Subject: Re: [PATCH v5 24/27] cxl: add region flag for precluding a device memory to be used for dax

From: Ben Cheatham <[email-masked]>
Date: Fri, 22 Nov 2024 14:46:13 -0600

On 11/18/24 10:44 AM, [email-masked] wrote:
> From: Alejandro Lucero <[email-masked]>
> 
> By definition a type2 cxl device will use the host managed memory for
> specific functionality, therefore it should not be available to other
> uses. However, a dax interface could be just good enough in some cases.
> 
> Add a flag to a cxl region for specifically state to not create a dax
> device. Allow a Type2 driver to set that flag at region creation time.
> 
> Signed-off-by: Alejandro Lucero <[email-masked]>
> ---
>  drivers/cxl/core/region.c | 10 +++++++++-
>  drivers/cxl/cxl.h         |  3 +++
>  drivers/cxl/cxlmem.h      |  3 ++-
>  include/cxl/cxl.h         |  3 ++-
>  4 files changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 70549d42c2e3..eff3ad788077 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -3558,7 +3558,8 @@ __construct_new_region(struct cxl_root_decoder *cxlrd,
>   * cxl_region driver.
>   */
>  struct cxl_region *cxl_create_region(struct cxl_root_decoder *cxlrd,
> -				     struct cxl_endpoint_decoder *cxled)
> +				     struct cxl_endpoint_decoder *cxled,
> +				     bool avoid_dax)
>  {
>  	struct cxl_region *cxlr;
>  
> @@ -3574,6 +3575,10 @@ struct cxl_region *cxl_create_region(struct cxl_root_decoder *cxlrd,
>  		drop_region(cxlr);
>  		return ERR_PTR(-ENODEV);
>  	}
> +
> +	if (avoid_dax)
> +		set_bit(CXL_REGION_F_AVOID_DAX, &cxlr->flags);
> +
>  	return cxlr;
>  }
>  EXPORT_SYMBOL_NS_GPL(cxl_create_region, CXL);
> @@ -3713,6 +3718,9 @@ static int cxl_region_probe(struct device *dev)
>  	case CXL_DECODER_PMEM:
>  		return devm_cxl_add_pmem_region(cxlr);
>  	case CXL_DECODER_RAM:
> +		if (test_bit(CXL_REGION_F_AVOID_DAX, &cxlr->flags))
> +			return 0;

I think it's possible for a type 2 device to have pmem as well, and
it looks like these are the only two options at the moment, so I would
just move this check to before the switch statement.

> +
>  		/*
>  		 * The region can not be manged by CXL if any portion of
>  		 * it is already online as 'System RAM'
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index 1e0e797b9303..ee3385db5663 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -512,6 +512,9 @@ struct cxl_region_params {
>   */
>  #define CXL_REGION_F_NEEDS_RESET 1
>  
> +/* Allow Type2 drivers to specify if a dax region should not be created. */
> +#define CXL_REGION_F_AVOID_DAX 2
> +

I would like to see flags such that the device could choose the region type 
(system ram, device-dax, or none). I think that adding the ability
for device-dax would add a patch or two, so that may be a good follow up
patch.

>  /**
>   * struct cxl_region - CXL region
>   * @dev: This region's device
> diff --git a/drivers/cxl/cxlmem.h b/drivers/cxl/cxlmem.h
> index 9d874f1cb3bf..cc2e2a295f3d 100644
> --- a/drivers/cxl/cxlmem.h
> +++ b/drivers/cxl/cxlmem.h
> @@ -875,5 +875,6 @@ struct seq_file;
>  struct dentry *cxl_debugfs_create_dir(const char *dir);
>  void cxl_dpa_debug(struct seq_file *file, struct cxl_dev_state *cxlds);
>  struct cxl_region *cxl_create_region(struct cxl_root_decoder *cxlrd,
> -				     struct cxl_endpoint_decoder *cxled);
> +				     struct cxl_endpoint_decoder *cxled,
> +				     bool avoid_dax);
>  #endif /* __CXL_MEM_H__ */
> diff --git a/include/cxl/cxl.h b/include/cxl/cxl.h
> index d295af4f5f9e..2a8ebabfc1dd 100644
> --- a/include/cxl/cxl.h
> +++ b/include/cxl/cxl.h
> @@ -73,7 +73,8 @@ struct cxl_endpoint_decoder *cxl_request_dpa(struct cxl_memdev *cxlmd,
>  					     resource_size_t max);
>  int cxl_dpa_free(struct cxl_endpoint_decoder *cxled);
>  struct cxl_region *cxl_create_region(struct cxl_root_decoder *cxlrd,
> -				     struct cxl_endpoint_decoder *cxled);
> +				     struct cxl_endpoint_decoder *cxled,
> +				     bool avoid_dax);
>  
>  int cxl_accel_region_detach(struct cxl_endpoint_decoder *cxled);
>  #endif