linux-kernel - Re: [RFC PATCH] fpga: bridge: return errors in the show() method of the "state" attribute

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Message-ID: <Y8/0WRFLTh5p/BfN@yilunxu-OptiPlex-7050>
Date:   Tue, 24 Jan 2023 23:08:09 +0800
From:   Xu Yilun <yilun.xu@...el.com>
To:     Marco Pagani <marpagan@...hat.com>
Cc:     Moritz Fischer <mdf@...nel.org>, Wu Hao <hao.wu@...el.com>,
        Tom Rix <trix@...hat.com>, linux-fpga@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fpga: bridge: return errors in the show() method of
 the "state" attribute

On 2023-01-24 at 12:30:50 +0100, Marco Pagani wrote:
> This patch changes the show() method of the "state" sysfs attribute to
> return an error if the bridge's enable_show() op returns an error code.
> In this way, the userspace can distinguish between when the bridge is
> actually "enabled" (i.e., allowing signals to pass) or "disabled"
> (i.e., gating signals), or when there is an error (e.g., the state of
> the bridge cannot be determined).

Maybe we remove the example for "error" here. This patch is just to
propagate the errors from low level drivers.

> 
> Currently, enable_show() returns an integer representing the bridge's
> state (enabled or disabled) or an error code. However, this integer
> value is interpreted in state_show() as a bool, resulting in the method
> printing "enabled" (i.e., the bridge allows signals to pass), without
> propagating the error, even when enable_show() returns an error code.
> 
> Another possibility could be to change the signature of enable_show()
> to return a bool for symmetry with the enable_set() "setter" method.
> However, this would prevent enable_show() from returning error codes

Returning error codes is good to me.

> and may break the HPS bridge driver (altera-hps2fpga.c +53).
> 
> Thanks
> 
> Signed-off-by: Marco Pagani <marpagan@...hat.com>
> ---
>  drivers/fpga/fpga-bridge.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/fpga/fpga-bridge.c b/drivers/fpga/fpga-bridge.c
> index 727704431f61..5cd40acab5bf 100644
> --- a/drivers/fpga/fpga-bridge.c
> +++ b/drivers/fpga/fpga-bridge.c
> @@ -293,12 +293,15 @@ static ssize_t state_show(struct device *dev,
>  			  struct device_attribute *attr, char *buf)
>  {
>  	struct fpga_bridge *bridge = to_fpga_bridge(dev);
> -	int enable = 1;
> +	int state = 1;
>  
> -	if (bridge->br_ops && bridge->br_ops->enable_show)
> -		enable = bridge->br_ops->enable_show(bridge);
> +	if (bridge->br_ops && bridge->br_ops->enable_show) {
> +		state = bridge->br_ops->enable_show(bridge);
> +		if (state < 0)
> +			return state;
> +	}
>  
> -	return sprintf(buf, "%s\n", enable ? "enabled" : "disabled");
> +	return sysfs_emit(buf, "%s\n", state ? "enabled" : "disabled");

The code looks good to me. You may remove RFC prefix in next version.

Thanks,
Yilun

>  }
>  
>  static DEVICE_ATTR_RO(name);
> -- 
> 2.39.1
>