Re: [PATCH] applesmc: Re-work SMC comms v2

From: Guenter Roeck <linux@roeck-us.net>
To: Brad Campbell <brad@fnarfbargle.com>,
	Andreas Kemnade <andreas@kemnade.info>,
	Jean Delvare <jdelvare@suse.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
	rydberg@bitmath.org, linux-hwmon@vger.kernel.org,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	hns@goldelico.com
Subject: Re: [PATCH] applesmc: Re-work SMC comms v2
Date: Thu, 5 Nov 2020 08:12:28 -0800	[thread overview]
Message-ID: <e59440cb-72bc-4e48-4ff0-54ceb8e3ae91@roeck-us.net> (raw)
In-Reply-To: <3c72ccc3-4de1-b5d0-423d-7b8c80991254@fnarfbargle.com>

On 11/4/20 11:26 PM, Brad Campbell wrote:
> Commit fff2d0f701e6 ("hwmon: (applesmc) avoid overlong udelay()") introduced
> an issue whereby communication with the SMC became unreliable with write
> errors like :
> 
> [  120.378614] applesmc: send_byte(0x00, 0x0300) fail: 0x40
> [  120.378621] applesmc: LKSB: write data fail
> [  120.512782] applesmc: send_byte(0x00, 0x0300) fail: 0x40
> [  120.512787] applesmc: LKSB: write data fail
> 
> The original code appeared to be timing sensitive and was not reliable with
> the timing changes in the aforementioned commit.
> 
> This patch re-factors the SMC communication to remove the timing 
> dependencies and restore function with the changes previously committed.
> 

Still a few formatting problems, but I like this version. Id take
care of the formatting myself, but send_command() will need a change.

Subject should be
	[PATCH v<version>] hwmon: (applesmc) ...

> v2 : Address logic and coding style

Change log should be after '---'

> 
> Reported-by: Andreas Kemnade <andreas@kemnade.info>
> Fixes: fff2d0f701e6 ("hwmon: (applesmc) avoid overlong udelay()")
> Signed-off-by: Brad Campbell <brad@fnarfbargle.com>
> 
> ---
> diff --git a/drivers/hwmon/applesmc.c b/drivers/hwmon/applesmc.c
> index a18887990f4a..de890f3ec12f 100644
> --- a/drivers/hwmon/applesmc.c
> +++ b/drivers/hwmon/applesmc.c
> @@ -42,6 +42,11 @@
>  
>  #define APPLESMC_MAX_DATA_LENGTH 32
>  
> +/* Apple SMC status bits */
> +#define SMC_STATUS_AWAITING_DATA  BIT(0) /* SMC has data waiting */
> +#define SMC_STATUS_IB_CLOSED      BIT(1) /* Will ignore any input */
> +#define SMC_STATUS_BUSY           BIT(2) /* Command in progress */
> +

Hah, tricked you here ;-). Using "BIT()" requires

#include <linux/bits.h>

>  /* wait up to 128 ms for a status change. */
>  #define APPLESMC_MIN_WAIT	0x0010
>  #define APPLESMC_RETRY_WAIT	0x0100
> @@ -151,65 +156,69 @@ static unsigned int key_at_index;
>  static struct workqueue_struct *applesmc_led_wq;
>  
>  /*
> - * wait_read - Wait for a byte to appear on SMC port. Callers must
> - * hold applesmc_lock.
> + * Wait for specific status bits with a mask on the SMC
> + * Used before and after writes, and before reads
>   */
> -static int wait_read(void)
> +
> +static int wait_status(u8 val, u8 mask)
>  {
>  	unsigned long end = jiffies + (APPLESMC_MAX_WAIT * HZ) / USEC_PER_SEC;
>  	u8 status;
>  	int us;
>  
>  	for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
> -		usleep_range(us, us * 16);
>  		status = inb(APPLESMC_CMD_PORT);
> -		/* read: wait for smc to settle */
> -		if (status & 0x01)
> +		if ((status & mask) == val)
>  			return 0;
>  		/* timeout: give up */
>  		if (time_after(jiffies, end))
>  			break;
> -	}
> -
> -	pr_warn("wait_read() fail: 0x%02x\n", status);
> +		usleep_range(us, us * 16);
> +		}

Bad indentation of "}"

