Re: [PATCH v2 6/7] USB: serial: ftdi_sio: Fix custom_divisor and c_*speed for ASYNC_SPD_CUST

[Johan Hovold <johan@kernel.org>]

On Sun, Jul 24, 2022 at 02:59:08PM +0200, Pali Rohár wrote:
> On Sunday 24 July 2022 14:54:58 Johan Hovold wrote:
> > On Sun, Jul 24, 2022 at 02:33:51PM +0200, Pali Rohár wrote:

> > > What is updated is the real baudrate in c_ispeed and c_ospeed
> > > extensions.
> > > 
> > > It is really wrong? I thought that c_cflag should stay unchanged at
> > > B38400 when ASYNC_SPD_CUST is used.
> > 
> > Yeah, cflags stay unchanged, but you shouldn't touch those fields when
> > using the deprecated ASYNC_SPD_CUST hack.
> 
> Hm... Why? I thought that new extended fields (c_ispeed and c_ospeed)
> should contain current real speed. What is the reason that c_*speed
> fields should have 38400 when ASYNC_SPD_CUST hack is set?

Because we shouldn't go adding new features built around the deprecated
ASYNC_SPD_CUST hack.

User picks 38400, sets that flag and magic happens with some drivers for
a while still while we look the other way.

This is not something that we should need to care about when using the
new interfaces.

> > Note that this currently only works because the ftdi driver uses
> > tty_get_baud_rate() instead of c_ospeed directly which is the
> > recommended (new) way.
> 
> Yes, tty_get_baud_rate() helper function is there for this purpose,
> right?

No.

Johan