Re: [PATCH v7 8/8] cxl: Allow auto-committed BI hdm decoders

[Richard Cheng <icheng@nvidia.com>]

On Tue, Jul 28, 2026 at 07:41:36AM +0800, Davidlohr Bueso wrote:
> Allow auto-committed BI hdm decoders on sane platforms, rejecting
> only broken paths (ie: one that cannot route BISnp, or BI paired
> with a host-only target range type).
> 
> The respective region creation is done like any other committed
> decoder - with cxlds->bi set by the time an decoder attaches.
> 
> A committed BI decoder under a window without the BI restriction is
> refused (undefined behavior per the CFMWS Window Restrictions), as
> is a committed decoder attaching to a region of a different
> coherency model.
> 
> Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
> ---
>  drivers/cxl/core/hdm.c    | 24 +++++++++++++++++-------
>  drivers/cxl/core/pci.c    | 36 +++++++++++++++++++++++++++++-------
>  drivers/cxl/core/region.c | 33 +++++++++++++++++++++++++++++++++
>  drivers/cxl/port.c        |  4 ++++
>  4 files changed, 83 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
> index f437fe15c6df..c5be6fe4c77a 100644
> --- a/drivers/cxl/core/hdm.c
> +++ b/drivers/cxl/core/hdm.c
> @@ -1061,13 +1061,23 @@ static int init_hdm_decoder(struct cxl_port *port, struct cxl_decoder *cxld,
>  		else
>  			cxld->target_type = CXL_DECODER_DEVMEM;
>  
> -		/*
> -		 * Autocommit BI-enabled decoders is not supported.
> -		 * At this point cxlds->bi is not yet setup, so there
> -		 * are no guarantees that the platform supports BI.
> -		 */
> -		if (FIELD_GET(CXL_HDM_DECODER0_CTRL_BI, ctrl))
> -			return -ENXIO;
> +		if (FIELD_GET(CXL_HDM_DECODER0_CTRL_BI, ctrl)) {
> +			struct cxl_dev_state *cxlds = cxled ?
> +				cxled_to_memdev(cxled)->cxlds : NULL;
> +
> +			if (cxld->target_type == CXL_DECODER_HOSTONLYMEM) {
> +				dev_warn(&port->dev,
> +					 "decoder%d.%d: BI with host-only\n",
> +					 port->id, cxld->id);
> +				return -ENXIO;
> +			}
> +			if (cxlds && !cxlds->bi_capable) {
> +				dev_warn(&port->dev,
> +					 "decoder%d.%d: path not BI capable\n",
> +					 port->id, cxld->id);
> +				return -ENXIO;
> +			}
> +		}
>  
>  		guard(rwsem_write)(&cxl_rwsem.region);
>  		if (cxld->id != cxl_num_decoders_committed(port)) {
> diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
> index 554058ccb1e9..8d2651e06a79 100644
> --- a/drivers/cxl/core/pci.c
> +++ b/drivers/cxl/core/pci.c
> @@ -1083,6 +1083,18 @@ static int __cxl_bi_commit_decoder(struct device *dev, void __iomem *bi)
>  				    scale, base);
>  }
>  
> +/* Committed, or no explicit commit required */
> +static bool cxl_bi_decoder_committed(void __iomem *bi)
> +{
> +	u32 caps = readl(bi + CXL_BI_DECODER_CAPS_OFFSET);
> +	u32 sts = readl(bi + CXL_BI_DECODER_STATUS_OFFSET);
> +
> +	if (!FIELD_GET(CXL_BI_DECODER_CAPS_EXPLICIT_COMMIT_REQ, caps))
> +		return true;
> +
> +	return FIELD_GET(CXL_BI_DECODER_STATUS_BI_COMMITTED, sts);
> +}
> +
>  /* Enable or dealloc BI-ID changes in the given level of the topology. */
>  static int __cxl_bi_ctrl_dport(struct cxl_dport *dport, bool enable)
>  {
> @@ -1127,6 +1139,16 @@ static int __cxl_bi_ctrl_dport(struct cxl_dport *dport, bool enable)
>  		return 0;
>  	case PCI_EXP_TYPE_DOWNSTREAM:
>  		if (enable) {
> +			/*
> +			 * Adopt a dport that was already programmed and
> +			 * committed by firmware: nothing new is enabled
> +			 * below it, so no commit is due.
> +			 */
> +			if (FIELD_GET(CXL_BI_DECODER_CTRL_BI_ENABLE, ctrl) &&
> +			    !FIELD_GET(CXL_BI_DECODER_CTRL_BI_FW, ctrl) &&
> +			    cxl_bi_decoder_committed(bi))
> +				return 0;
> +

Small question, when DSP's BI related registers are committed by FW, does it guaranteed
that all the intermediate switch on the path are also committed with BI enabled ?