>  	return -EIO;
>  }
>  
>  /*
> - * send_byte - Write to SMC port, retrying when necessary. Callers
> + * send_byte_data - Write to SMC data port. Callers
>   * must hold applesmc_lock.
> + * Parameter skip must be true on the last write of any
> + * command or it'll time out.
>   */
> -static int send_byte(u8 cmd, u16 port)
> +
> +static int send_byte_data(u8 cmd, u16 port, bool skip)
>  {
> -	u8 status;
> -	int us;
> -	unsigned long end = jiffies + (APPLESMC_MAX_WAIT * HZ) / USEC_PER_SEC;
> +	int ret;
>  
> +	ret = wait_status(SMC_STATUS_BUSY, SMC_STATUS_BUSY | SMC_STATUS_IB_CLOSED);
> +	if (ret)
> +		return ret;
>  	outb(cmd, port);
> -	for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
> -		usleep_range(us, us * 16);
> -		status = inb(APPLESMC_CMD_PORT);
> -		/* write: wait for smc to settle */
> -		if (status & 0x02)
> -			continue;
> -		/* ready: cmd accepted, return */
> -		if (status & 0x04)
> -			return 0;
> -		/* timeout: give up */
> -		if (time_after(jiffies, end))
> -			break;
> -		/* busy: long wait and resend */
> -		udelay(APPLESMC_RETRY_WAIT);
> -		outb(cmd, port);
> -	}
> +	return wait_status(skip ? 0 : SMC_STATUS_BUSY, SMC_STATUS_BUSY);
> +}
>  
> -	pr_warn("send_byte(0x%02x, 0x%04x) fail: 0x%02x\n", cmd, port, status);
> -	return -EIO;
> +static int send_byte(u8 cmd, u16 port)
> +{
> +	return send_byte_data(cmd, port, false);
>  }
>  
> +/*
> + * send_command - Write a command to the SMC. Callers must hold applesmc_lock.
> + * If SMC is in undefined state, any new command write resets the state machine.
> + */
> +
>  static int send_command(u8 cmd)
>  {
> -	return send_byte(cmd, APPLESMC_CMD_PORT);
> +	u8 status;
> +	int ret;
> +
> +	ret = wait_status(0, SMC_STATUS_IB_CLOSED);
> +	if (ret)
> +		return ret;
> +
> +	status = inb(APPLESMC_CMD_PORT);
> +

Is this read necessary ? 'status' isn't used subsequently, meaning we'll
probably get static analyzer warnings about a variable which is assigned
but unused. If the read is necessary, just don't assign it to a variable.

> +	outb(cmd, APPLESMC_CMD_PORT);
> +	return wait_status(SMC_STATUS_BUSY, SMC_STATUS_BUSY);
>  }
>  
>  static int send_argument(const char *key)
> @@ -239,7 +248,9 @@ static int read_smc(u8 cmd, const char *key, u8 *buffer, u8 len)
>  	}
>  
>  	for (i = 0; i < len; i++) {
> -		if (wait_read()) {
> +		if (wait_status(SMC_STATUS_AWAITING_DATA | SMC_STATUS_BUSY,
> +				SMC_STATUS_AWAITING_DATA | SMC_STATUS_BUSY |
> +				SMC_STATUS_IB_CLOSED)) {
>  			pr_warn("%.4s: read data[%d] fail\n", key, i);
>  			return -EIO;
>  		}
> @@ -250,7 +261,7 @@ static int read_smc(u8 cmd, const char *key, u8 *buffer, u8 len)
>  	for (i = 0; i < 16; i++) {
>  		udelay(APPLESMC_MIN_WAIT);
>  		status = inb(APPLESMC_CMD_PORT);
> -		if (!(status & 0x01))
> +		if (!(status & SMC_STATUS_AWAITING_DATA))
>  			break;
>  		data = inb(APPLESMC_DATA_PORT);
>  	}
> @@ -275,7 +286,7 @@ static int write_smc(u8 cmd, const char *key, const u8 *buffer, u8 len)
>  	}
>  
>  	for (i = 0; i < len; i++) {
> -		if (send_byte(buffer[i], APPLESMC_DATA_PORT)) {
> +		if (send_byte_data(buffer[i], APPLESMC_DATA_PORT, i == len - 1)) {
>  			pr_warn("%s: write data fail\n", key);
>  			return -EIO;
>  		}
> 