Re: [PATCH 1/4] cxl: Remove the CXL_DECODER_MIXED mistake

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Thu, 16 Jan 2025 22:10:32 -0800
Dan Williams <dan.j.williams@intel.com> 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.
> 
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  drivers/cxl/core/hdm.c    |    8 ++++----
>  drivers/cxl/core/region.c |   12 ------------
>  drivers/cxl/cxl.h         |    4 +---
>  3 files changed, 5 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
> index 28edd5822486..be8556119d94 100644
> --- a/drivers/cxl/core/hdm.c
> +++ b/drivers/cxl/core/hdm.c
> @@ -329,12 +329,12 @@ static int __cxl_dpa_reserve(struct cxl_endpoint_decoder *cxled,
>  
>  	if (resource_contains(&cxlds->pmem_res, res))
>  		cxled->mode = CXL_DECODER_PMEM;
> -	else if (resource_contains(&cxlds->ram_res, res))
> +	if (resource_contains(&cxlds->ram_res, res))

Logic of removing the else?  I assume there is 0 chance that both conditions
match, but doesn't this mean if the res is not in ram_res we always hit the next
else and print the warning?

>  		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/cxl.h b/drivers/cxl/cxl.h
> index f6015f24ad38..0fb8d70fa3e5 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -379,7 +379,6 @@ enum cxl_decoder_mode {
>  	CXL_DECODER_NONE,
>  	CXL_DECODER_RAM,
>  	CXL_DECODER_PMEM,
> -	CXL_DECODER_MIXED,
>  	CXL_DECODER_DEAD,
>  };
>  
> @@ -389,10 +388,9 @@ 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_PMEM)
Maybe just < DEAD is simpler?
>  		return names[mode];
>  	return "mixed";
>  }
> 
>