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Message-ID: <c0f52a2d-19ff-acb2-92ac-cec87819a2cf@endrift.com>
Date:   Wed, 3 Aug 2022 17:05:55 -0700
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Subject: Re: [PATCH BlueZ] hog-lib: Increase maximum report map size
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To:     Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Cc:     "linux-bluetooth@vger.kernel.org" <linux-bluetooth@vger.kernel.org>
References: <20220803225716.1287921-1-vi@endrift.com>
 <CABBYNZKcAZeDqjeHzTfqmvKS3Tbur-rRi6+uPL4x4t8MnWp5ug@mail.gmail.com>
From:   Vicki Pfau <vi@endrift.com>
In-Reply-To: <CABBYNZKcAZeDqjeHzTfqmvKS3Tbur-rRi6+uPL4x4t8MnWp5ug@mail.gmail.com>
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On 8/3/22 16:55, Luiz Augusto von Dentz wrote:
> Hi Vicki,
> 
> On Wed, Aug 3, 2022 at 4:07 PM Vicki Pfau <vi@endrift.com> wrote:
>>
>> Though a 512 byte report map size seems plenty large, there exist some devices
>> (e.g. Brydge W-Touch) that send larger reports. There is no protocol-defined
>> maximum size so doubling the maximum size is safe, and should hopefully fix
>> most real-world failures.
>> ---
>>  profiles/input/hog-lib.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/profiles/input/hog-lib.c b/profiles/input/hog-lib.c
>> index 4a9c60185..9f3eb428c 100644
>> --- a/profiles/input/hog-lib.c
>> +++ b/profiles/input/hog-lib.c
>> @@ -64,7 +64,7 @@
>>  #define HOG_PROTO_MODE_BOOT    0
>>  #define HOG_PROTO_MODE_REPORT  1
>>
>> -#define HOG_REPORT_MAP_MAX_SIZE        512
>> +#define HOG_REPORT_MAP_MAX_SIZE        1024
>>  #define HID_INFO_SIZE                  4
>>  #define ATT_NOTIFICATION_HEADER_SIZE   3
> 
> Afaik 512 is the maximum length an attribute can have even when using
> read long procedure:
> 
> BLUETOOTH CORE SPECIFICATION Version 5.3 | Vol 3, Part F
> page 1416:
> 
> The maximum length of an attribute value shall be 512 octets.
> 
> And
> 
> BLUETOOTH SPECIFICATION
> HID Service Specification
> Page 16 of 26
> 
> 2.6.1 Report Map Characteristic Behavior
> The GATT Read Characteristic Value or Read Long Characteristic Values sub-
> procedures are used to read the Report Map characteristic value.
> The length of the Report Map characteristic value is limited to 512 octets.
> 
> So I believe the device is not compliant and very likely needs to have
> multiple instances of HID Service instead of combining everything in a
> single instance.
> 
>> --
>> 2.37.1
>>
> 
> 

Ah, that's strange. I looked through the spec but didn't see those. That said, while the device may be non-compliant, the device is on the market and I doubt I could get them to update the firmware as a random third party. It works on Windows, so clearly Windows doesn't have a problem with its noncompliance. So this raises the question, how should Linux handle non-compliant hardware, especially when it could easily be made to work just by bending the rules in this one instance? I can absolutely change the commit message since it's erroneous, but the question then comes down to how should it be handled at all.