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Date:   Fri, 2 Sep 2022 10:07:41 +0200
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Subject: Re: [PATCH v2] platform/x86: wmi: Allow duplicate GUIDs for drivers
 that use struct wmi_driver
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To:     "Limonciello, Mario" <Mario.Limonciello@amd.com>
Cc:     Mark Gross <markgross@kernel.org>,
        Platform Driver <platform-driver-x86@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andy Shevchenko <andy.shevchenko@gmail.com>
References: <20220829201500.6341-1-mario.limonciello@amd.com>
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 <MN0PR12MB6101441AA29CE815DCAD26AEE27B9@MN0PR12MB6101.namprd12.prod.outlook.com>
From:   Hans de Goede <hdegoede@redhat.com>
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Hi,

On 9/1/22 23:39, Limonciello, Mario wrote:
> [Public]
> 
> 
> 
>> -----Original Message-----
>> From: Andy Shevchenko <andy.shevchenko@gmail.com>
>> Sent: Thursday, September 1, 2022 12:17
>> To: Limonciello, Mario <Mario.Limonciello@amd.com>
>> Cc: Hans de Goede <hdegoede@redhat.com>; Mark Gross
>> <markgross@kernel.org>; Platform Driver <platform-driver-
>> x86@vger.kernel.org>; Linux Kernel Mailing List <linux-
>> kernel@vger.kernel.org>
>> Subject: Re: [PATCH v2] platform/x86: wmi: Allow duplicate GUIDs for drivers
>> that use struct wmi_driver
>>
>> On Mon, Aug 29, 2022 at 11:20 PM Mario Limonciello
>> <mario.limonciello@amd.com> wrote:
>>>
>>> The WMI subsystem in the kernel currently tracks WMI devices by
>>> a GUID string not by ACPI device.  The GUID used by the `wmi-bmof`
>>> module however is available from many devices on nearly every machine.
>>>
>>> This originally was though to be a bug, but as it happens on most
>>
>> thought
>>
>>> machines it is a design mistake.  It has been fixed by tying an ACPI
>>> device to the driver with struct wmi_driver. So drivers that have
>>> moved over to struct wmi_driver can actually support multiple
>>> instantiations of a GUID without any problem.
>>>
>>> Add an allow list into wmi.c for GUIDs that the drivers that are known
>>> to use struct wmi_driver.  The list is populated with `wmi-bmof` right
>>> now. The additional instances of that in sysfs with be suffixed with -%d
>>
>> ...
>>
>>> +/* allow duplicate GUIDs as these device drivers use struct wmi_driver */
>>> +static const char * const allow_duplicates[] = {
>>> +       "05901221-D566-11D1-B2F0-00A0C9062910", /* wmi-bmof */
>>> +       NULL,
>>
>> No comma for the terminator.
>>
>>> +};
>>
>> ...
>>
>>> +static int guid_count(const guid_t *guid)
>>> +{
>>> +       struct wmi_block *wblock;
>>> +       int count = 0;
>>> +
>>> +       list_for_each_entry(wblock, &wmi_block_list, list) {
>>> +               if (guid_equal(&wblock->gblock.guid, guid))
>>> +                       count++;
>>> +       }
>>> +
>>> +       return count;
>>> +}
>>
>> I haven't deeply checked the code, but this kind of approach is
>> fragile and proven to be error prone as shown in practice. The
>> scenario is (again, not sure if it's possible, need a comment in the
>> code if it's not possible) removing an entry from the list in the
>> middle and trying to add it again. you will see the duplicate count
>> values. That's why in the general case we use IDA or similar
>> approaches.
> 
> It shouldn't be possible to add/remove from the list, they're fixed
> lists that were parsed from _WDG.
> 
> Hans - since you already took this into your review queue, can you
> land fixes for the 3 things Andy pointed out before it goes to -next
> or do you want me to do a manual follow up for them?

I can do a local fix and squash it into the original commit.

> 1) Spelling error in commit message
> 2) Remove comma on terminator

Ack, will fix.

> 3) Add a comment why guid_count is safe (if you agree with me it is)

I agree it is safe.

Can you suggest some wording for the comment please ?

Regards,

Hans