Re: [PATCH v3] applesmc: Re-work SMC comms

[Henrik Rydberg <rydberg@bitmath.org>]

Hi Brad,

On 2020-11-08 02:00, Brad Campbell wrote:
> G'day Henrik,
> 
> I noticed you'd also loosened up the requirement for SMC_STATUS_BUSY in read_smc(). I assume
> that causes problems on the early Macbook. This is revised on the one sent earlier.
> If you could test this on your Air1,1 it'd be appreciated.

No, I managed to screw up the patch; you can see that I carefully added 
the same treatment for the read argument, being unsure if the BUSY state 
would remain during the AVAILABLE data phase. I can check that again, 
but unfortunately the patch in this email shows the same problem.

I think it may be worthwhile to rethink the behavior of wait_status() 
here. If one machine shows no change after a certain status bit change, 
then perhaps the others share that behavior, and we are waiting in vain. 
Just imagine how many years of cpu that is, combined. ;-)

Henrik

> 
> 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
> 
> 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>
> 
> ---
> Changelog :
> v1 : Inital attempt
> v2 : Address logic and coding style
> v3 : Removed some debug hangover. Added tested-by. Modifications for MacBookAir1,1
> 
> diff --git a/drivers/hwmon/applesmc.c b/drivers/hwmon/applesmc.c
> index a18887990f4a..3e968abb37aa 100644
> --- a/drivers/hwmon/applesmc.c
> +++ b/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
> @@ -42,6 +43,11 @@
>   
>   #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 */
> +
>   /* wait up to 128 ms for a status change. */
>   #define APPLESMC_MIN_WAIT	0x0010
>   #define APPLESMC_RETRY_WAIT	0x0100
> @@ -151,65 +157,73 @@ 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;
>   
> +	udelay(APPLESMC_MIN_WAIT);
>   	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;
> +		usleep_range(us, us * 16);
>   	}
> -
> -	pr_warn("wait_read() fail: 0x%02x\n", status);
>   	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);
> +	int ret;
> +	int i;
> +
> +	for (i = 0; i < 16; i++) {
> +		ret = wait_status(0, SMC_STATUS_IB_CLOSED);
> +		if (ret)
> +			return ret;
> +
> +		outb(cmd, APPLESMC_CMD_PORT);
> +		ret = wait_status(SMC_STATUS_BUSY, SMC_STATUS_BUSY);
> +		if (!ret)
> +			return ret;
> +	}
> +	return -EIO;
>   }
>   
>   static int send_argument(const char *key)
> @@ -239,7 +253,8 @@ 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_AWAITING_DATA | SMC_STATUS_IB_CLOSED)) {
>   			pr_warn("%.4s: read data[%d] fail\n", key, i);
>   			return -EIO;
>   		}
> @@ -250,7 +265,7 @@ 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 +290,7 @@ 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;
>   		}
>