| Message ID | 20240913140538.221708-2-john.ogness@linutronix.de |
|---|---|
| State | New |
| Headers | show |
| Series | convert 8250 to nbcon | expand |
On Fri 2024-09-13 16:11:35, John Ogness wrote: > Move IER handling out of rs485_start_tx() callback and into a new > wrapper serial8250_rs485_start_tx(). Replace all callback call sites > with wrapper, except for the console write() callback, where it is > inappropriate to modify IER. Sigh, I am trying to review this patch but I am not familiar with the code. Feel free to ignore me when the questions are completely off. > --- a/drivers/tty/serial/8250/8250_port.c > +++ b/drivers/tty/serial/8250/8250_port.c > @@ -1370,7 +1370,6 @@ static void serial8250_stop_rx(struct uart_port *port) > serial8250_rpm_get(up); > > up->ier &= ~(UART_IER_RLSI | UART_IER_RDI); > - up->port.read_status_mask &= ~UART_LSR_DR; > serial_port_out(port, UART_IER, up->ier); > > serial8250_rpm_put(up); > @@ -1543,16 +1542,20 @@ static inline void __start_tx(struct uart_port *port) > * > * Generic callback usable by 8250 uart drivers to start rs485 transmission. > * Assumes that setting the RTS bit in the MCR register means RTS is high. > - * (Some chips use inverse semantics.) Further assumes that reception is > - * stoppable by disabling the UART_IER_RDI interrupt. (Some chips set the > - * UART_LSR_DR bit even when UART_IER_RDI is disabled, foiling this approach.) > + * (Some chips use inverse semantics.) > + * It does not disable RX interrupts. Use the wrapper function > + * serial8250_rs485_start_tx() if that is also needed. > */ > void serial8250_em485_start_tx(struct uart_8250_port *up) > { > unsigned char mcr = serial8250_in_MCR(up); > > + /* > + * Some chips set the UART_LSR_DR bit even when UART_IER_RDI is > + * disabled, so explicitly mask it. > + */ > if (!(up->port.rs485.flags & SER_RS485_RX_DURING_TX)) > - serial8250_stop_rx(&up->port); > + up->port.read_status_mask &= ~UART_LSR_DR; This change is related to disabling UART_IER_RDI but we do not longer disable it in this code path. Why do we need to do it here, please? Why is it needed only in the em485-specific path, please? I tried to understand the code and am in doubts: On one hand, the comment talks about UART_LSR_DR and UART_IER_RDI so seems to be relater. But the "Some chips set..." comment has been added by the commit 058bc104f7ca5c83d81 ("serial: 8250: Generalize rs485 software emulation"). And I do not see any explanation why it was added in this code path even though UART_LSR_DR and UART_IER_RDI were manipulated in serial8250_stop_rx() which can be called also in other code paths via uport->ops->stop_rx(). Also the comment suggests that this fixes a bug in some chips but the line has been added into 1.1.60 back in 2007. --- a/drivers/char/ChangeLog +++ b/drivers/char/ChangeLog @@ -1,3 +1,28 @@ +Sat Oct 29 18:17:34 1994 Theodore Y. Ts'o (tytso@rt-11) + + * serial.c (rs_ioctl, get_lsr_info): Added patch suggested by Arne + Riiber so that user mode programs can tell when the + transmitter shift register is empty. + +Thu Oct 27 23:14:29 1994 Theodore Y. Ts'o (tytso@rt-11) + + * tty_ioctl.c (wait_until_sent): Added debugging printk statements + (under the #ifdef TTY_DEBUG_WAIT_UNTL_SENT) + + * serial.c (rs_interrupt, rs_interrupt_single, receive_chars, + change_speed, rs_close): rs_close now disables receiver + interrupts when closing the serial port. This allows the + serial port to close quickly when Linux and a modem (or a + mouse) are engaged in an echo war; when closing the serial + port, we now first stop listening to incoming characters, + and *then* wait for the transmit buffer to drain. + + In order to make this change, the info->read_status_mask + is now used to control what bits of the line status + register are looked at in the interrupt routine in all + cases; previously it was only used in receive_chars to + select a few of the status bits. + --- a/drivers/char/serial.c +++ b/drivers/char/serial.c [...] @@ -1780,6 +1830,15 @@ static void rs_close(struct tty_struct *tty, struct file * filp) info->normal_termios = *tty->termios; if (info->flags & ASYNC_CALLOUT_ACTIVE) info->callout_termios = *tty->termios; + /* + * At this point we stop accepting input. To do this, we + * disable the receive line status interrupts, and tell the + * interrut driver to stop checking the data ready bit in the + * line status register. + */ + info->IER &= ~UART_IER_RLSI; + serial_out(info, UART_IER, info->IER); + info->read_status_mask &= ~UART_LSR_DR; if (info->flags & ASYNC_INITIALIZED) { wait_until_sent(tty, 3000); /* 30 seconds timeout */ /* => It looks like it was not a fix for a "buggy chips". It looks like it was part of the design. > > if (up->port.rs485.flags & SER_RS485_RTS_ON_SEND) > mcr |= UART_MCR_RTS; > @@ -1562,6 +1565,18 @@ void serial8250_em485_start_tx(struct uart_8250_port *up) > } > EXPORT_SYMBOL_GPL(serial8250_em485_start_tx); > > +/** > + * serial8250_rs485_start_tx() - stop rs485 reception, enable transmission > + * @up: uart 8250 port > + */ > +void serial8250_rs485_start_tx(struct uart_8250_port *up) > +{ > + if (!(up->port.rs485.flags & SER_RS485_RX_DURING_TX)) > + serial8250_stop_rx(&up->port); > + > + up->rs485_start_tx(up); > +} > + > /* Returns false, if start_tx_timer was setup to defer TX start */ > static bool start_tx_rs485(struct uart_port *port) > { > @@ -1585,7 +1600,7 @@ static bool start_tx_rs485(struct uart_port *port) > if (em485->tx_stopped) { > em485->tx_stopped = false; > > - up->rs485_start_tx(up); > + serial8250_rs485_start_tx(up); If I get this correctly then this keeps the existing behavior when up->rs485_start_tx == serial8250_em485_start_tx Is this always the case, please? The callback has been added by the commit 058bc104f7ca5c83d81 ("serial: 8250: Generalize rs485 software emulation") because 8250_bcm2835aux.c driver needed to do something else. Can start_tx_rs485() be called for the 8250_bcm2835aux.c driver? Will it still work as expected? > > if (up->port.rs485.delay_rts_before_send > 0) { > em485->active_timer = &em485->start_tx_timer; Best Regards, Petr
On 2024-09-17, Petr Mladek <pmladek@suse.com> wrote: > Sigh, I am trying to review this patch but I am not familiar with the > code. Feel free to ignore me when the questions are completely off. I appreciate you researching where the code came from. I made my changes based on what I see the code doing now. >> --- a/drivers/tty/serial/8250/8250_port.c >> +++ b/drivers/tty/serial/8250/8250_port.c >> void serial8250_em485_start_tx(struct uart_8250_port *up) >> { >> unsigned char mcr = serial8250_in_MCR(up); >> >> + /* >> + * Some chips set the UART_LSR_DR bit even when UART_IER_RDI is >> + * disabled, so explicitly mask it. >> + */ >> if (!(up->port.rs485.flags & SER_RS485_RX_DURING_TX)) >> - serial8250_stop_rx(&up->port); >> + up->port.read_status_mask &= ~UART_LSR_DR; > > This change is related to disabling UART_IER_RDI but we do not longer > disable it in this code path. Correct. It will be disabled in the new wrapper serial8250_em485_start_tx(). For the console write() callback, RDI is already being disabled (IER is cleared). It will not use the wrapper. > Why do we need to do it here, please? Because the console write() callback also needs to clear LSR_DR. That part of the callback needs to stay. > Why is it needed only in the em485-specific path, please? Only RS485 deals with controlling TX/RX directions. > On one hand, the comment talks about UART_LSR_DR and UART_IER_RDI > so seems to be relater. I do not know if the LSR_DR modify is strictly necessary. I am just preserving the existing behavior (and related comment). The disabling of IER_RDI will still happen (via wrapper or explicitly as in the console write() callback). >> static