>  			value = ctrl & ~CXL_BI_DECODER_CTRL_BI_FW;
>  			value |= CXL_BI_DECODER_CTRL_BI_ENABLE;
>  		} else {
> @@ -1173,11 +1195,11 @@ static int __cxl_bi_ctrl_endpoint(struct cxl_dev_state *cxlds, bool enable)
>  
>  	if (enable) {
>  		if (FIELD_GET(CXL_BI_DECODER_CTRL_BI_ENABLE, ctrl)) {
> -			if (cxlds->bi)
> -				return 0;
> -			dev_err(cxlds->dev,
> -				"BI already enabled in hardware\n");
> -			return -EBUSY;
> +			/* adopt firmware enabled */
> +			if (!cxlds->bi)
> +				dev_dbg(cxlds->dev,
> +					"adopting firmware-enabled BI\n");
> +			goto done;
>  		}
>  		val = ctrl | CXL_BI_DECODER_CTRL_BI_ENABLE;
>  	} else {
> @@ -1192,11 +1214,11 @@ static int __cxl_bi_ctrl_endpoint(struct cxl_dev_state *cxlds, bool enable)
>  	}
>  
>  	writel(val, bi + CXL_BI_DECODER_CTRL_OFFSET);
> -	cxlds->bi = enable;
>  
>  	dev_dbg(cxlds->dev, "BI requests %s\n",
>  		str_enabled_disabled(enable));
> -
> +done:
> +	cxlds->bi = enable;
>  	return 0;
>  }
>  
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 76c6dc28a407..5578ef68034d 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -1827,6 +1827,21 @@ static int cxl_region_attach_position(struct cxl_region *cxlr,
>  	return rc;
>  }
>  
> +/* Read back the committed BI bit of an auto-discovered decoder */
> +static bool cxled_committed_bi(struct cxl_endpoint_decoder *cxled)
> +{
> +	struct cxl_port *port = cxled_to_port(cxled);
> +	struct cxl_hdm *cxlhdm = dev_get_drvdata(&port->dev);
> +	u32 ctrl;
> +
> +	if (!cxlhdm || !cxlhdm->regs.hdm_decoder)
> +		return false;
> +
> +	ctrl = readl(cxlhdm->regs.hdm_decoder +
> +		     CXL_HDM_DECODER0_CTRL_OFFSET(cxled->cxld.id));
> +	return FIELD_GET(CXL_HDM_DECODER0_CTRL_BI, ctrl);
> +}
> +
>  static int cxl_region_attach_auto(struct cxl_region *cxlr,
>  				  struct cxl_endpoint_decoder *cxled, int pos)
>  {
> @@ -1839,6 +1854,16 @@ static int cxl_region_attach_auto(struct cxl_region *cxlr,
>  		return -EINVAL;
>  	}
>  
> +	/* A committed decoder may only join a region of its own flavor */
> +	if (cxled_committed_bi(cxled) !=
> +	    (cxlr->type == CXL_DECODER_DEVMEM &&
> +	     cxl_root_decoder_is_bi(cxlr->cxlrd))) {
> +		dev_err(&cxlr->dev, "%s:%s coherency model mismatch\n",
> +			dev_name(&cxled_to_memdev(cxled)->dev),
> +			dev_name(&cxled->cxld.dev));
> +		return -ENXIO;
> +	}
> +
>  	if (pos >= 0) {
>  		dev_dbg(&cxlr->dev, "%s: expected auto position, not %d\n",
>  			dev_name(&cxled->cxld.dev), pos);
> @@ -3806,6 +3831,14 @@ static struct cxl_region *construct_region(struct cxl_root_decoder *cxlrd,
>  	if (part < 0)
>  		return ERR_PTR(-EBUSY);
>  
> +	/* avoid UB */
> +	if (cxled_committed_bi(cxled) && !cxl_root_decoder_is_bi(cxlrd)) {
> +		dev_err(cxlmd->dev.parent,
> +			"%s:%s BI decoder in a non-BI window\n",
> +			dev_name(&cxlmd->dev), dev_name(&cxled->cxld.dev));
> +		return ERR_PTR(-ENXIO);
> +	}
> +

Should we also check the inverse mismatch case here ?

if (!cxled_committed_bi() && cxl_root_decoder_is_bi()) {}

thoughts ?

>  	do {
>  		cxlr = __create_region(cxlrd, cxlds->part[part].mode,
>  				       atomic_read(&cxlrd->region_id),
> diff --git a/drivers/cxl/port.c b/drivers/cxl/port.c
> index ab3317fc1388..0c666ce00aa2 100644
> --- a/drivers/cxl/port.c
> +++ b/drivers/cxl/port.c
> @@ -181,6 +181,10 @@ static int cxl_endpoint_port_probe(struct cxl_port *port)
>  	if (rc)
>  		return rc;
>  
> +	/*
> +	 * Must precede region discovery so that any firmware-committed BI
> +	 * decoder is adopted before its region is assembled.
> +	 */
>  	rc = cxl_bi_setup(port);
>  	if (rc)
>  		dev_dbg(&port->dev, "BI setup failed rc=%d\n", rc);
> -- 
> 2.39.5
>

Hmmm just curious are intermediate switch are guaranteed to be valid in AUTO discovery
scenario , seems that the implementation only checks DSP.

Best regards,
Richard Cheng.