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commit c284cf062191237abfda57dd526cd10afc8abb19
parent 45d9ab8a306bd096faad32cd43a814b40e91870a
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 14 Sep 2018 05:58:12 -1000

Merge tag 'tty-4.19-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty

Pull tty fixes from Greg KH:
 "Here are three small HVC tty driver fixes to resolve a reported
  regression from 4.19-rc1.

  All of these have been in linux-next for a while with no reported
  issues"

* tag 'tty-4.19-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty:
  tty: hvc: hvc_write() fix break condition
  tty: hvc: hvc_poll() fix read loop batching
  tty: hvc: hvc_poll() fix read loop hang

Diffstat:
Mdrivers/tty/hvc/hvc_console.c | 38+++++++++++++++++++++++++++++++++-----
1 file changed, 33 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c @@ -522,6 +522,8 @@ static int hvc_write(struct tty_struct *tty, const unsigned char *buf, int count return -EIO; while (count > 0) { + int ret = 0; + spin_lock_irqsave(&hp->lock, flags); rsize = hp->outbuf_size - hp->n_outbuf; @@ -537,10 +539,13 @@ static int hvc_write(struct tty_struct *tty, const unsigned char *buf, int count } if (hp->n_outbuf > 0) - hvc_push(hp); + ret = hvc_push(hp); spin_unlock_irqrestore(&hp->lock, flags); + if (!ret) + break; + if (count) { if (hp->n_outbuf > 0) hvc_flush(hp); @@ -623,6 +628,15 @@ static int hvc_chars_in_buffer(struct tty_struct *tty) #define MAX_TIMEOUT (2000) static u32 timeout = MIN_TIMEOUT; +/* + * Maximum number of bytes to get from the console driver if hvc_poll is + * called from driver (and can't sleep). Any more than this and we break + * and start polling with khvcd. This value was derived from from an OpenBMC + * console with the OPAL driver that results in about 0.25ms interrupts off + * latency. + */ +#define HVC_ATOMIC_READ_MAX 128 + #define HVC_POLL_READ 0x00000001 #define HVC_POLL_WRITE 0x00000002 @@ -669,8 +683,8 @@ static int __hvc_poll(struct hvc_struct *hp, bool may_sleep) if (!hp->irq_requested) poll_mask |= HVC_POLL_READ; + read_again: /* Read data if any */ - count = tty_buffer_request_room(&hp->port, N_INBUF); /* If flip is full, just reschedule a later read */ @@ -717,9 +731,23 @@ static int __hvc_poll(struct hvc_struct *hp, bool may_sleep) #endif /* CONFIG_MAGIC_SYSRQ */ tty_insert_flip_char(&hp->port, buf[i], 0); } - if (n == count) - poll_mask |= HVC_POLL_READ; - read_total = n; + read_total += n; + + if (may_sleep) { + /* Keep going until the flip is full */ + spin_unlock_irqrestore(&hp->lock, flags); + cond_resched(); + spin_lock_irqsave(&hp->lock, flags); + goto read_again; + } else if (read_total < HVC_ATOMIC_READ_MAX) { + /* Break and defer if it's a large read in atomic */ + goto read_again; + } + + /* + * Latency break, schedule another poll immediately. + */ + poll_mask |= HVC_POLL_READ; out: /* Wakeup write queue if necessary */