linux-kernel - Re: [PATCH v2] Documentation/driver-api/cxl: device hotplug section

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Message-ID: <aUVhxIJzGiiGs9ee@gourry-fedora-PF4VCD3F>
Date: Fri, 19 Dec 2025 09:31:32 -0500
From: Gregory Price <gourry@...rry.net>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: linux-cxl@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	dave@...olabs.net, dave.jiang@...el.com, alison.schofield@...el.com,
	vishal.l.verma@...el.com, ira.weiny@...el.com,
	dan.j.williams@...el.com, corbet@....net
Subject: Re: [PATCH v2] Documentation/driver-api/cxl: device hotplug section

On Fri, Dec 19, 2025 at 10:55:18AM +0000, Jonathan Cameron wrote:
> On Thu, 18 Dec 2025 12:07:47 -0500
> Gregory Price <gourry@...rry.net> wrote:
> 
> > +
> > +Multi-Endpoint Memory Device Present at Boot
> > +--------------------------------------------
> > +A hot-plug capable CXL memory device, such as one which presents multiple
> > +expanders as a single large-capacity device, should report the maximum
> > +*possible* capacity for the device at boot. ::
> > +
> > +                  HB0
> > +                  RP0
> > +                   |
> > +     [Multi-Endpoint Memory Device]
> 
> So this is the weird switch as end point thing?   Maybe a reference.
> My guess is these will go away as switch and memory device vendors catch
> up with the spec, but maybe I'm wrong.
> 

I guess I just don't want to dictate the innards of a multi-endpoint
memory device.  It *really really* implies there must be some kind of
switch inside - but that switch might not even be runtime programmable
or discoverable (basically all the settings get locked on boot and it
becomes passthrough).

If you'd rather just not have this section at all, I'm ok with that.
The switch case below this covers the base case for a switch-based
device where everything is programmable.

> > +              _____|_____
> > +             |          |
> > +        [Endpoint0]   [Empty]
> > +
> > +
> > +Limiting the size to the capacity preset at boot will limit hot-add support
> > +to replacing capacity that was present at boot.
> 
>