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

Vicki Pfau
2022-08-04T00:18:23Z

On 8/3/22 17:16, Luiz Augusto von Dentz wrote:
> Hi Vicki,
>
> On Wed, Aug 3, 2022 at 5:05 PM Vicki Pfau <vi@endrift.com> wrote:
>>
>>
>>
>> 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.
>
> While I agree this could be worked around it is probably worth
> checking with the manufacturer if it is aware of the problem because
> even if we were to allow reading past 512 bytes offset in the future
> there may be qualification tests enforcing not to do so, besides
> versions up to BlueZ 5.65 would still not work anyway so I thing
> letting the manufacturer know there is a problem with their
> implementation is actually worth a shot here.
>

That's fair enough. I'll see if I can find an email address for them.