Re: [PATCH v4 RESEND] hid-ft260: Add serial driver

[Michael Zaidman <michael.zaidman@gmail.com>]

On Wed, Jan 17, 2024 at 10:43:31AM +1300, Daniel Beer wrote:
> On Tue, Jan 16, 2024 at 11:34:32PM +0200, Michael Zaidman wrote:
> > > +/* The FT260 has a "power saving mode" that causes the device to switch
> > > + * to a 30 kHz oscillator if there's no activity for 5 seconds.
> > > + * Unfortunately this mode can only be disabled by reprogramming
> > > + * internal fuses, which requires an additional programming voltage.
> > > + *
> > > + * One effect of this mode is to cause data loss on a fast UART that
> > > + * transmits after being idle for longer than 5 seconds. We work around
> > > + * this by sending a dummy report at least once per 4 seconds if the
> > > + * UART is in use.
> > > + */
> > 
> > For I2C, we addressed a similar issue in
> > https://lore.kernel.org/all/20221105211151.7094-8-michael.zaidman@gmail.com/

Link to the correct patch
https://lore.kernel.org/all/20221105211151.7094-11-michael.zaidman@gmail.com/

> > commit. But we did it per IO synchronously when the distance between this and
> > the previous IO exceeded 5 seconds. In this way, the chip can still sleep
> > between the IOs. On the contrary, the suggested workaround prevents the chip
> > from entering the power saving mode during active TTY sessions regardless of
> > the traffic intensity on the UART bus.
> > 
> > I cannot reproduce the issue with 1K Tx bursts at 921600 baud rate sent every
> > 10 seconds with the disabled chip wakeup workaround.
> > 
> > Can you guide me on how to reproduce the data loss you observed?
> 
> Hi Michael,
> 
> This was my comment originally. It's been a long time (at least a year),
> but from memory I had an FT260 attached to a UART console on an MCU dev
> kit, which would print messages at 115200.
> 
> If the MCU sat idle for more than 5 seconds and then printed a message,
> the first few characters of the line would be missing in picocom. If the
> MCU kept busy, printing more frequently than once every 5 seconds, the
> problem did not occur.
> 
> Cheers,
> Daniel
> 

Hi Daniel,

Thanks for the clarification. It was not clear from the issue description
in the commit whether it happens on the ft260 Tx or Rx line, and I assumed
it is Tx. Also, the periodic dummy report workaround is not active in the
submitted patch. I reproduced the issue on the Rx line. And confirm the
workaround works as expected when enabled.

May I suggest modifying the description to clarify that the data loss
happens on the Rx line and state that the current dummy report period is
4.8 seconds?

Also, please enable the reschedule_work flag in the ft260_uart_probe
routine to activate the periodic dummy reports.

--Michael