Re: [PATCH v3] cxl: check decoder count for end device

[Dave Jiang <dave.jiang@intel.com>]

On 10/25/2022 10:38 AM, Dan Williams wrote:
> Jonathan Cameron wrote:
>> On Mon, 24 Oct 2022 08:43:30 -0700
>> Dave Jiang <dave.jiang@intel.com> wrote:
>>
>>> CXL spec rev3.0 8.2.4.19.1 added definition for up to 32 decoders. It also
>>> indicates that for devices, only 10 decoders should be advertised. Add
>>> check on number of decoders greater than 10 for devices and emit warning.
>>>
>>> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
>> I wonder... Should warning say CXL r3.0 spec violation?
>>
>> Seems possible this might grow in future versions and so it
>> might be nice to give a hint that it 'might' be valid in a higher spec version?
>>
>> I don't care strongly either way though.
>>
>> FWIW
>> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
>>
>>> ---
>>>
>>> v3:
>>> - Fix commit header and output message to reflect code change from v2.
>>>
>>> v2:
>>> - Remove decoder count reassignment from violation (Dan)
>>>
>>>   drivers/cxl/core/hdm.c |   10 ++++++++++
>>>   1 file changed, 10 insertions(+)
>>>
>>> diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
>>> index d1d2caea5c62..c3b756f93261 100644
>>> --- a/drivers/cxl/core/hdm.c
>>> +++ b/drivers/cxl/core/hdm.c
>>> @@ -71,9 +71,19 @@ EXPORT_SYMBOL_NS_GPL(devm_cxl_add_passthrough_decoder, CXL);
>>>   static void parse_hdm_decoder_caps(struct cxl_hdm *cxlhdm)
>>>   {
>>>   	u32 hdm_cap;
>>> +	struct device *dev = &cxlhdm->port->dev;
>>>   
>>>   	hdm_cap = readl(cxlhdm->regs.hdm_decoder + CXL_HDM_DECODER_CAP_OFFSET);
>>>   	cxlhdm->decoder_count = cxl_hdm_decoder_count(hdm_cap);
>>> +	/*
>>> +	 * CXL spec rev3.0 8.2.4.19.1 indicates CXL devices shall not advertise
>>> +	 * more than 10 decoders. Switches and Host Bridges may advertise up to
>>> +	 * 32 decoders. Set the decoders to 10 for devices if more than 10 are
>>> +	 * found.
> Stale comment, the "Set the decoders to 10" is no longer done.
>
>>> +	 */
>>> +	if (is_cxl_endpoint(cxlhdm->port) && cxlhdm->decoder_count > 10)
>>> +		dev_warn(dev, "Endpoint decoder count (%d) > 10, spec violation!\n",
>>> +			 cxlhdm->decoder_count);
> This is still too scary for how the kernel is going to handle it. I
> would expect:
>
> dev_dbg(dev, "Endpoint decoder count %d, expected max 10\n")
>
> ...what downside are you seeing that's prompting this patch? Sometimes
> Linux might want to constrain what hardware can do to avoid code
> maintenance burden, but I do not see that harm here. Otherwise, there
> are users that treat any log message above KERN_NOTICE priority as a
> sign of a broken system. Is a platform with an endpoint with 16 decoders
> broken? Also, if we do this bounds check for endpoints why are we not
> doing it for bridges for the > 32 case?

This is purely the spec changed for 3.0. Should I add check for bridges 
as well and add debug emit for both or just drop it entirely?