Hi!

> [ Adding Pavel who disabled EFR at one point to CC. ]
> 
> On Mon, May 25, 2020 at 07:49:54PM +0200, Denis Ahrens wrote:
> > 
> > 
> > > On 25. May 2020, at 10:27, Johan Hovold <johan@kernel.org> wrote:
> > > 
> > > On Sun, May 24, 2020 at 06:31:44PM +0200, Denis Ahrens wrote:
> > >> From: Denis Ahrens <denis@h3q.com>
> > >> 
> > >> Enable Automatic RTS/CTS flow control for the 16C950 UART in Enhanced Mode
> > >> like described in the Data Sheet Revision 1.2 page 28 and 29.
> > >> 
> > >> Without this change normal console output works, but everything putting
> > >> a little more pressure on the UART simply overruns the FIFO.
> > >> 
> > >> Signed-off-by: Denis Ahrens <denis@h3q.com>
> > >> ---
> > >> 
> > >> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> > >> index f77bf820b7a3..024235946f4d 100644
> > >> --- a/drivers/tty/serial/8250/8250_port.c
> > >> +++ b/drivers/tty/serial/8250/8250_port.c
> > >> @@ -2168,7 +2168,9 @@ int serial8250_do_startup(struct uart_port *port)
> > >>                serial_port_out(port, UART_LCR, 0);
> > >>                serial_icr_write(up, UART_CSR, 0); /* Reset the UART */
> > >>                serial_port_out(port, UART_LCR, UART_LCR_CONF_MODE_B);
> > >> -               serial_port_out(port, UART_EFR, UART_EFR_ECB);
> > >> +               serial_port_out(port, UART_EFR, UART_EFR_ECB |
> > >> +                                               UART_EFR_RTS |
> > >> +                                               UART_EFR_CTS);
> > >>                serial_port_out(port, UART_LCR, 0);
> > >>        }
> > > 
> > > This doesn't look right as you're now enabling automatic flow control
> > > for everyone.
> > > 
> > > Try adding this to set_termios() instead when enabling flow control.
> > 
> > The part in set_termios() is never reached because the UART_CAP_EFR
> > capability was removed ca. 10 years ago. The code fails to preserve
> > the UART_EFR_ECB bit which is in the same register as UART_EFR_CTS.
> > Also for some reason UART_EFR_RTS is not set.
> 
> The EFR capability was added by commit 7a56aa45982b ("serial: add
> UART_CAP_EFR and UART_CAP_SLEEP flags to 16C950 UARTs definition") and
> then removed half a year later by commit 0694e2aeb81 ("serial: unbreak
> billionton CF card") -- you should mention that in the commit message
> too.

Hmm, that's compact flash card, and long time ago. I probably still
have it somewhere, but I have not used it in a long long time...

Best regards,

									Pavel


-- 
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