Re: [PATCH v8 23/27] cxl: add region flag for precluding a device memory to be used for dax

[Alejandro Lucero Palau <alucerop@amd.com>]

On 12/24/24 18:04, Jonathan Cameron wrote:
> On Mon, 16 Dec 2024 16:10:38 +0000
> <alejandro.lucero-palau@amd.com> wrote:
>
>> From: Alejandro Lucero <alucerop@amd.com>
>>
>> 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.
> So this is presented as something a type 2 driver would chose to set
> or not. That feels premature if for now they are all going to set
> it?


I would say, by definition, a type2 should avoid dax, but it could be 
dax just what is needed.

I was told to add this option in previous reviews, and I considered it 
more flexible and harmless.


>> Signed-off-by: Alejandro Lucero <alucerop@amd.com>
>> Reviewed-by: Zhi Wang <zhiw@nvidia.com>
>> ---
>>   drivers/cxl/core/region.c | 11 ++++++++++-
>>   drivers/cxl/cxl.h         |  3 +++
>>   drivers/cxl/cxlmem.h      |  3 ++-
>>   include/cxl/cxl.h         |  3 ++-
>>   4 files changed, 17 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
>> index a24d8678e8dc..aeaa6868e556 100644
>> --- a/drivers/cxl/core/region.c
>> +++ b/drivers/cxl/core/region.c
>> @@ -3557,12 +3557,14 @@ __construct_new_region(struct cxl_root_decoder *cxlrd,
>>    * cxl_create_region - Establish a region given an endpoint decoder
>>    * @cxlrd: root decoder to allocate HPA
>>    * @cxled: endpoint decoder with reserved DPA capacity
>> + * @no_dax: if true no DAX device should be created
>>    *
>>    * Returns a fully formed region in the commit state and attached to the
>>    * 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 no_dax)
>>   {
>>   	struct cxl_region *cxlr;
>>   
>> @@ -3578,6 +3580,10 @@ struct cxl_region *cxl_create_region(struct cxl_root_decoder *cxlrd,
>>   		drop_region(cxlr);
>>   		return ERR_PTR(-ENODEV);
>>   	}
>> +
>> +	if (no_dax)
>> +		set_bit(CXL_REGION_F_NO_DAX, &cxlr->flags);
>> +
>>   	return cxlr;
>>   }
>>   EXPORT_SYMBOL_NS_GPL(cxl_create_region, "CXL");
>> @@ -3713,6 +3719,9 @@ static int cxl_region_probe(struct device *dev)
>>   	if (rc)
>>   		return rc;
>>   
>> +	if (test_bit(CXL_REGION_F_NO_DAX, &cxlr->flags))
>> +		return 0;
>> +
>>   	switch (cxlr->mode) {
>>   	case CXL_DECODER_PMEM:
>>   		return devm_cxl_add_pmem_region(cxlr);
>> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
>> index 57d6dda3fb4a..cc9e3d859fa6 100644
>> --- a/drivers/cxl/cxl.h
>> +++ b/drivers/cxl/cxl.h
>> @@ -521,6 +521,9 @@ 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_NO_DAX 2
>> +
>>   /**
>>    * struct cxl_region - CXL region
>>    * @dev: This region's device
>> diff --git a/drivers/cxl/cxlmem.h b/drivers/cxl/cxlmem.h
>> index 9d874f1cb3bf..712f25f494e0 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 no_dax);
>>   #endif /* __CXL_MEM_H__ */
>> diff --git a/include/cxl/cxl.h b/include/cxl/cxl.h
>> index e0ea5b801a2e..14be26358f9c 100644
>> --- a/include/cxl/cxl.h
>> +++ b/include/cxl/cxl.h
>> @@ -61,7 +61,8 @@ 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 no_dax);
>>   
>>   int cxl_accel_region_detach(struct cxl_endpoint_decoder *cxled);
>>   #endif