Re: [PATCH] ACPI: PPTT: Dump PPTT table when error detected

[Jeremy Linton <jeremy.linton@arm.com>]

Hi,

On 10/14/25 3:51 AM, Feng Tang wrote:
> There was warning message about PPTT table:
> "ACPI PPTT: PPTT table found, but unable to locate core 1 (1)",
> and it in turn caused scheduler warnings when building up the system.
> It took a while to root cause the problem be related a broken PPTT
> table which has wrong cache information.
> 
> To speedup debugging similar issues, dump the PPTT table when there
> is warning about it.
> 
> The dumped info on a ARM server is like:
> 
>      ACPI PPTT: Processors:
>      P[  0][0x0024]: parent=0x0000 acpi_proc_id=  0 num_res=1 flags=0x11(package)
>      P[  1][0x005a]: parent=0x0024 acpi_proc_id=  0 num_res=1 flags=0x12()
>      P[  2][0x008a]: parent=0x005a acpi_proc_id=  0 num_res=3 flags=0x1a(leaf)
>      P[  3][0x00f2]: parent=0x005a acpi_proc_id=  1 num_res=3 flags=0x1a(leaf)
>      P[  4][0x015a]: parent=0x005a acpi_proc_id=  2 num_res=3 flags=0x1a(leaf)
>      ...
>      ACPI PPTT: Caches:
>      C[   0][0x0072]: flags=0x7f next_level=0x0000 size=0x4000000  sets=65536  way=16 attribute=0xa  line_size=64
>      C[   1][0x00aa]: flags=0x7f next_level=0x00da size=0x10000    sets=256    way=4  attribute=0x4  line_size=64
>      C[   2][0x00c2]: flags=0x7f next_level=0x00da size=0x10000    sets=256    way=4  attribute=0x2  line_size=64
>      C[   3][0x00da]: flags=0x7f next_level=0x0000 size=0x100000   sets=2048   way=8  attribute=0xa  line_size=64


This is an interesting idea, particularly if the dump is smarter and 
acts like a linter, but i'm not sure the output right now would have 
helped with the original problem. In that respect simply sprinkling a 
lot of pr_debug information around during the parsing might be more 
helpful to understand the context around parse errors.

I was under the impression doing that was frowned on, as I ended up 
removing quite a number of them during the initial reviews.


I wonder if simply print_hex_dump'ing the table on certain errors with 
some acpi=debug like option is just as useful. Particularly if someone 
writes a better userspace pptt pretty printer, since the iasl output 
isn't helpful to understand the tree relationships.


>      ...
> 
>  From it, we can see the processor/cache info and the hierarchy relation
> between them.
> 
> Signed-off-by: Feng Tang <feng.tang@linux.alibaba.com>
> ---
>   drivers/acpi/pptt.c | 75 +++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 75 insertions(+)
> 
> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
> index 54676e3d82dd..e7b48a77a12f 100644
> --- a/drivers/acpi/pptt.c
> +++ b/drivers/acpi/pptt.c
> @@ -498,6 +498,79 @@ static void acpi_pptt_warn_missing(void)
>   	pr_warn_once("No PPTT table found, CPU and cache topology may be inaccurate\n");
>   }
>   
> +static void acpi_dump_pptt_table(struct acpi_table_header *table_hdr)
> +{
> +	struct acpi_subtable_header *entry, *entry_start;
> +	unsigned long end;
> +	struct acpi_pptt_processor *cpu;
> +	struct acpi_pptt_cache *cache;
> +	u32 entry_sz, i;
> +	u8 len;
> +	static bool dumped;
> +
> +	/* PPTT table could be pretty big, no need to dump it twice */
> +	if (dumped)
> +		return;
> +	dumped = true;
> +
> +	end = (unsigned long)table_hdr + table_hdr->length;
> +	entry_start = ACPI_ADD_PTR(struct acpi_subtable_header, table_hdr,
> +			     sizeof(struct acpi_table_pptt));
> +
> +	pr_info("Processors:\n");
> +	entry_sz = sizeof(struct acpi_pptt_processor);
> +	entry = entry_start;
> +	i = 0;
> +	while ((unsigned long)entry + entry_sz <= end) {
> +		len = entry->length;
> +		if (!len) {
> +			pr_warn("Invalid zero length subtable\n");
> +			return;
> +		}
> +
> +		cpu = (struct acpi_pptt_processor *)entry;
> +		entry = ACPI_ADD_PTR(struct acpi_subtable_header, entry, len);
> +
> +		if (cpu->header.type != ACPI_PPTT_TYPE_PROCESSOR)
> +			continue;
> +
> +		printk(KERN_INFO "P[%3d][0x%04lx]: parent=0x%04x acpi_proc_id=%3d num_res=%d flags=0x%02x(%s%s%s)\n",
> +			i++, (unsigned long)cpu - (unsigned long)table_hdr,
> +			cpu->parent, cpu->acpi_processor_id,
> +			cpu->number_of_priv_resources, cpu->flags,
> +			cpu->flags & ACPI_PPTT_PHYSICAL_PACKAGE ? "package" : "",
> +			cpu->flags & ACPI_PPTT_ACPI_LEAF_NODE ? "leaf" : "",
> +			cpu->flags & ACPI_PPTT_ACPI_PROCESSOR_IS_THREAD ? ", thread" : ""
> +			);
> +
> +	}
> +
> +	pr_info("Caches:\n");
> +	entry_sz = sizeof(struct acpi_pptt_cache);
> +	entry = entry_start;
> +	i = 0;
> +	while ((unsigned long)entry + entry_sz <= end) {
> +		len = entry->length;
> +		if (!len) {
> +			pr_warn("Invalid zero length subtable\n");
> +			return;
> +		}
> +
> +		cache = (struct acpi_pptt_cache *)entry;
> +		entry = ACPI_ADD_PTR(struct acpi_subtable_header, entry, len);
> +
> +		if (cache->header.type != ACPI_PPTT_TYPE_CACHE)
> +			continue;
> +
> +		printk(KERN_INFO "C[%4d][0x%04lx]: flags=0x%02x next_level=0x%04x size=0x%-8x sets=%-6d way=%-2d attribute=0x%-2x line_size=%d\n",
> +			i++, (unsigned long)cache - (unsigned long)table_hdr,
> +			cache->flags, cache->next_level_of_cache, cache->size,
> +			cache->number_of_sets, cache->associativity,
> +			cache->attributes, cache->line_size
> +			);
> +	}
> +}
> +
>   /**
>    * topology_get_acpi_cpu_tag() - Find a unique topology value for a feature
>    * @table: Pointer to the head of the PPTT table
> @@ -534,6 +607,8 @@ static int topology_get_acpi_cpu_tag(struct acpi_table_header *table,
>   	}
>   	pr_warn_once("PPTT table found, but unable to locate core %d (%d)\n",
>   		    cpu, acpi_cpu_id);
> +
> +	acpi_dump_pptt_table(table);
>   	return -ENOENT;
>   }
>