Re: [PATCH v18 13/13] Documentation: cxl: Document CXL protocol error handling

[Jonathan Cameron]

On Fri, 17 Jul 2026 17:27:06 -0500
Terry Bowman <terry.bowman@xxxxxxx> wrote:

> Add Documentation/driver-api/cxl/linux/protocol-error-handling.rst
> describing the end-to-end CXL protocol error path: AER ingress, the
> AER-CXL kfifo handoff, the cxl_core consumer worker, RCD/RCH special
> cases, severity policy, trace events, and a source code map.
> 
> This documents the architecture introduced by the preceding patches in
> this series.
> 
> Assisted-by: Claude:claude-opus-4.7
> Signed-off-by: Terry Bowman <terry.bowman@xxxxxxx>
A couple of trivial things inline to tidy up. Actual text seems good
to me.

Reviewed-by: Jonathan Cameron <jonathan.cameron@xxxxxxxxxxxxxxxx>

> 
> ---
> Changes in v17->v18:
> - Simplify document for readability (Jonathan)
> - Drop historical context that goes stale (Jonathan)
> - Shorten ASCII flow diagram (Jonathan)
> - Drop manual backtick markup, use automarkup (Jonathan)
> - Clarify USP/DSP as single switch component (Dave)
> - Fix line wrapping to 80 chars (Jonathan)
> ---
>  Documentation/driver-api/cxl/index.rst        |   1 +
>  .../cxl/linux/protocol-error-handling.rst     | 222 ++++++++++++++++++
>  2 files changed, 223 insertions(+)
>  create mode 100644 Documentation/driver-api/cxl/linux/protocol-error-handling.rst
> 
> diff --git a/Documentation/driver-api/cxl/index.rst b/Documentation/driver-api/cxl/index.rst
> index 3dfae1d310ca5..6861b2e5726a3 100644
> --- a/Documentation/driver-api/cxl/index.rst
> +++ b/Documentation/driver-api/cxl/index.rst
> @@ -42,6 +42,7 @@ that have impacts on each other.  The docs here break up configurations steps.
>     linux/dax-driver
>     linux/memory-hotplug
>     linux/access-coordinates
> +   linux/protocol-error-handling
>  
>  .. toctree::
>     :maxdepth: 2
> diff --git a/Documentation/driver-api/cxl/linux/protocol-error-handling.rst b/Documentation/driver-api/cxl/linux/protocol-error-handling.rst
> new file mode 100644
> index 0000000000000..67f0492e56702
> --- /dev/null
> +++ b/Documentation/driver-api/cxl/linux/protocol-error-handling.rst
> @@ -0,0 +1,222 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +==============================
> +CXL Protocol Error Handling
> +==============================
> +
> +CXL devices report protocol-layer failures (CXL.cachemem RAS) as PCIe

Why short wrap?  Docs are 80 chars I think.

> +AER Internal Errors: PCI_ERR_COR_INTERNAL for correctable events and
> +PCI_ERR_UNC_INTN for uncorrectable events.  The actual fault
> +information lives in CXL RAS capability registers, not in the PCIe AER
> +status registers.

> +Error flow
> +==========
> +
> +.. code-block:: text
> +
> +   CXL device raises AER Internal Error
> +   (PCI_ERR_COR_INTERNAL or PCI_ERR_UNC_INTN)
> +                   |
> +                   v
> +   +--------------------------------------+
> +   | AER core (aer.c)                     |
> +   |  aer_irq() -> aer_isr()             |
> +   |  -> find_source_device()             |
> +   |  -> handle_error_source(dev, info)   |
> +   +--------------------------------------+
> +                   |
> +                   v
> +   +--------------------------------------+
> +   | handle_error_source() dispatch       |
> +   |                                      |
> +   |  1. cxl_rch_handle_error()           |
> +   |     [always; filters internally]     |
> +   |                                      |
> +   |  2. if is_cxl_error():              |

Smells like a missing space.

> +   |       cxl_forward_error()            |
> +   |       [enqueue to kfifo]             |
> +   |                                      |
> +   |  3. if cxl_pending && non-CE:        |
> +   |       cxl_proto_err_flush()          |
> +   |       [sync drain before recovery]   |
> +   |                                      |
> +   |  4. pci_aer_handle_error() [always]  |
> +   +--------------------------------------+
> +                   |
> +          (kfifo -> workqueue)
> +                   |
> +                   v
> +   +--------------------------------------+
> +   | __cxl_proto_err_work_fn() consumer   |
> +   |                                      |
> +   |  if is_cxl_restricted(pdev):         |
> +   |    cxl_handle_rdport_errors()        |
> +   |    [RCH dport RAS first]             |
> +   |                                      |
> +   |  port = find_cxl_port_by_dev(        |
> +   |           &pdev->dev, NULL)           |
> +   |  dport = cxl_find_dport_by_dev(      |
> +   |           port, &pdev->dev)           |
> +   |  [dport NULL for EP/USP; set RP/DSP] |

check the alignment here as well.  A couple of extra spaces in the
lines above I think.

> +   |                                      |
> +   |  cxl_handle_proto_error()            |
> +   +--------------------------------------+
> +            |                |
> +            v                v
> +   +-----------------+  +--------------------+
> +   | CE              |  | UCE                |
> +   | cxl_handle_     |  | cxl_do_recovery()  |
> +   |   cor_ras()     |  |  read RAS status   |
> +   | trace + clear   |  |  trace + panic     |
> +   +-----------------+  +--------------------+
> +
> +cxl_do_recovery() reads the CXL RAS uncorrectable status register.
> +If UE bits are set, it emits the trace event and panics.  If no bits
> +are set (e.g. RAS mapped but error already cleared), it logs a
> +diagnostic and defers to AER recovery.
> +
> +
> +Severity policy
> +===============
> +
> +**CE** - cxl_handle_cor_ras() reads the CXL RAS correctable status
> +register, clears set bits, and emits a cxl_aer_correctable_error
> +trace event.  No recovery action.
> +
> +**UCE (non-fatal, and fatal on Root Port/Downstream Port)** - cxl_do_recovery() reads the CXL RAS

Wrap needs an update here.

> +uncorrectable status register.  If UE bits are set, the kernel panics.
> +CXL.cachemem traffic cannot be safely recovered once an uncorrectable
> +error is signaled; continuing risks silent data corruption across
> +interleaved HDM regions.  This panic policy applies to the native AER
> +path.  On firmware-first (CPER/GHES) platforms the CPER handler emits
> +trace events only and does not call cxl_do_recovery().