bool start_tx_rs485(struct uart_port *port) >> { >> @@ -1585,7 +1600,7 @@ static bool start_tx_rs485(struct uart_port *port) >> if (em485->tx_stopped) { >> em485->tx_stopped = false; >> >> - up->rs485_start_tx(up); >> + serial8250_rs485_start_tx(up); > > If I get this correctly then this keeps the existing behavior when > > up->rs485_start_tx == serial8250_em485_start_tx Correct. > Is this always the case, please? Yes. > Can start_tx_rs485() be called for the 8250_bcm2835aux.c driver? Yes. > Will it still work as expected? Yes, but it does perform an extra read. And since someone added a comment just to mention that, I assume it was important for some use case. John
On Wed 2024-09-18 17:10:53, John Ogness wrote: > On 2024-09-17, Petr Mladek <pmladek@suse.com> wrote: > > Sigh, I am trying to review this patch but I am not familiar with the > > code. Feel free to ignore me when the questions are completely off. > > I appreciate you researching where the code came from. I made my changes > based on what I see the code doing now. > > >> --- a/drivers/tty/serial/8250/8250_port.c > >> +++ b/drivers/tty/serial/8250/8250_port.c > >> void serial8250_em485_start_tx(struct uart_8250_port *up) > >> { > >> unsigned char mcr = serial8250_in_MCR(up); > >> > >> + /* > >> + * Some chips set the UART_LSR_DR bit even when UART_IER_RDI is > >> + * disabled, so explicitly mask it. > >> + */ > >> if (!(up->port.rs485.flags & SER_RS485_RX_DURING_TX)) > >> - serial8250_stop_rx(&up->port); > >> + up->port.read_status_mask &= ~UART_LSR_DR; > > > > This change is related to disabling UART_IER_RDI but we do not longer > > disable it in this code path. > > Correct. It will be disabled in the new wrapper > serial8250_em485_start_tx(). For the console write() callback, RDI is > already being disabled (IER is cleared). It will not use the wrapper. > > > Why do we need to do it here, please? > > Because the console write() callback also needs to clear LSR_DR. That > part of the callback needs to stay. > > > Why is it needed only in the em485-specific path, please? > > Only RS485 deals with controlling TX/RX directions. > > > On one hand, the comment talks about UART_LSR_DR and UART_IER_RDI > > so seems to be relater. > > I do not know if the LSR_DR modify is strictly necessary. I am just > preserving the existing behavior (and related comment). The disabling of > IER_RDI will still happen (via wrapper or explicitly as in the console > write() callback). > > >> static bool start_tx_rs485(struct uart_port *port) > >> { > >> @@ -1585,7 +1600,7 @@ static bool start_tx_rs485(struct uart_port *port) > >> if (em485->tx_stopped) { > >> em485->tx_stopped = false; > >> > >> - up->rs485_start_tx(up); > >> + serial8250_rs485_start_tx(up); > > > > If I get this correctly then this keeps the existing behavior when > > > > up->rs485_start_tx == serial8250_em485_start_tx > > Correct. > > > Is this always the case, please? > > Yes. > > > Can start_tx_rs485() be called for the 8250_bcm2835aux.c driver? > > Yes. IMHO, the answer "Yes" to both last questions can't be valid. The 8250_bcm2835aux driver does: static int bcm2835aux_serial_probe(struct platform_device *pdev) { [...] up.rs485_start_tx = bcm2835aux_rs485_start_tx; [...] } As a result, the 1st "Yes" was not correct: up->rs485_start_tx != serial8250_em485_start_tx and this patch would change the behavior for the 8250_bcm2835aux driver. Before, start_tx_rs485() called directly: up->rs485_start_tx(up); Newly, it would call: void serial8250_rs485_start_tx(struct uart_8250_port *up) { if (!