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Sender: Guenter Roeck <groeck7@gmail.com>
Subject: Re: [PATCH v6 1/1] applesmc: Re-work SMC comms
To:     Brad Campbell <brad@fnarfbargle.com>, linux-hwmon@vger.kernel.org
Cc:     Arnd Bergmann <arnd@arndb.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        hns@goldelico.com, Andreas Kemnade <andreas@kemnade.info>,
        Jean Delvare <jdelvare@suse.com>,
        Henrik Rydberg <rydberg@bitmath.org>
References: <20200930105442.3f642f6c@aktux> <20201104142057.62493c12@aktux>
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From:   Guenter Roeck <linux@roeck-us.net>
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Message-ID: <711f71f8-4709-4555-32f6-bab993a8d3f2@roeck-us.net>
Date:   Thu, 12 Nov 2020 09:20:47 -0800
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On 11/11/20 7:08 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.
> 
> Tested on : MacbookAir6,2 MacBookPro11,1 iMac12,2, MacBookAir1,1,
> MacBookAir3,1
> 
> Fixes: fff2d0f701e6 ("hwmon: (applesmc) avoid overlong udelay()")
> Reported-by: Andreas Kemnade <andreas@kemnade.info>
> Tested-by: Andreas Kemnade <andreas@kemnade.info> # MacBookAir6,2
> Acked-by: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Brad Campbell <brad@fnarfbargle.com>
> Signed-off-by: Henrik Rydberg <rydberg@bitmath.org>

Applied.
Guenter

> 
> ---
> Changelog : 
> v1 : Initial attempt
> v2 : Address logic and coding style based on comments received
> v3 : Removed some debug hangover. Added tested-by. Modifications for MacBookAir1,1
> - Significant rework of wait logic by Henrik Rydberg <rydberg@bitmath.org> to
>   make it function at all on the MacBookAir1,1.
> 
> v4 : Re-factored logic based on Apple driver. Simplified wait_status loop
> - Re-factored the logic again, this time based on the Apple driver. This
>   functioned without error on all tested Macs. wait_status() contributed
>   by Henrik Rydberg <rydberg@bitmath.org>
> 
> v5 : Re-wrote status loop. Simplified busy check in send_byte()
> - Re-wrote wait_status() based on feedback from Guenter Roeck <linux@roeck-us.net>
> - Added additional comments to explain multiple tests in send_byte()
> - Addressed repeated indentation issues
> 
> v6 : Reverted simplification of busy check in send_byte()
> - The logic simplification in v5 send_byte() caused a few read failures in
>   stress testing on a fast SMC (3 in 5.6million). Reverting that change passed 
>   5 million reads with 0 errors.
> 
> Index: linux-stable/drivers/hwmon/applesmc.c
> ===================================================================
> --- linux-stable.orig/drivers/hwmon/applesmc.c
> +++ linux-stable/drivers/hwmon/applesmc.c
> @@ -32,6 +32,7 @@
>  #include <linux/hwmon.h>
>  #include <linux/workqueue.h>
>  #include <linux/err.h>
> +#include <linux/bits.h>
>  
>  /* data port used by Apple SMC */
>  #define APPLESMC_DATA_PORT	0x300
> @@ -42,10 +43,13 @@
>  
>  #define APPLESMC_MAX_DATA_LENGTH 32
>  
> -/* wait up to 128 ms for a status change. */
> -#define APPLESMC_MIN_WAIT	0x0010
> -#define APPLESMC_RETRY_WAIT	0x0100
> -#define APPLESMC_MAX_WAIT	0x20000
> +/* Apple SMC status bits */
> +#define SMC_STATUS_AWAITING_DATA  BIT(0) /* SMC has data waiting to be read */
> +#define SMC_STATUS_IB_CLOSED      BIT(1) /* Will ignore any input */
> +#define SMC_STATUS_BUSY           BIT(2) /* Command in progress */
> +
> +/* Initial wait is 8us */
> +#define APPLESMC_MIN_WAIT      0x0008
>  
>  #define APPLESMC_READ_CMD	0x10
>  #define APPLESMC_WRITE_CMD	0x11
> @@ -151,65 +155,84 @@ 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 all transactions.
> + * This does 10 fast loops of 8us then exponentially backs off for a
> + * minimum total wait of 262ms. Depending on usleep_range this could
> + * run out past 500ms.
