[PATCH net] net: phy: sfp: fix the BR,min computation

[PATCH net] net: phy: sfp: fix the BR,min computation

[Antoine Tenart]

In an SFP EEPROM values can be read to get information about a given SFP
module. One of those is the bitrate, which can be determined using a
nominal bitrate in addition with min and max values (in %). The SFP code
currently compute both BR,min and BR,max values thanks to this nominal
and min,max values.

This patch fixes the BR,min computation as the min value should be
subtracted to the nominal one, not added.

Fixes: 9962acf7fb8c ("sfp: add support for 1000Base-PX and 1000Base-BX10")
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
---
 drivers/net/phy/sfp-bus.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/phy/sfp-bus.c b/drivers/net/phy/sfp-bus.c
index 0381da78d228..fd6c23f69c2f 100644
--- a/drivers/net/phy/sfp-bus.c
+++ b/drivers/net/phy/sfp-bus.c
@@ -125,7 +125,7 @@ void sfp_parse_support(struct sfp_bus *bus, const struct sfp_eeprom_id *id,
 	if (id->base.br_nominal) {
 		if (id->base.br_nominal != 255) {
 			br_nom = id->base.br_nominal * 100;
-			br_min = br_nom + id->base.br_nominal * id->ext.br_min;
+			br_min = br_nom - id->base.br_nominal * id->ext.br_min;
 			br_max = br_nom + id->base.br_nominal * id->ext.br_max;
 		} else if (id->ext.br_max) {
 			br_nom = 250 * id->ext.br_max;
-- 
2.17.0

Re: [PATCH net] net: phy: sfp: fix the BR,min computation

[David Miller]

From: Antoine Tenart <antoine.tenart@bootlin.com>
Date: Fri,  4 May 2018 17:10:54 +0200

> In an SFP EEPROM values can be read to get information about a given SFP
> module. One of those is the bitrate, which can be determined using a
> nominal bitrate in addition with min and max values (in %). The SFP code
> currently compute both BR,min and BR,max values thanks to this nominal
> and min,max values.
> 
> This patch fixes the BR,min computation as the min value should be
> subtracted to the nominal one, not added.
> 
> Fixes: 9962acf7fb8c ("sfp: add support for 1000Base-PX and 1000Base-BX10")
> Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>

Applied and queued up for -stable, thanks.

Re: [PATCH net] net: phy: sfp: fix the BR,min computation

[Russell King - ARM Linux]

On Fri, May 04, 2018 at 05:10:54PM +0200, Antoine Tenart wrote:
> In an SFP EEPROM values can be read to get information about a given SFP
> module. One of those is the bitrate, which can be determined using a
> nominal bitrate in addition with min and max values (in %). The SFP code
> currently compute both BR,min and BR,max values thanks to this nominal
> and min,max values.
> 
> This patch fixes the BR,min computation as the min value should be
> subtracted to the nominal one, not added.
> 
> Fixes: 9962acf7fb8c ("sfp: add support for 1000Base-PX and 1000Base-BX10")
> Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>

I know David has already applied it, but for the record, your fix looks
correct, thanks.

> ---
>  drivers/net/phy/sfp-bus.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/phy/sfp-bus.c b/drivers/net/phy/sfp-bus.c
> index 0381da78d228..fd6c23f69c2f 100644
> --- a/drivers/net/phy/sfp-bus.c
> +++ b/drivers/net/phy/sfp-bus.c
> @@ -125,7 +125,7 @@ void sfp_parse_support(struct sfp_bus *bus, const struct sfp_eeprom_id *id,
>  	if (id->base.br_nominal) {
>  		if (id->base.br_nominal != 255) {
>  			br_nom = id->base.br_nominal * 100;
> -			br_min = br_nom + id->base.br_nominal * id->ext.br_min;
> +			br_min = br_nom - id->base.br_nominal * id->ext.br_min;
>  			br_max = br_nom + id->base.br_nominal * id->ext.br_max;
>  		} else if (id->ext.br_max) {
>  			br_nom = 250 * id->ext.br_max;
> -- 
> 2.17.0
> 

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