linux-kernel - Re: [PATCH v1] cxl/port: Fix target list setup for multiple decoders sharing the same dport

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Message-ID: <aV9CMxeE2KMlbXHH@aschofie-mobl2.lan>
Date: Wed, 7 Jan 2026 21:35:47 -0800
From: Alison Schofield <alison.schofield@...el.com>
To: Robert Richter <rrichter@....com>
CC: Davidlohr Bueso <dave@...olabs.net>, Jonathan Cameron
	<jonathan.cameron@...wei.com>, Dave Jiang <dave.jiang@...el.com>, "Vishal
 Verma" <vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>, "Dan
 Williams" <dan.j.williams@...el.com>, <linux-cxl@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] cxl/port: Fix target list setup for multiple decoders
 sharing the same dport

On Wed, Jan 07, 2026 at 11:03:55AM +0100, Robert Richter wrote:
> If a switch port has more than one decoder that is using the same
> downstream port, the enumeration of the target lists may fail with:
> 
>  # dmesg | grep target.list
>  update_decoder_targets: cxl decoder1.0: dport3 found in target list, index 3
>  update_decoder_targets: cxl decoder1.0: dport2 found in target list, index 2
>  update_decoder_targets: cxl decoder1.0: dport0 found in target list, index 0
>  update_decoder_targets: cxl decoder2.0: dport3 found in target list, index 1
>  update_decoder_targets: cxl decoder4.0: dport3 found in target list, index 1
>  cxl_mem mem6: failed to find endpoint12:0000:00:01.4 in target list of decoder2.1
>  cxl_mem mem8: failed to find endpoint13:0000:20:01.4 in target list of decoder4.1
> 
> The case, that the same downstream port can be used in multiple target
> lists, is allowed and possible.
> 
> Fix the update of the target list. Enumerate all children of the
> switch port and do not stop the iteration after the first matching
> target was found.
> 
> With the fix applied:
> 
>  # dmesg | grep target.list
>  update_decoder_targets: cxl decoder1.0: dport2 found in target list, index 2
>  update_decoder_targets: cxl decoder1.0: dport0 found in target list, index 0
>  update_decoder_targets: cxl decoder1.0: dport3 found in target list, index 3
>  update_decoder_targets: cxl decoder2.0: dport3 found in target list, index 1
>  update_decoder_targets: cxl decoder2.1: dport3 found in target list, index 1
>  update_decoder_targets: cxl decoder4.0: dport3 found in target list, index 1
>  update_decoder_targets: cxl decoder4.1: dport3 found in target list, index 1
> 
> Analyzing the conditions when this happens:
> 
> 1) A dport is shared by multiple decoders.
> 
> 2) The decoders have interleaving configured (ways > 1).
> 
> The configuration above has the following hierarchy details (fixed
> version):
> 
>  root0
>  |_
>  | |
>  | decoder0.1
>  | ways: 2
>  | target_list: 0,1
>  |_______________________________________
>  |                                       |
>  | dport0                                | dport1
>  |                                       |
>  port2                                   port4
>  |                                       |
>  |___________________                    |_____________________
>  | |                 |                   | |                   |
>  | decoder2.0        decoder2.1          | decoder4.0          decoder4.1
>  | ways: 2           ways: 2             | ways: 2             ways: 2
>  | target_list: 2,3  target_list: 2,3    | target_list: 2,3    target_list: 2,3
>  |___________________                    |___________________
>  |                   |                   |                   |
>  | dport2            | dport3            | dport2            | dport3
>  |                   |                   |                   |
>  endpoint7           endpoint12          endpoint9           endpoint13
>  |_                  |_                  |_                  |_
>  | |                 | |                 | |                 | |
>  | decoder7.0        | decoder12.0       | decoder9.0        | decoder13.0
>  | decoder7.2        | decoder12.2       | decoder9.2        | decoder13.2
>  |                   |                   |                   |
>  mem3                mem5                mem6                mem8
> 
> Note: Device numbers vary for every boot.
> 
> Current kernel fails to enumerate endpoint12 and endpoint13 as the
> target list is not updated for the second decoder.

Reviewed-by: Alison Schofield <alison.schofield@...el.com>

Using the cxl-debug messaging you used to explain this, I was able to
see this in the default cxl-test topology. At 6.18 we stopped seeing
all the expected dports and with this patch, they're back.