On 22/08/2025 11:56, Koralahalli Channabasappa, Smita wrote:
>>
>>>
>>>> ```
>>>>
>>>> 3. When CXL_REGION is disabled, there is a failure to fallback to
>>>> dax_hmem, in which case only CXL Window X is visible.
>>>
>>> Haven't tested !CXL_REGION yet.
>
> When CXL_REGION is disabled, DEV_DAX_CXL will also be disabled. So dax_hmem
> should handle it.
Yes, falling back to dax_hmem/kmem is the result we expect.
I haven't figured out the root cause of the issue yet, but I can tell you that
in my QEMU environment,
there is currently a certain probability that it cannot fall back to
dax_hmem/kmem.
Upon its failure, I observed the following warnings and errors (with my local
fixup kernel).
[ 12.203254] kmem dax0.0: mapping0: 0x5d0000000-0x7cfffffff could not reserve
region
[ 12.203437] kmem dax0.0: probe with driver kmem failed with error -16
> I was able to fallback to dax_hmem. But let me know if I'm missing something.
>
> config DEV_DAX_CXL
> tristate "CXL DAX: direct access to CXL RAM regions"
> depends on CXL_BUS && CXL_REGION && DEV_DAX
> ..
>
>>>
>>>> On failure:
>>>> ```
>>>> 100000000-27ffffff : System RAM
>>>> 5c0001128-5c00011b7 : port1
>>>> 5c0011128-5c00111b7 : port2
>>>> 5d0000000-6cffffff : CXL Window 0
>>>> 6d0000000-7cffffff : CXL Window 1
>>>> 7000000000-700000ffff : PCI Bus 0000:0c
>>>> 7000000000-700000ffff : 0000:0c:00.0
>>>> 7000001080-70000010d7 : mem1
>>>> ```
>>>>
>>>> On success:
>>>> ```
>>>> 5d0000000-7cffffff : dax0.0
>>>> 5d0000000-7cffffff : System RAM (kmem)
>>>> 5d0000000-6cffffff : CXL Window 0
>>>> 6d0000000-7cffffff : CXL Window 1
>>>> ```
>>>>
>>>> In term of issues 1 and 2, this arises because hmem_register_device()
>>>> attempts to register resources of all "HMEM devices," whereas we only need
>>>> to register the IORES_DESC_SOFT_RESERVED resources. I believe resolving
>>>> the current TODO will address this.
>>>>
>>>> ```
>>>> - rc = region_intersects(res->start, resource_size(res), IORESOURCE_MEM,
>>>> - IORES_DESC_SOFT_RESERVED);
>>>> - if (rc != REGION_INTERSECTS)
>>>> - return 0;
>>>> + /* TODO: insert "Soft Reserved" into iomem here */
>>>> ```
>>>
>>> Above makes sense.
>>
>> I think the subroutine add_soft_reserved() in your previous patchset[1] are
>> able to cover this TODO
>>
>>>
>>> I'll probably wait for an update from Smita to test again, but if you
>>> or Smita have anything you want me to try out on my hardwware in the
>>> meantime, let me know.
>>>
>>
>> Here is my local fixup based on Dan's RFC, it can resovle issue 1 and 2.
>
> I almost have the same approach 🙂 Sorry, I missed adding your
> "Signed-off-by".. Will include for next revision..
Never mind.
Glad to see your V6, I will test and take a look at soon
>
>>
>>
>> -- 8< --
>> commit e7ccd7a01e168e185971da66f4aa13eb451caeaf
>> Author: Li Zhijian <lizhij...@fujitsu.com>
>> Date: Fri Aug 20 11:07:15 2025 +0800
>>
>> Fix probe-order TODO
>> Signed-off-by: Li Zhijian <lizhij...@fujitsu.com>
>>
>> diff --git a/drivers/dax/hmem/hmem.c b/drivers/dax/hmem/hmem.c
>> index 754115da86cc..965ffc622136 100644
>> --- a/drivers/dax/hmem/hmem.c
>> +++ b/drivers/dax/hmem/hmem.c
>> @@ -93,6 +93,26 @@ static void process_defer_work(struct work_struct *_work)
>> walk_hmem_resources(&pdev->dev, handle_deferred_cxl);
>> }
>> +static int add_soft_reserved(resource_size_t start, resource_size_t len,
>> + unsigned long flags)
>> +{
>> + struct resource *res = kzalloc(sizeof(*res), GFP_KERNEL);
>> + int rc;
>> +
>> + if (!res)
>> + return -ENOMEM;
>> +
>> + *res = DEFINE_RES_NAMED_DESC(start, len, "Soft Reserved",
>> + flags | IORESOURCE_MEM,
>> + IORES_DESC_SOFT_RESERVED);
>> +
>> + rc = insert_resource(&iomem_resource, res);
>> + if (rc)
>> + kfree(res);
>> +
>> + return rc;
>> +}
>> +
>> static int hmem_register_device(struct device *host, int target_nid,
>> const struct resource *res)
>> {
>> @@ -102,6 +122,10 @@ static int hmem_register_device(struct device *host,
>> int target_nid,
>> long id;
>> int rc;
>>
> > + if (soft_reserve_res_intersects(res->start, resource_size(res),
>> + IORESOURCE_MEM, IORES_DESC_NONE) == REGION_DISJOINT)
>> + return 0;
>> +
>
> Should also handle CONFIG_EFI_SOFT_RESERVE not enabled case..
I think it’s unnecessary. For !CONFIG_EFI_SOFT_RESERVE, it will return directly
because soft_reserve_res_intersects() will always return REGION_DISJOINT.
Thanks
Zhijian
>
>
> Thanks
> Smita
