Re: About PPTT find_acpi_cpu_topology_package()

[John Garry <john.garry@huawei.com>]

On 12/02/2020 13:55, Sudeep Holla wrote:
> On Wed, Feb 12, 2020 at 12:48:33PM +0000, John Garry wrote:
>> On 12/02/2020 11:59, Sudeep Holla wrote:
> 
> [...]
> 

Hi Sudeep,

>>> Yes, as mentioned above. We are not going to do extra work for lazy firmware.
>>
>> I don't think it's reasonable to just label this as lazy. The table may just
>> not have the flag set unintentionally. FW and software guys make mistakes,
>> like the mistakes in PPTT, itself.
>>
> 
> We are not talking about flags, it's UID and it is pretty important if
> there are more than one objects of same time.
> 

I am talking about the Processor ID valid flag, which is specifically 
related.

>>> Linux also will be lazy on such platform and provide weird unique numbers
>>> like in the above case you have mentioned.
>>
>> Personally I think that the kernel can be do better than provide meaningless
>> values like this, since it knows the processor IDs and which physical
>> package they belong to.
>>
> 
> This was discussed quite a lot, I can dig and point you to it. That's the
> reason for choosing offset. We are *not going back* to this again. Fix the
> firmware before it gets copied for all future platforms and Linux has to
> deal with that *forever*.

I would liked to have been made aware earlier of the oversight. Quite 
often we only find problems when someone or something complains.

It is a strange API to provide offsets like this, and I did not realize 
that they were actually being exposed to userspace.

> 
>> If not, at least make the user know of potential deficiencies in the table.
>>
> 
> How ? What are your suggestions ? Does adding a warning or note that UID
> is missing and offset is chosen help ? 

I'd say so. I know now, but let's save others the potential hassle. And 
having this debate again.

I am kind of fine with that.

How about something like this:

--- a/drivers/acpi/pptt.c
+++ b/drivers/acpi/pptt.c
@@ -515,6 +515,8 @@ static int topology_get_acpi_cpu_tag(struct 
acpi_table_header *table,
   if (level == 0 || cpu_node->flags & ACPI_PPTT_ACPI_PROCESSOR_ID_VALID)
	return cpu_node->acpi_processor_id;
+   if (level == PPTT_ABORT_PACKAGE)
+	pr_warn_once("ACPI Processor ID valid not set for physical package 
node, will use node table offset as substitute for UID\n");
                 return ACPI_PTR_DIFF(cpu_node, table);
         }
         pr_warn_once("PPTT table found, but unable to locate core %d 
(%d)\n",

Thanks,
John