Hello
i'd like to ask to questions related to serial communication.
1. Is there any reason why DMA is disabled by default in beagle bone black
serial_omap driver (CONFIG_SERIAL_8250_DMA=n) ?
Is it safe to recompile it with CONFIG_SERIAL_8250_DMA=y?
2. I encountered board freeze (most probably deadlock but I'm not sure)
when using pppd and n_gsm to communicate with GSM modem over serial line.
When I connected to the board from internet and issued some command
wich output a lot of data, board got stuck but when output little data
board behaved correctly. After digging into n_gsm driver I discovered
that board gets stuck when 8250 driver calls gsmld_write_wakeu() indicating
upper layer (n_gsm)
that is has room again for new data. n_gsm in return sends more data to
8250 driver directly in the gsmld_write_wakeup() , which is called from
interrupt. I thought this could be the source of the deadlog but I didn't
discover anything wrong. However the problem got treated when I instead of
directly sending data in gsmld_write_wakeup() scheduled tasklet and only
inside it I sent the data.
Any hint what could be wrong with the original code? The behaviour was the
same both in 4.19.94-ti-r37 and 4.14.49-ti-r54.
Tank you for any help
regards
Pavel
Instead of
static void gsmld_write_wakeup(struct tty_struct *tty)
{
struct gsm_mux *gsm = tty->disc_data;
unsigned long flags;
/* Queue poll */
clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
spin_lock_irqsave(&gsm->tx_lock, flags);
gsm_data_kick(gsm);
if (gsm->tx_bytes < TX_THRESH_LO) {
gsm_dlci_data_sweep(gsm);
}
spin_unlock_irqrestore(&gsm->tx_lock, flags);
}
this
static void gsmld_write_wakeup(struct tty_struct *tty)
{
struct gsm_mux *gsm = tty->disc_data;
/* Queue poll */
clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
tasklet_schedule(&gsm->wkup_tasklet);
}
static void wkup_tasklet_handler(unsigned long data)
{
struct gsm_mux *gsm = (struct gsm_mux *)data;
unsigned long flags;
spin_lock_irqsave(&gsm->tx_lock, flags);
gsm_data_kick(gsm);
if (gsm->tx_bytes < TX_THRESH_LO) {
gsm_dlci_data_sweep(gsm);
}
spin_unlock_irqrestore(&gsm->tx_lock, flags);
}
this is call stack trace from omap driver up to n_gsm driver resulting in
freez
[ 361.652243] [<bf309b98>] (gsmld_write_wakeup [n_gsm]) from [<c07f7e5c>]
(tty_wakeup+0x60/0x6c)
[ 361.652255] [<c07f7e5c>] (tty_wakeup) from [<c0802ac4>]
(tty_port_default_wakeup+0x24/0x30)
[ 361.652266] [<c0802ac4>] (tty_port_default_wakeup) from [<c0802614>]
(tty_port_tty_wakeup+0x20/0x24)
[ 361.652277] [<c0802614>] (tty_port_tty_wakeup) from [<c081c074>]
(uart_write_wakeup+0x24/0x2c)
[ 361.652288] [<c081c074>] (uart_write_wakeup) from [<c0820460>]
(serial8250_tx_chars+0x104/0x208)
[ 361.652300] [<c0820460>] (serial8250_tx_chars) from [<c0821280>]
(serial8250_handle_irq.part.11+0xd8/0xdc)
[ 361.652312] [<c0821280>] (serial8250_handle_irq.part.11) from
[<c08212ac>] (serial8250_handle_irq+0x28/0x2c)
[ 361.652323] [<c08212ac>] (serial8250_handle_irq) from [<c082359c>]
(omap8250_irq+0x3c/0x54)
[ 361.652336] [<c082359c>] (omap8250_irq) from [<c01af844>]
(__handle_irq_event_percpu+0x84/0x2d0)
[ 361.652349] [<c01af844>] (__handle_irq_event_percpu) from [<c01afacc>]
(handle_irq_event_percpu+0x3c/0x90)
[ 361.652361] [<c01afacc>] (handle_irq_event_percpu) from [<c01afb68>]
(handle_irq_event+0x48/0x6c)
[ 361.652373] [<c01afb68>] (handle_irq_event) from [<c01b3aa8>]
(handle_level_irq+0xdc/0x158)
[ 361.652384] [<c01b3aa8>] (handle_level_irq) from [<c01ae8d4>]
(generic_handle_irq+0x34/0x44)
[ 361.652396] [<c01ae8d4>] (generic_handle_irq) from [<c01aefd4>]
(__handle_domain_irq+0x8c/0xfc)
[ 361.652408] [<c01aefd4>] (__handle_domain_irq) from [<c01022c0>]
(omap_intc_handle_irq+0x44/0x9c)
[ 361.652419] [<c01022c0>] (omap_intc_handle_irq) from [<c0101a0c>]
(__irq_svc+0x6c/0xa8)
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