linux-kernel - Re: [PATCH v3 4/9] cxl/mem: Skip intermediate port enumeration of restricted endpoints (RCDs)

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Date:   Tue, 15 Nov 2022 10:09:57 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Robert Richter <rrichter@....com>,
        Dan Williams <dan.j.williams@...el.com>
CC:     Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        "Jonathan Cameron" <Jonathan.Cameron@...wei.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Dave Jiang <dave.jiang@...el.com>
Subject: Re: [PATCH v3 4/9] cxl/mem: Skip intermediate port enumeration of
 restricted endpoints (RCDs)

Robert Richter wrote:
> On 14.11.22 16:24:06, Dan Williams wrote:
> > Robert Richter wrote:
> > > When an endpoint is found, all ports in beetween the endpoint and the
> > > CXL host bridge need to be created. In the RCH case there are no ports
> > > in between a host bridge and the endpoint. Skip the enumeration of
> > > intermediate ports.
> > > 
> > > The port enumeration does not only create all ports, it also
> > > initializes the endpoint chain by adding the endpoint to every
> > > downstream port up to the root bridge. This must be done also in RCD
> > > mode, but is much more simple as the endpoint only needs to be added
> > > to the host bridge's dport.
> > > 
> > > Note: For endpoint removal the cxl_detach_ep() is not needed as it is
> > > released in cxl_port_release().
> > > 
> > > Signed-off-by: Robert Richter <rrichter@....com>
> > > ---
> > >  drivers/cxl/core/port.c | 18 ++++++++++++++++--
> > >  1 file changed, 16 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/cxl/core/port.c b/drivers/cxl/core/port.c
> > > index d10c3580719b..e21a9c3fe4da 100644
> > > --- a/drivers/cxl/core/port.c
> > > +++ b/drivers/cxl/core/port.c
> > > @@ -1366,8 +1366,24 @@ int devm_cxl_enumerate_ports(struct cxl_memdev *cxlmd)
> > >  {
> > >  	struct device *dev = &cxlmd->dev;
> > >  	struct device *iter;
> > > +	struct cxl_dport *dport;
> > > +	struct cxl_port *port;
> > >  	int rc;
> > >  
> > > +	/*
> > > +	 * Skip intermediate port enumeration in the RCH case, there
> > > +	 * are no ports in between a host bridge and an endpoint. Only
> > > +	 * initialize the EP chain.
> > > +	 */
> > > +	if (is_cxl_restricted(cxlmd)) {
> > > +		port = cxl_mem_find_port(cxlmd, &dport);
> > > +		if (!port)
> > > +			return -ENXIO;
> > > +		rc = cxl_add_ep(dport, &cxlmd->dev);
> > 
> > On second look, this seems problematic. While yes it will be deleted
> > when the root CXL port dies, it will not be deleted if the cxl_pci
> > driver is reloaded. I will code up a unit test to double check.
> > 
> > I note that cxl_add_ep() for the VH case is skipped for the root CXL
> > port, so I do not suspect it is needed here either. Did you add it for a
> > specific reason?
> 
> Yes, all endpoint iterators need to be reworked. Also true, the first
> endpoint is skipped in the chain. So only intermediate EP structs are
> touched by the loops actually.
> 
> In particular, cxl_ep_load() returned NULL for the first lookup if the
> ep is missing. We could stop the iteration then. I tried to avoid a
> rework here, but maybe it is not too extensive as I expected first.

Hmm, ok, let me get the unit test topology working for this to make sure
my assumptions are correct about when an @ep reference is used / needed.