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

Dave Jiang
2025-11-13T20:36:30Z

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@gourry.net>
>>>>> Signed-off-by: Robert Richter <rrichter@amd.com>
>>>>> ---
>>>>> 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?

>
> -Robert