linux-kernel - Re: [PATCH v2] ACPI: PPTT: Dump PPTT table when error detected

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Message-ID: <aWUpQ04uNcXtp0wR@bogus>
Date: Mon, 12 Jan 2026 17:02:59 +0000
From: Sudeep Holla <sudeep.holla@....com>
To: Feng Tang <feng.tang@...ux.alibaba.com>
Cc: "Rafael J . Wysocki" <rafael@...nel.org>, Len Brown <lenb@...nel.org>,
	Sudeep Holla <sudeep.holla@....com>,
	Jeremy Linton <jeremy.linton@....com>,
	Hanjun Guo <guohanjun@...wei.com>,
	James Morse <james.morse@....com>,
	Joanthan Cameron <Jonathan.Cameron@...wei.com>,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ACPI: PPTT: Dump PPTT table when error detected

On Wed, Dec 31, 2025 at 06:49:09PM +0800, 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, which makes
> the warning more noticeable and helps bug hunting.
> 
> The dumped info format 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
>     ...
> 
> It provides a global and straightforward view of the hierarchy of the
> processor and caches info of the platform, and from the offset info
> (the 3rd column), the child-parent relation could be checked.
> 
> With this, the root cause of the original issue was pretty obvious,
> that there were some caches items missing which caused the issue when
> building up scheduler domain.
> 

While this may sound like a good idea, it deviates from how errors in other
table-parsing code are handled. Instead of dumping the entire table, it would
be preferable to report the specific issue encountered during parsing.

I do not have a strong objection if Rafael is comfortable with this approach;
however, it does differ from the established pattern used by similar code.
Dumping the entire table in a custom manner is not the standard way of
handling parsing errors. Just my opinion.

-- 
Regards,
Sudeep