Re: [PATCH v2] cxl/hdm: Avoid incorrect DVSEC fallback when HDM decoders are enabled

[Dave Jiang]

On 3/16/26 1:19 PM, Smita Koralahalli wrote:
> Check the global CXL_HDM_DECODER_ENABLE bit instead of looping over
> per-decoder COMMITTED bits to determine whether to fall back to DVSEC
> range emulation. When the HDM decoder capability is globally enabled,
> ignore DVSEC range registers regardless of individual decoder commit
> state.
> 
> should_emulate_decoders() currently loops over per-decoder COMMITTED
> bits, which leads to an incorrect DVSEC fallback when those bits are
> zero. One way to trigger this is to destroy a region and bounce the
> memdev:
> 
>   cxl disable-region region0
>   cxl destroy-region region0
>   cxl disable-memdev mem0
>   cxl enable-memdev mem0
> 
> Region teardown zeroes the HDM decoder registers including the committed
> bits. The subsequent memdev re-probe finds uncommitted decoders and falls
> back to DVSEC emulation, even though HDM remains globally enabled.
> 
> Observed failures:
> 
>   should_emulate_decoders: cxl_port endpoint6: decoder6.0: committed: 0 base: 0x0_00000000 size: 0x0_00000000
>   devm_cxl_setup_hdm: cxl_port endpoint6: Fallback map 1 range register
>   ..
>   devm_cxl_add_region: cxl_acpi ACPI0017:00: decoder0.0: created region0
>   __construct_region: cxl_pci 0000:e1:00.0: mem1:decoder6.0:
>   __construct_region region0 res: [mem 0x850000000-0x284fffffff flags 0x200] iw: 1 ig: 4096
>   cxl region0: pci0000:e0:port1 cxl_port_setup_targets expected iw: 1 ig: 4096 ..
>   cxl region0: pci0000:e0:port1 cxl_port_setup_targets got iw: 1 ig: 256 state: disabled ..
>   cxl_port endpoint6: failed to attach decoder6.0 to region0: -6
>   ..
>   devm_cxl_add_region: cxl_acpi ACPI0017:00: decoder0.0: created region4
>   alloc_hpa: cxl region4: HPA allocation error (-34) ..
> 
> Fixes: 52cc48ad2a76 ("cxl/hdm: Limit emulation to the number of range registers")
> Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@xxxxxxx>
> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>

Smita,

Applied to cxl/fixes
75cea0776de5 cxl/hdm: Avoid incorrect DVSEC fallback when HDM decoders are enabled

Any chance to add the issue reproduce steps to a regression test script in CXL CLI test please? Thanks!

> ---
> v2:
>   Reworded commit message to focus on root cause per Dan's feedback.
>   Dropped downstream details (IG/construct_region/hpa_alloc)
>   Added error signatures.
>   Link to v1: https://lore.kernel.org/r/20260212223800.23624-1-Smita.KoralahalliChannabasappa@xxxxxxx
> ---
>  drivers/cxl/core/hdm.c | 25 +++++++++----------------
>  1 file changed, 9 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
> index c222e98ae736..cb5d5a047a9d 100644
> --- a/drivers/cxl/core/hdm.c
> +++ b/drivers/cxl/core/hdm.c
> @@ -94,7 +94,6 @@ static bool should_emulate_decoders(struct cxl_endpoint_dvsec_info *info)
>  	struct cxl_hdm *cxlhdm;
>  	void __iomem *hdm;
>  	u32 ctrl;
> -	int i;
>  
>  	if (!info)
>  		return false;
> @@ -113,22 +112,16 @@ static bool should_emulate_decoders(struct cxl_endpoint_dvsec_info *info)
>  		return false;
>  
>  	/*
> -	 * If any decoders are committed already, there should not be any
> -	 * emulated DVSEC decoders.
> +	 * If HDM decoders are globally enabled, do not fall back to DVSEC
> +	 * range emulation. Zeroed decoder registers after region teardown
> +	 * do not imply absence of HDM capability.
> +	 *
> +	 * Falling back to DVSEC here would treat the decoder as AUTO and
> +	 * may incorrectly latch default interleave settings.
>  	 */
> -	for (i = 0; i < cxlhdm->decoder_count; i++) {
> -		ctrl = readl(hdm + CXL_HDM_DECODER0_CTRL_OFFSET(i));
> -		dev_dbg(&info->port->dev,
> -			"decoder%d.%d: committed: %ld base: %#x_%.8x size: %#x_%.8x\n",
> -			info->port->id, i,
> -			FIELD_GET(CXL_HDM_DECODER0_CTRL_COMMITTED, ctrl),
> -			readl(hdm + CXL_HDM_DECODER0_BASE_HIGH_OFFSET(i)),
> -			readl(hdm + CXL_HDM_DECODER0_BASE_LOW_OFFSET(i)),
> -			readl(hdm + CXL_HDM_DECODER0_SIZE_HIGH_OFFSET(i)),
> -			readl(hdm + CXL_HDM_DECODER0_SIZE_LOW_OFFSET(i)));
> -		if (FIELD_GET(CXL_HDM_DECODER0_CTRL_COMMITTED, ctrl))
> -			return false;
> -	}
> +	ctrl = readl(hdm + CXL_HDM_DECODER_CTRL_OFFSET);
> +	if (ctrl & CXL_HDM_DECODER_ENABLE)
> +		return false;
>  
>  	return true;
>  }
> 
> base-commit: 93d0fcdddc9e7be9d4f42acbe57bc90dbb0fe75d