Re: [PATCH BlueZ] hog-lib: Increase maximum report map size

Vicki Pfau
2022-08-04T00:07:12Z

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.