Re: [PATCH v4 11/14] cxl/atl: Lock decoders that need address translation

[Dave Jiang]

On 11/14/25 12:34 AM, Robert Richter wrote:
> On Thu, Nov 13, 2025 at 01:36:26PM -0700, Dave Jiang wrote:
>>
>>
>> On 11/13/25 1:05 PM, Robert Richter wrote:
>>> On 12.11.25 09:34:34, Dave Jiang wrote:
>>>>
>>>>
>>>> On 11/11/25 5:54 AM, Robert Richter wrote:
>>>>> On 04.11.25 10:13:34, Dave Jiang wrote:
>>>>>>
>>>>>>
>>>>>> On 11/3/25 11:47 AM, Robert Richter wrote:
>>>>>>> There is only support to translate addresses from an endpoint to its
>>>>>>> CXL host bridge, but not in the opposite direction from the bridge to
>>>>>>> the endpoint. Thus, the endpoint address range cannot be determined
>>>>>>> and setup manually for a given SPA range of a region. If the endpoint
>>>>>>> has address translation enabled, lock it to prevent the kernel from
>>>>>>> reconfiguring it.
>>>>>>>
>>>>>>> Reviewed-by: Gregory Price <gourry@xxxxxxxxxx>
>>>>>>> Signed-off-by: Robert Richter <rrichter@xxxxxxx>
>>>>>>> ---
>>>>>>>  drivers/cxl/core/atl.c | 10 ++++++++++
>>>>>>>  1 file changed, 10 insertions(+)
>>>>>>>
>>>>>>> diff --git a/drivers/cxl/core/atl.c b/drivers/cxl/core/atl.c
>>>>>>> index d6aa7e6d0ac5..5c15e4d12193 100644
>>>>>>> --- a/drivers/cxl/core/atl.c
>>>>>>> +++ b/drivers/cxl/core/atl.c
>>>>>>> @@ -158,6 +158,16 @@ static int cxl_prm_translate_hpa_range(struct cxl_root *cxl_root, void *data)
>>>>>>>  		return -ENXIO;
>>>>>>>  	}
>>>>>>>  
>>>>>>> +	/*
>>>>>>> +	 * There is only support to translate from the endpoint to its
>>>>>>> +	 * parent port, but not in the opposite direction from the
>>>>>>> +	 * parent to the endpoint. Thus, the endpoint address range
>>>>>>> +	 * cannot be determined and setup manually. If the address range
>>>>>>> +	 * was translated and modified, forbid reprogramming of the
>>>>>>> +	 * decoders and lock them.
>>>>>>> +	 */
>>>>>>> +	cxld->flags |= CXL_DECODER_F_LOCK;
>>>>>>
>>>>>  
>>>>>> Feels like this should be something the BIOS should enforce if that
>>>>>> is the expectation? And the kernel checks and warns if that is not
>>>>>> the case.
>>>>>
>>>>> I think this is more a limitation of the kernel implementation rather
>>>>> than the BIOS. The BIOS provides enought information by CFMWS, PRM,
>>>>> HDM and PCI topology. In theory and if there is demand for it, support
>>>>> could be added for driver region setup.
>>>>
>>>
>>>> But shouldn't the BIOS set the decoder lock rather than the kernel
>>>> setting a software lock flag based on assumption of the PRM based
>>>> setup?
>>>
>>> If BIOS locks the decoders, it cannot be removed even for the case
>>> there the OS can actually handle it.
>>
> 
>> Oh so the current implementation is auto region by BIOS but in the
>> future it may not be? But if you add a lock flag, you wouldn't be
>> able to remove it later anyhow since it's presented as locked?
> 
> The BIOS provides all necessary data for address translation, so that
> decoders can be reconfigured (including normalized endpoint
> addresses). There is no reason to lock the decoders by the BIOS, as
> otherwise, with a capable kernel (or other OS), it would not be
> possible to shutdown auto-generated regions.
> 
> However, current kernel implementation does not support this and is
> unable to create the region. That is why the kernel and not the BIOS
> should lock the decoders.

Ok I see what you are saying. Fair enough. As long as we have a comment that makes note of this detail.

> 
> -Robert