MSI/MSI-X)
> > +but also in the post probe phase where actual demand is available. Thus, a
> > +new API, platform_msi_domain_alloc_irqs_group is introduced which drivers
> > +using IMS would be able to call multiple times. The vectors allocated each
> > +time this API is called are associated with a group ID. To free the vectors
> > +associated with a particular group, the platform_msi_domain_free_irqs_group
> > +API can be called. The existing drivers using platform-msi infrastructure
> > +will continue to use the existing alloc (platform_msi_domain_alloc_irqs)
> > +and free (platform_msi_domain_free_irqs) APIs and are assigned a default
> > +group ID of 0.
> > +
> > +Thus, platform-msi.c provides the generic methods which can be used by any
> > +non-pci MSI interrupt type while the newly created ims-msi.c provides IMS
> > +specific callbacks that can be used by drivers capable of generating IMS
> > +interrupts.
>
> How exactly is an IMS interrupt is different from a platform msi?
>
> It looks like it is just some thin wrapper around msi_domain - what is
> it for?

So I think conceptually, there is no difference between platform-msi and 
IMS. (Just thinking out loud).

From a code stand-point, currently
1. Allocation of interrupts is static. I don't think the 
platform-msi-domain_alloc_irqs can be called multiple times.
2. only a write-msg callback is present and they use the parent IRQ 
chip's mask/unmask functions
3. IMS needs interrupt remapping support to be enabled (this is 
independent of the above 2).

If 1 and 2 is all that you are looking for, then we can split the code 
such that we have a generic platform_msi_domain_alloc_irqs_dyn, which 
will be used for the dynamic allocation of IRQs and another 
platform_msi_domain_alloc_irqs_ims (or whatever the name IMS will boil 
down to) which will use interrupt remapping support to get the IRQ 
domain etc.

> > +FAQs and general misconceptions:
> > +================================
> > +
> > +** There were some concerns raised by Thomas Gleixner and Marc Zyngier
> > +during Linux plumbers conference 2019:
> > +
> > +1. Enumeration of IMS needs to be done by PCI core code and not by
> > +   individual device drivers:
> > +
> > +   Currently, if the kernel needs a generic way to discover IMS capability
> > +   without host driver dependency, the PCIE Designated Vendor specific
> > +
> > +   However, we cannot have a standard way of enumerating the IMS size
> > +   because for context based devices, the interrupt message is part of
> > +   the context itself which is managed entirely by the driver. Since
> > +   context creation is done on demand, there is no way to tell during boot
> > +   time, the maximum number of contexts (and hence the number of interrupt
> > +   messages)that the device can support.
>
> FWIW, I agree with this
>
> Like platform-msi, IMS should be controlled entirely by the driver.
yup!

> > +2. Why is Intel designing a new interrupt mechanism rather than extending
> > +   MSI-X to address its limitations? Isn't 2048 device interrupts enough?
> > +
> > +   MSI-X has a rigid definition of one-table and on-device storage and does
> > +   not provide the full flexibility required for future multi-tile
> > +   accelerator designs.
> > +   IMS was envisioned to be used with large number of ADIs in devices where
> > +   each will need unique interrupt resources. For example, a DSA shared
> > +   work queue can support large number of clients where each client can
> > +   have its own interrupt. In future, with user interrupts, we expect the
> > +   demand for messages to increase further.
>
> Generally agree
ok!

> > +Device Driver Changes:
> > +=====================
> > +
> > +1. platform_msi_domain_alloc_irqs_group (struct device *dev, unsigned int
> > +   nvec, const struct platform_msi_ops *platform_ops, int *group_id)
> > +   to allocate IMS interrupts, where:
> > +
> > +   dev: The device for which to allocate interrupts
> > +   nvec: The number of interrupts to allocate
> > +   platform_ops: Callbacks for platform MSI ops (to be provided by driver)
> > +   group_id: returned by the call, to be used to free IRQs of a certain type
> > +
> > +   eg: static struct platform_msi_ops ims_ops  = {
> > +        .irq_mask               = ims_irq_mask,
> > +        .irq_unmask             = ims_irq_unmask,
> > +        .write_msg              = ims_write_msg,
> > +        };
> > +
> > +        int group;
> > +        platform_msi_domain_alloc_irqs_group (dev, nvec, platform_ops, &group)
> > +
> > +   where, struct platform_msi_ops:
> > +   irq_mask:   mask an interrupt source
> > +   irq_unmask: unmask an interrupt source
> > +   irq_write_msi_msg: write message content
> > +
> > +   This API can be called multiple times. Every time a new group will be
> > +   associated with the allocated vectors. Group ID starts from 0.
>
> Need much more closer look, but this seems conceptually fine to me.
>
> As above the API here is called platform_msi - which seems good to
> me. Again not sure why the word IMS is needed

well, in this case, ims_ops, ims_mask etc are just example names.

> Jason