Re: USB 1.1 Full Speed OHCI slow/high latency

[ValdikSS <iam@valdikss.org.ru>]

On 22.09.2025 5:16 PM, Alan Stern wrote:
> On Mon, Sep 22, 2025 at 02:11:43AM +0300, ValdikSS wrote:
>> Alan, Greg, I found the reason for 1 ms delay: it is artificial, added in
>> v2.5.21 as a possible race condition fix:
> 
> Good work tracking that down!
> 
>> I removed "+ 1" and it removed that 1ms delay. Doesn't seem to break
>> anything in my setup. UHCI code doesn't seem to have any similar delays.
>>
>> Could it be relevant only for hardware of its era? If I add "no +1" code
>> quirk as a module option, disabled by default, does it sounds sane to you?
> 
> I'm not sure that there is any OHCI hardware from later than that era.
> But regardless...
> 
> Module options are frowned upon these days.  Instead you could add a
> sysfs (or even debugfs) attribute file for controlling that "+ 1".

I just implemented another approach: do not unlink the ED right away, 
instead mark it IDLE and unlink only if no consecutive transfers are 
performed after the timeout. This still preserves one-frame delay while 
unlinking, should be safe.

Please take a look if you have free time. I'm no kernel developer by any 
means :)