>   */
> -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;
> +	int i;
>  
> -	for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
> -		usleep_range(us, us * 16);
> +	us = APPLESMC_MIN_WAIT;
> +	for (i = 0; i < 24 ; i++) {
>  		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;
> +		usleep_range(us, us * 2);
> +		if (i > 9)
> +			us <<= 1;
>  	}
> -
> -	pr_warn("wait_read() fail: 0x%02x\n", status);
>  	return -EIO;
>  }
>  
> -/*
> - * send_byte - Write to SMC port, retrying when necessary. Callers
> - * must hold applesmc_lock.
> - */
> +/* send_byte - Write to SMC data port. Callers must hold applesmc_lock. */
> +
>  static int send_byte(u8 cmd, u16 port)
>  {
> -	u8 status;
> -	int us;
> -	unsigned long end = jiffies + (APPLESMC_MAX_WAIT * HZ) / USEC_PER_SEC;
> +	int status;
>  
> -	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);
> -	}
> +	status = wait_status(0, SMC_STATUS_IB_CLOSED);
> +	if (status)
> +		return status;
> +	/*
> +	 * This needs to be a separate read looking for bit 0x04
> +	 * after bit 0x02 falls. If consolidated with the wait above
> +	 * this extra read may not happen if status returns both
> +	 * simultaneously and this would appear to be required.
> +	 */
> +	status = wait_status(SMC_STATUS_BUSY, SMC_STATUS_BUSY);
> +	if (status)
> +		return status;
>  
> -	pr_warn("send_byte(0x%02x, 0x%04x) fail: 0x%02x\n", cmd, port, status);
> -	return -EIO;
> +	outb(cmd, port);
> +	return 0;
>  }
>  
> +/* send_command - Write a command to the SMC. Callers must hold applesmc_lock. */
> +
>  static int send_command(u8 cmd)
>  {
> -	return send_byte(cmd, APPLESMC_CMD_PORT);
> +	int ret;
> +
> +	ret = wait_status(0, SMC_STATUS_IB_CLOSED);
> +	if (ret)
> +		return ret;
> +	outb(cmd, APPLESMC_CMD_PORT);
> +	return 0;
> +}
> +
> +/*
> + * Based on logic from the Apple driver. This is issued before any interaction
> + * If busy is stuck high, issue a read command to reset the SMC state machine.
> + * If busy is stuck high after the command then the SMC is jammed.
> + */
> +
> +static int smc_sane(void)
> +{
> +	int ret;
> +
> +	ret = wait_status(0, SMC_STATUS_BUSY);
> +	if (!ret)
> +		return ret;
> +	ret = send_command(APPLESMC_READ_CMD);
> +	if (ret)
> +		return ret;
> +	return wait_status(0, SMC_STATUS_BUSY);
>  }
>  
>  static int send_argument(const char *key)
> @@ -226,6 +249,11 @@ static int read_smc(u8 cmd, const char *
>  {
>  	u8 status, data = 0;
>  	int i;
> +	int ret;
> +
> +	ret = smc_sane();
> +	if (ret)
> +		return ret;
>  
>  	if (send_command(cmd) || send_argument(key)) {
>  		pr_warn("%.4s: read arg fail\n", key);
> @@ -239,7 +267,8 @@ static int read_smc(u8 cmd, const char *
>  	}
>  
>  	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)) {
>  			pr_warn("%.4s: read data[%d] fail\n", key, i);
>  			return -EIO;
>  		}
> @@ -250,19 +279,24 @@ static int read_smc(u8 cmd, const char *
>  	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);
>  	}
>  	if (i)
>  		pr_warn("flushed %d bytes, last value is: %d\n", i, data);
>  
> -	return 0;
> +	return wait_status(0, SMC_STATUS_BUSY);
>  }
>  
>  static int write_smc(u8 cmd, const char *key, const u8 *buffer, u8 len)
>  {
>  	int i;
> +	int ret;
> +
> +	ret = smc_sane();
> +	if (ret)
> +		return ret;
>  
>  	if (send_command(cmd) || send_argument(key)) {
>  		pr_warn("%s: write arg fail\n", key);
> @@ -281,7 +315,7 @@ static int write_smc(u8 cmd, const char
>  		}
>  	}
>  
> -	return 0;
> +	return wait_status(0, SMC_STATUS_BUSY);
>  }
>  
>  static int read_register_count(unsigned int *count)
>