Re: [PATCH] HID: logitech-hidpp: Enable HID++ for all the Logitech Bluetooth devices

["Peter F. Patel-Schneider" <pfpschneider@gmail.com>]

On 8/29/22 09:41, Bastien Nocera wrote:
> On Fri, 2022-08-26 at 11:37 -0400, Peter F. Patel-Schneider wrote:
>> Looking at
>> https://bugzilla.kernel.org/show_bug.cgi?id=215699 reminded me of
>> another problem with the driver code.   Several HID++ 2.0 features,
>> including
>> the HiRes Wheel, have commands with bit fields in them and set all
>> the bits in
>> the bit field at once. But when the driver code sets the high-
>> resolution bit
>> for this feature it also sets two other bits, ignoring their current
>> setting.
>> This prevents other software from reliably using these two other
>> bits, one of
>> which is for reporting wheel movement in the opposite direction, a.
>> k. a.
>> natural scrolling.
>>
>> It would be useful for the driver code to first get the other bits
>> and set
>> them to their retrieved values.
> Do you have any other examples of this? For the classic Linux desktop,
> this particular bug isn't something we'd hit, as natural scrolling is
> implemented at the higher levels (libinput) rather than something we
> expect the driver to support.


I looked and I didn't find any other examples of this in the driver.  The only 
HID++ command with this feature that is used by the driver, as far as I could 
tell, is

#define CMD_HIRES_WHEEL_SET_WHEEL_MODE 0x20

of

#define HIDPP_PAGE_HIRES_WHEEL 0x2121


The way Solaar users see this bug is that if they use this feature to set 
reverse scrolling sometimes the driver flips the bit off after Solaar flips 
the bit on.  If a user doesn't use Solaar or some other software that changes 
devices behaviour using HID++ commands then they won't see any problem.  (The 
could have a multi-host mouse, use Logitech software to flip the bit, and then 
when they change hosts to a Linux host the Linux driver would flip the bit.)


peter