Re: [PATCH v11 3/9] ACPI: x86: s2idle: Fix a logic error parsing AMD constraints table

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Aug 09, 2023 at 01:54:47PM -0500, Mario Limonciello wrote:
> The constraints table should be resetting the `list` object
> after running through all of `info_obj` iterations.

This *looks* like it should fix a real problem (see below), but not
the one mentioned here.  But maybe I'm missing something because the
code that looks broken to me has been there since 146f1ed852a8 ("ACPI:
PM: s2idle: Add AMD support to handle _DSM"), which appeared in v5.11
in 2021.

> This adjusts whitespace as well as less code will now be included
> with each loop.
>
> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> ---
> v9->v10:
>  * split from other patches
> ---
>  drivers/acpi/x86/s2idle.c | 35 +++++++++++++++++------------------
>  1 file changed, 17 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/acpi/x86/s2idle.c b/drivers/acpi/x86/s2idle.c
> index ce62e61a9605e..b566b3aa09388 100644
> --- a/drivers/acpi/x86/s2idle.c
> +++ b/drivers/acpi/x86/s2idle.c
> @@ -129,12 +129,12 @@ static void lpi_device_get_constraints_amd(void)
>  				struct lpi_constraints *list;
>  				acpi_status status;
>  
> +				list = &lpi_constraints_table[lpi_constraints_table_size];
> +				list->min_dstate = -EINVAL;

I really have no idea what's going on here, but the code still looks
weird:

  1) Moving the "list" update:

       for (j = 0; j < package->package.count; ++j) {
     +   list = &lpi_constraints_table[lpi_constraints_table_size];
         for (k = 0; k < info_obj->package.count; ++k) {
     -     list = &lpi_constraints_table[lpi_constraints_table_size];
           ...
         }
         lpi_constraints_table_size++;
       }

     looks fine, but lpi_constraints_table_size isn't updated inside
     the "k" loop, and "list" isn't otherwise updated, so it shouldn't
     make any functional difference.

     HOWEVER, this patch also moves all the
     dev_info.enabled/name/min_dstate tests outside the "k" loop, so
     they're only done after the "k" loop has completed and they've
     all been set, which looks like it DOES fix a problem and is not
     mentioned in the commit log.

  2) Both lpi_device_get_constraints_amd() and
     lpi_device_get_constraints() overwrite the global
     lpi_constraints_table for each PNP0D80 device.  I assume there's
     some higher-level constraint that there can only be one such
     device, but the code doesn't enforce that.

  3) It's obvious that lpi_device_get_constraints() can only allocate
     lpi_constraints_table once per call.  It's NOT obvious for
     lpi_device_get_constraints_amd(), because the alloc is inside a
     loop:

       for (i = 0; i < out_obj->package.count; i++) {
         lpi_constraints_table = kcalloc(...);

     If the AMD _DSM returns more than one package, we'll leak all but
     the last one.

  4) Both lpi_device_get_constraints_amd() and
     lpi_device_get_constraints() use pre- and post-increment in the
     "for" loops for no apparent reason:

       for (i = 0; i < out_obj->package.count; i++)
         for (j = 0; j < package->package.count; ++j)
           for (k = 0; k < info_obj->package.count; ++k)  # AMD only

     I'd say they should all use the same (I vote for post-increment).

>  				for (k = 0; k < info_obj->package.count; ++k) {
>  					union acpi_object *obj = &info_obj->package.elements[k];
>  
> -					list = &lpi_constraints_table[lpi_constraints_table_size];
> -					list->min_dstate = -1;
> -
>  					switch (k) {
>  					case 0:
>  						dev_info.enabled = obj->integer.value;
> @@ -149,26 +149,25 @@ static void lpi_device_get_constraints_amd(void)
>  						dev_info.min_dstate = obj->integer.value;
>  						break;
>  					}
> +				}
>  
> -					if (!dev_info.enabled || !dev_info.name ||
> -					    !dev_info.min_dstate)
> -						continue;
> +				if (!dev_info.enabled || !dev_info.name ||
> +				    !dev_info.min_dstate)
> +					continue;
>  
> -					status = acpi_get_handle(NULL, dev_info.name,
> -								 &list->handle);
> -					if (ACPI_FAILURE(status))
> -						continue;
> +				status = acpi_get_handle(NULL, dev_info.name, &list->handle);
> +				if (ACPI_FAILURE(status))
> +					continue;
>  
> -					acpi_handle_debug(lps0_device_handle,
> -							  "Name:%s\n", dev_info.name);
> +				acpi_handle_debug(lps0_device_handle,
> +						  "Name:%s\n", dev_info.name);
>  
> -					list->min_dstate = dev_info.min_dstate;
> +				list->min_dstate = dev_info.min_dstate;
>  
> -					if (list->min_dstate < 0) {
> -						acpi_handle_debug(lps0_device_handle,
> -								  "Incomplete constraint defined\n");
> -						continue;
> -					}
> +				if (list->min_dstate < 0) {
> +					acpi_handle_debug(lps0_device_handle,
> +							  "Incomplete constraint defined\n");
> +					continue;
>  				}
>  				lpi_constraints_table_size++;
>  			}
> -- 
> 2.34.1
>