Re: [PATCH] applesmc: Re-work SMC comms v1

From: Guenter Roeck
Date: Thu Nov 05 2020 - 01:32:21 EST

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On Thu, Nov 05, 2020 at 04:47:25PM +1100, Brad Campbell wrote:
> Commit fff2d0f701e6 ("hwmon: (applesmc) avoid overlong udelay()") introduced
> an issue whereby communication with the SMC became unreliable with write
> errors :
>
> [ 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.
>
> Reported-by: Andreas Kemnade <andreas@xxxxxxxxxxxx>

Add

Fixes: fff2d0f701e6 ("hwmon: (applesmc) avoid overlong udelay()")

> Signed-off-by: Brad Campbell <brad@xxxxxxxxxxxxxxx>
>
> ---
> diff --git a/drivers/hwmon/applesmc.c b/drivers/hwmon/applesmc.c
> index a18887990f4a..22cc5122ce9a 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 from VirtualSMC */
> +#define SMC_STATUS_AWAITING_DATA 0x01 ///< Data waiting to be read
> +#define SMC_STATUS_IB_CLOSED 0x02 /// A write is pending / will ignore input
> +#define SMC_STATUS_BUSY 0x04 ///< Busy in the middle of a command.
> +

Maybe consider using BIT() while at it.

/* Please use standard comments */

Also, what does the "<" mean ? Is that supposed to be negated
(ie bit set means not busy) ? If so, that isn't a standard notation
that I am aware of. Maybe "not set if busy in the middle of a command"
would be better in this case.

> /* wait up to 128 ms for a status change. */
> #define APPLESMC_MIN_WAIT 0x0010
> #define APPLESMC_RETRY_WAIT 0x0100
> @@ -151,65 +156,77 @@ 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);
> + }
> + pr_warn("wait_status timeout: 0x%02x, 0x%02x, 0x%02x\n", status, val, mask);
> 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)

I would suggest to keep send_byte() and change it to the following.

static int send_byte(u8 cmd, u16 port)
{
return send_byte_data(cmd, port, false);
}

That would limit the number of changes needed later in the code
(it is never called with a hard 'true' as parameter).

> +
> +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;
> + u8 wstat = SMC_STATUS_BUSY;
>
> + if (skip)
> + wstat = 0;

u8 wstat = skip ? 0 : SMC_STATUS_BUSY;

> + if (wait_status(SMC_STATUS_BUSY,
> + SMC_STATUS_BUSY | SMC_STATUS_IB_CLOSED))

This fits one line, and the error code
should really not be overwritten.

ret = wait_status(SMC_STATUS_BUSY, SMC_STATUS_BUSY | SMC_STATUS_IB_CLOSED);
if (ret)
return ret;

> + goto fail;
> 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);
> - }
> -
> - pr_warn("send_byte(0x%02x, 0x%04x) fail: 0x%02x\n", cmd, port, status);
> + if (!wait_status(wstat,
> + SMC_STATUS_BUSY))

That really fits into one line.

> + return 0;
> +fail:
> + pr_warn("send_byte_data(0x%02x, 0x%04x) fail\n", cmd, APPLESMC_CMD_PORT);

Can you drop this message ? wait_status() already displays a message,
after all. Also, please reverse error handling, and don't overwrite
error codes.

ret = wait_status(wstat, SMC_STATUS_BUSY)
if (ret)
return ret;

Actually, this can be simplified to
return wait_status(wstat, SMC_STATUS_BUSY);
or, since wstat is only used once,
return wait_status(skip ? 0 : SMC_STATUS_BUSY, SMC_STATUS_BUSY);

> return -EIO;
> }
>
> +/*
> + * 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;
> +
> + if (wait_status(0,
> + SMC_STATUS_IB_CLOSED)) {

Another one of those odd continuation lines.

> + pr_warn("send_command SMC was busy\n");

and logging noise. As for error handling, same as above, please

ret = wait_status(0, SMC_STATUS_IB_CLOSED);
if (ret)
return ret;

> + goto fail; }
> +
> + status = inb(APPLESMC_CMD_PORT);
> +
> + outb(cmd, APPLESMC_CMD_PORT);
> + if (!wait_status(SMC_STATUS_BUSY,
> + SMC_STATUS_BUSY))

Odd/unnecessary continuation line again.

> + return 0;
> +fail:
> + pr_warn("send_cmd(0x%02x, 0x%04x) fail\n", cmd, APPLESMC_CMD_PORT);

Wow, up to three messages on failure. Please, don't do that.
One message per failure is really enough. Please simplify to

return wait_status(SMC_STATUS_BUSY, SMC_STATUS_BUSY);

Actually, I notice that the callers of send_command()
log yet again. Maybe it is time to drop all the messages
from here and from send_argument() and only log in the
calling code.

> + return -EIO;
> }
>
> static int send_argument(const char *key)
> @@ -217,7 +234,8 @@ static int send_argument(const char *key)
> int i;
>
> for (i = 0; i < 4; i++)
> - if (send_byte(key[i], APPLESMC_DATA_PORT))
> + /* Parameter skip is false as we always send data after an argument */

Please align comments with code. Maybe move the comment ahead
of the for statement. Or drop it entirely - it doesn't add that
much value. Actually, this blob would go away if you keep
send_byte().

> + if (send_byte_data(key[i], APPLESMC_DATA_PORT, false))
> return -EIO;
> return 0;
> }
> @@ -233,13 +251,15 @@ static int read_smc(u8 cmd, const char *key, u8 *buffer, u8 len)
> }
>
> /* This has no effect on newer (2012) SMCs */
> - if (send_byte(len, APPLESMC_DATA_PORT)) {
> + if (send_byte_data(len, APPLESMC_DATA_PORT, false)) {
> pr_warn("%.4s: read len fail\n", key);
> return -EIO;
> }
>
> 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)) {

Align continuatiuon lines with preceding '('. "checkpatch --strict"
reports all those alignment issues.

> pr_warn("%.4s: read data[%d] fail\n", key, i);
> return -EIO;
> }
> @@ -250,7 +270,7 @@ 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);
> }
> @@ -263,20 +283,21 @@ static int read_smc(u8 cmd, const char *key, u8 *buffer, u8 len)
> static int write_smc(u8 cmd, const char *key, const u8 *buffer, u8 len)
> {
> int i;
> + u8 end = len-1;

space before and after '-', please. checkpatch --strict will tell.

>
> if (send_command(cmd) || send_argument(key)) {
> pr_warn("%s: write arg fail\n", key);
> return -EIO;

I notice the driver keeps overwriting error codes. Oh well.
I can't expect you to fix that, and it should not be fixed as part
of this patch, but please don't make it worse (not here, but above
where calls are changed).

> }
>
> - if (send_byte(len, APPLESMC_DATA_PORT)) {
> + if (send_byte_data(len, APPLESMC_DATA_PORT, false)) {
> pr_warn("%.4s: write len fail\n", key);
> return -EIO;
> }
>
> for (i = 0; i < len; i++) {
> - if (send_byte(buffer[i], APPLESMC_DATA_PORT)) {
> - pr_warn("%s: write data fail\n", key);
> + if (send_byte_data(buffer[i], APPLESMC_DATA_PORT, (i == end))) {

Unnecessary ( ) around i == end. Not sure if the 'end' variable
is worth it. Might as well make it "i == len - 1" and let the compiler
optimize it at will.

> + pr_warn("%s: write data fail at %i\n", key, i);
> return -EIO;
> }
> }

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