Re: [PATCH v3 1/6] cxl: Remove the CXL_DECODER_MIXED mistake

[Fan Ni <nifan.cxl@gmail.com>]

On Mon, Feb 03, 2025 at 08:24:06PM -0800, Dan Williams wrote:
> CXL_DECODER_MIXED is a safety mechanism introduced for the case where
> platform firmware has programmed an endpoint decoder that straddles a
> DPA partition boundary. While the kernel is careful to only allocate DPA
> capacity within a single partition there is no guarantee that platform
> firmware, or anything that touched the device before the current kernel,
> gets that right.
> 
> However, __cxl_dpa_reserve() will never get to the CXL_DECODER_MIXED
> designation because of the way it tracks partition boundaries. A
> request_resource() that spans ->ram_res and ->pmem_res fails with the
> following signature:
> 
>     __cxl_dpa_reserve: cxl_port endpoint15: decoder15.0: failed to reserve allocation
> 
> CXL_DECODER_MIXED is dead defensive programming after the driver has
> already given up on the device. It has never offered any protection in
> practice, just delete it.
> 
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Reviewed-by: Alejandro Lucero <alucerop@amd.com>
> Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Reviewed-by: Fan Ni <fan.ni@samsung.com>

> ---
>  drivers/cxl/core/hdm.c    |    6 +++---
>  drivers/cxl/core/region.c |   12 ------------
>  drivers/cxl/cxl.h         |    4 +---
>  3 files changed, 4 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
> index 50e6a45b30ba..b7f6a2d69f78 100644
> --- a/drivers/cxl/core/hdm.c
> +++ b/drivers/cxl/core/hdm.c
> @@ -332,9 +332,9 @@ static int __cxl_dpa_reserve(struct cxl_endpoint_decoder *cxled,
>  	else if (resource_contains(&cxlds->ram_res, res))
>  		cxled->mode = CXL_DECODER_RAM;
>  	else {
> -		dev_warn(dev, "decoder%d.%d: %pr mixed mode not supported\n",
> -			 port->id, cxled->cxld.id, cxled->dpa_res);
> -		cxled->mode = CXL_DECODER_MIXED;
> +		dev_warn(dev, "decoder%d.%d: %pr does not map any partition\n",
> +			 port->id, cxled->cxld.id, res);
> +		cxled->mode = CXL_DECODER_NONE;
>  	}
>  
>  	port->hdm_end++;
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index e8d11a988fd9..34aa3d45fafa 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -2738,18 +2738,6 @@ static int poison_by_decoder(struct device *dev, void *arg)
>  	if (!cxled->dpa_res || !resource_size(cxled->dpa_res))
>  		return rc;
>  
> -	/*
> -	 * Regions are only created with single mode decoders: pmem or ram.
> -	 * Linux does not support mixed mode decoders. This means that
> -	 * reading poison per endpoint decoder adheres to the requirement
> -	 * that poison reads of pmem and ram must be separated.
> -	 * CXL 3.0 Spec 8.2.9.8.4.1
> -	 */
> -	if (cxled->mode == CXL_DECODER_MIXED) {
> -		dev_dbg(dev, "poison list read unsupported in mixed mode\n");
> -		return rc;
> -	}
> -
>  	cxlmd = cxled_to_memdev(cxled);
>  	if (cxled->skip) {
>  		offset = cxled->dpa_res->start - cxled->skip;
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index bbbaa0d0a670..9fd1524ed150 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -380,7 +380,6 @@ enum cxl_decoder_mode {
>  	CXL_DECODER_NONE,
>  	CXL_DECODER_RAM,
>  	CXL_DECODER_PMEM,
> -	CXL_DECODER_MIXED,
>  	CXL_DECODER_DEAD,
>  };
>  
> @@ -390,10 +389,9 @@ static inline const char *cxl_decoder_mode_name(enum cxl_decoder_mode mode)
>  		[CXL_DECODER_NONE] = "none",
>  		[CXL_DECODER_RAM] = "ram",
>  		[CXL_DECODER_PMEM] = "pmem",
> -		[CXL_DECODER_MIXED] = "mixed",
>  	};
>  
> -	if (mode >= CXL_DECODER_NONE && mode <= CXL_DECODER_MIXED)
> +	if (mode >= CXL_DECODER_NONE && mode < CXL_DECODER_DEAD)
>  		return names[mode];
>  	return "mixed";
>  }
> 

-- 
Fan Ni