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

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Date:   Mon, 14 Nov 2022 16:24:06 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Robert Richter <rrichter@....com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "Ira Weiny" <ira.weiny@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>
CC:     <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>,
        Robert Richter <rrichter@....com>
Subject: RE: [PATCH v3 4/9] cxl/mem: Skip intermediate port enumeration of
 restricted endpoints (RCDs)

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?