Subject: Re: [PATCH 5/6] powerpc/powernv: implement opal_put_chars_nonatomic

From: Benjamin Herrenschmidt <[email-masked]>
Date: Mon, 09 Apr 2018 15:57:55 +1000

On Mon, 2018-04-09 at 15:40 +1000, Nicholas Piggin wrote:
> The RAW console does not need writes to be atomic, so implement a
> _nonatomic variant which does not take a spinlock. This API is used
> in xmon, so the less locking thta's used, the better chance there is
> that a crash can be debugged.

I find the term "nonatomic" confusing... don't we have a problem if we
start hitting OPAL without a lock where we can't trust
opal_console_write_buffer_space anymore ? I think we need to handle
partial writes in that case. Maybe we should return how much was
written and leave the caller to deal with it.

I was hoping (but that isn't the case) that by nonatomic you actually
meant calls that could be done in a non-atomic context, where we can do
msleep instead of mdelay. That would be handy for the console coming
from the hvc thread (the tty one).

Cheers,
Ben.

> 
> Cc: Benjamin Herrenschmidt <[email-masked]>
> Signed-off-by: Nicholas Piggin <[email-masked]>
> ---
>  arch/powerpc/include/asm/opal.h       |  1 +
>  arch/powerpc/platforms/powernv/opal.c | 35 +++++++++++++++++++--------
>  drivers/tty/hvc/hvc_opal.c            |  4 +--
>  3 files changed, 28 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/opal.h b/arch/powerpc/include/asm/opal.h
> index bbff49fab0e5..66954d671831 100644
> --- a/arch/powerpc/include/asm/opal.h
> +++ b/arch/powerpc/include/asm/opal.h
> @@ -303,6 +303,7 @@ extern void opal_configure_cores(void);
>  
>  extern int opal_get_chars(uint32_t vtermno, char *buf, int count);
>  extern int opal_put_chars(uint32_t vtermno, const char *buf, int total_len);
> +extern int opal_put_chars_nonatomic(uint32_t vtermno, const char *buf, int total_len);
>  extern int opal_flush_console(uint32_t vtermno);
>  
>  extern void hvc_opal_init_early(void);
> diff --git a/arch/powerpc/platforms/powernv/opal.c b/arch/powerpc/platforms/powernv/opal.c
> index b05500a70f58..dc77fc57d1e9 100644
> --- a/arch/powerpc/platforms/powernv/opal.c
> +++ b/arch/powerpc/platforms/powernv/opal.c
> @@ -344,9 +344,9 @@ int opal_get_chars(uint32_t vtermno, char *buf, int count)
>  	return 0;
>  }
>  
> -int opal_put_chars(uint32_t vtermno, const char *data, int total_len)
> +static int __opal_put_chars(uint32_t vtermno, const char *data, int total_len, bool atomic)
>  {
> -	unsigned long flags;
> +	unsigned long flags = 0 /* shut up gcc */;
>  	int written;
>  	__be64 olen;
>  	s64 rc;
> @@ -354,11 +354,8 @@ int opal_put_chars(uint32_t vtermno, const char *data, int total_len)
>  	if (!opal.entry)
>  		return -ENODEV;
>  
> -	/* We want put_chars to be atomic to avoid mangling of hvsi
> -	 * packets. To do that, we first test for room and return
> -	 * -EAGAIN if there isn't enough.
> -	 */
> -	spin_lock_irqsave(&opal_write_lock, flags);
> +	if (atomic)
> +		spin_lock_irqsave(&opal_write_lock, flags);
>  	rc = opal_console_write_buffer_space(vtermno, &olen);
>  	if (rc || be64_to_cpu(olen) < total_len) {
>  		/* Closed -> drop characters */
> @@ -391,14 +388,18 @@ int opal_put_chars(uint32_t vtermno, const char *data, int total_len)
>  
>  	written = be64_to_cpu(olen);
>  	if (written < total_len) {
> -		/* Should not happen */
> -		pr_warn("atomic console write returned partial len=%d written=%d\n", total_len, written);
> +		if (atomic) {
> +			/* Should not happen */
> +			pr_warn("atomic console write returned partial "
> +				"len=%d written=%d\n", total_len, written);
> +		}
>  		if (!written)
>  			written = -EAGAIN;
>  	}
>  
>  out:
> -	spin_unlock_irqrestore(&opal_write_lock, flags);
> +	if (atomic)
> +		spin_unlock_irqrestore(&opal_write_lock, flags);
>  
>  	/* In the -EAGAIN case, callers loop, so we have to flush the console
>  	 * here in case they have interrupts off (and we don't want to wait
> @@ -412,6 +413,20 @@ int opal_put_chars(uint32_t vtermno, const char *data, int total_len)
>  	return written;
>  }
>  
> +int opal_put_chars(uint32_t vtermno, const char *data, int total_len)
> +{
> +	/* We want put_chars to be atomic to avoid mangling of hvsi
> +	 * packets. To do that, we first test for room and return
> +	 * -EAGAIN if there isn't enough.
> +	 */
> +	return __opal_put_chars(vtermno, data, total_len, true);
> +}
> +
> +int opal_put_chars_nonatomic(uint32_t vtermno, const char *data, int total_len)
> +{
> +	return __opal_put_chars(vtermno, data, total_len, false);
> +}
> +
>  int opal_flush_console(uint32_t vtermno)
>  {
>  	s64 rc;
> diff --git a/drivers/tty/hvc/hvc_opal.c b/drivers/tty/hvc/hvc_opal.c
> index af122ad7f06d..e151cfacf2a7 100644
> --- a/drivers/tty/hvc/hvc_opal.c
> +++ b/drivers/tty/hvc/hvc_opal.c
> @@ -51,7 +51,7 @@ static u32 hvc_opal_boot_termno;
>  
>  static const struct hv_ops hvc_opal_raw_ops = {
>  	.get_chars = opal_get_chars,
> -	.put_chars = opal_put_chars,
> +	.put_chars = opal_put_chars_nonatomic,
>  	.notifier_add = notifier_add_irq,
>  	.notifier_del = notifier_del_irq,
>  	.notifier_hangup = notifier_hangup_irq,
> @@ -269,7 +269,7 @@ static void udbg_opal_putc(char c)
>  	do {
>  		switch(hvc_opal_boot_priv.proto) {
>  		case HV_PROTOCOL_RAW:
> -			count = opal_put_chars(termno, &c, 1);
> +			count = opal_put_chars_nonatomic(termno, &c, 1);
>  			break;
>  		case HV_PROTOCOL_HVSI:
>  			count = hvc_opal_hvsi_put_chars(termno, &c, 1);