(up->port.rs485.flags & SER_RS485_RX_DURING_TX)) serial8250_stop_rx(&up->port); up->rs485_start_tx(up); } It means that it could call serial8250_stop_rx() even when it was not called by the original code. And SER_RS485_RX_DURING_TX seems to be checked even in drivers/tty/serial/8250/8250_bcm2835aux.c. So, it looks like it might be (un)set even for this driver. Or is this code path prevented in start_tx_rs485()? I mean that em485->tx_stopped could never be true for the 8250_bcm2835aux driver? But I see static int bcm2835aux_serial_probe(struct platform_device *pdev) { [...] up.port.rs485_config = serial8250_em485_config; up.port.rs485_supported = serial8250_em485_supported; [...] } => It looks like even bcm2835aux driver could have the em485 thing. But it obviously wanted to something special in up->rs485_start_tx(). It looks to me that the change might either cause regression. Or it would deserve a comment unless the validity is obvious for people familiar with the code. Best Regards, Petr
diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h index e5310c65cf52..6e90223ba1d6 100644 --- a/drivers/tty/serial/8250/8250.h +++ b/drivers/tty/serial/8250/8250.h @@ -236,6 +236,8 @@ void serial8250_em485_stop_tx(struct uart_8250_port *p); void serial8250_em485_destroy(struct uart_8250_port *p); extern struct serial_rs485 serial8250_em485_supported; +void serial8250_rs485_start_tx(struct uart_8250_port *up); + /* MCR <-> TIOCM conversion */ static inline int serial8250_TIOCM_to_MCR(int tiocm) { diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index 2786918aea98..ba8d9cefc451 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -1370,7 +1370,6 @@ static void serial8250_stop_rx(struct uart_port *port) serial8250_rpm_get(up); up->ier &= ~(UART_IER_RLSI | UART_IER_RDI); - up->port.read_status_mask &= ~UART_LSR_DR; serial_port_out(port, UART_IER, up->ier); serial8250_rpm_put(up); @@ -1543,16 +1542,20 @@ static inline void __start_tx(struct uart_port *port) * * Generic callback usable by 8250 uart drivers to start rs485 transmission. * Assumes that setting the RTS bit in the MCR register means RTS is high. - * (Some chips use inverse semantics.) Further assumes that reception is - * stoppable by disabling the UART_IER_RDI interrupt. (Some chips set the - * UART_LSR_DR bit even when UART_IER_RDI is disabled, foiling this approach.) + * (Some chips use inverse semantics.) + * It does not disable RX interrupts. Use the wrapper function + * serial8250_rs485_start_tx() if that is also needed. */ void serial8250_em485_start_tx(struct uart_8250_port *up) { unsigned char mcr = serial8250_in_MCR(up); + /* + * Some chips set the UART_LSR_DR bit even when UART_IER_RDI is + * disabled, so explicitly mask it. + */ if (!(up->port.rs485.flags & SER_RS485_RX_DURING_TX)) - serial8250_stop_rx(&up->port); + up->port.read_status_mask &= ~UART_LSR_DR; if (up->port.rs485.flags & SER_RS485_RTS_ON_SEND) mcr |= UART_MCR_RTS; @@ -1562,6 +1565,18 @@ void serial8250_em485_start_tx(struct uart_8250_port *up) } EXPORT_SYMBOL_GPL(serial8250_em485_start_tx); +/** + * serial8250_rs485_start_tx() - stop rs485 reception, enable transmission + * @up: uart 8250 port + */ +void serial8250_rs485_start_tx(struct uart_8250_port *up) +{ + if (!(up->port.rs485.flags & SER_RS485_RX_DURING_TX)) + serial8250_stop_rx(&up->port); + + up->rs485_start_tx(up); +} + /* Returns false, if start_tx_timer was setup to defer TX start */ static bool start_tx_rs485(struct uart_port *port) { @@ -1585,7 +1600,7 @@ static bool start_tx_rs485(struct uart_port *port) if (em485->tx_stopped) { em485->tx_stopped = false; - up->rs485_start_tx(up); + serial8250_rs485_start_tx(up); if (up->port.rs485.delay_rts_before_send > 0) { em485->active_timer = &em485->start_tx_timer;
Move IER handling out of rs485_start_tx() callback and into a new wrapper serial8250_rs485_start_tx(). Replace all callback call sites with wrapper, except for the console write() callback, where it is inappropriate to modify IER. Signed-off-by: John Ogness <john.ogness@linutronix.de> --- drivers/tty/serial/8250/8250.h | 2 ++ drivers/tty/serial/8250/8250_port.c | 27 +++++++++++++++++++++------ 2 files changed, 23 insertions(+), 6 deletions(-)