Subject: Re: [PATCH 08/15] powerpc/powernv: implement opal_put_chars_atomic

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 01 May 2018 19:48:58 +1000

On Tue, 2018-05-01 at 00:55 +1000, Nicholas Piggin wrote:
> The RAW console does not need writes to be atomic, so relax
> opal_put_chars to be able to do partial writes, and implement an
> _atomic variant which does not take a spinlock. This API is used
> in xmon, so the less locking that is used, the better chance there
> is that a crash can be debugged.

Same comment I already had :-) "atomic" in Linux tends to mean
something else (ie, atomic context), so I'd rather have something
like opal_put_chars_sync() or such...

> 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 | 37 +++++++++++++++++++--------
>  drivers/tty/hvc/hvc_opal.c            | 18 +++++++++----
>  3 files changed, 41 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/opal.h b/arch/powerpc/include/asm/opal.h
> index bbff49fab0e5..5d7072411561 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_atomic(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 55d4b1983110..bcdb90ada938 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 +413,22 @@ 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)
> +{
> +	return __opal_put_chars(vtermno, data, total_len, false);
> +}
> +
> +/*
> + * opal_put_chars_atomic will not perform partial-writes. Data will be
> + * atomically written to the terminal or not at all. This is not strictly
> + * true at the moment because console space can race with OPAL's console
> + * writes.
> + */
> +int opal_put_chars_atomic(uint32_t vtermno, const char *data, int total_len)
> +{
> +	return __opal_put_chars(vtermno, data, total_len, true);
> +}
> +
>  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..0a72f98ee082 100644
> --- a/drivers/tty/hvc/hvc_opal.c
> +++ b/drivers/tty/hvc/hvc_opal.c
> @@ -183,9 +183,15 @@ static int hvc_opal_probe(struct platform_device *dev)
>  			return -ENOMEM;
>  		pv->proto = proto;
>  		hvc_opal_privs[termno] = pv;
> -		if (proto == HV_PROTOCOL_HVSI)
> -			hvsilib_init(&pv->hvsi, opal_get_chars, opal_put_chars,
> +		if (proto == HV_PROTOCOL_HVSI) {
> +			/*
> +			 * We want put_chars to be atomic to avoid mangling of
> +			 * hvsi packets.
> +			 */
> +			hvsilib_init(&pv->hvsi,
> +				     opal_get_chars, opal_put_chars_atomic,
>  				     termno, 0);
> +		}
>  
>  		/* Instanciate now to establish a mapping index==vtermno */
>  		hvc_instantiate(termno, termno, ops);
> @@ -376,8 +382,9 @@ void __init hvc_opal_init_early(void)
>  	else if (of_device_is_compatible(stdout_node,"ibm,opal-console-hvsi")) {
>  		hvc_opal_boot_priv.proto = HV_PROTOCOL_HVSI;
>  		ops = &hvc_opal_hvsi_ops;
> -		hvsilib_init(&hvc_opal_boot_priv.hvsi, opal_get_chars,
> -			     opal_put_chars, index, 1);
> +		hvsilib_init(&hvc_opal_boot_priv.hvsi,
> +			     opal_get_chars, opal_put_chars_atomic,
> +			     index, 1);
>  		/* HVSI, perform the handshake now */
>  		hvsilib_establish(&hvc_opal_boot_priv.hvsi);
>  		pr_devel("hvc_opal: Found HVSI console\n");
> @@ -409,7 +416,8 @@ void __init udbg_init_debug_opal_hvsi(void)
>  	hvc_opal_privs[index] = &hvc_opal_boot_priv;
>  	hvc_opal_boot_termno = index;
>  	udbg_init_opal_common();
> -	hvsilib_init(&hvc_opal_boot_priv.hvsi, opal_get_chars, opal_put_chars,
> +	hvsilib_init(&hvc_opal_boot_priv.hvsi,
> +		     opal_get_chars, opal_put_chars_atomic,
>  		     index, 1);
>  	hvsilib_establish(&hvc_opal_boot_priv.hvsi);
>  }