mmc: core: make pwrseq_emmc (partially) support sleepy GPIO controllers

pwrseq_emmc.c implements a HW reset procedure for eMMC chip by driving a
GPIO line.

It registers the .reset() cb on mmc_pwrseq_ops and it registers a system
restart notification handler; both of them perform reset by unconditionally
calling gpiod_set_value().

If the eMMC reset line is tied to a GPIO controller whose driver can sleep
(i.e. I2C GPIO controller), then the kernel would spit warnings when trying
to reset the eMMC chip by means of .reset() mmc_pwrseq_ops cb (that is
exactly what I'm seeing during boot).

Furthermore, on system reset we would gets to the system restart
notification handler with disabled interrupts - local_irq_disable() is
called in machine_restart() at least on ARM/ARM64 - and we would be in
trouble when the GPIO driver tries to sleep (which indeed doesn't happen
here, likely because in my case the machine specific code doesn't call
do_kernel_restart(), I guess..).

This patch fixes the .reset() cb to make use of gpiod_set_value_cansleep(),
so that the eMMC gets reset on boot without complaints, while, since there
isn't that much we can do, we avoid register the restart handler if the
GPIO controller has a sleepy driver (and we spit a dev_notice() message to
let people know)..

This had been tested on a downstream 4.9 kernel with backported
commit 83f37ee7ba33 ("mmc: pwrseq: Add reset callback to the struct
mmc_pwrseq_ops") and commit ae60fb031cf2 ("mmc: core: Don't do eMMC HW
reset when resuming the eMMC card"), because I couldn't boot my board
otherwise. Maybe worth to RFT.
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/pwrseq_emmc.c

@@ -30,19 +30,14 @@ struct mmc_pwrseq_emmc {
 
 #define to_pwrseq_emmc(p) container_of(p, struct mmc_pwrseq_emmc, pwrseq)
 
-static void __mmc_pwrseq_emmc_reset(struct mmc_pwrseq_emmc *pwrseq)
-{
-	gpiod_set_value(pwrseq->reset_gpio, 1);
-	udelay(1);
-	gpiod_set_value(pwrseq->reset_gpio, 0);
-	udelay(200);
-}
-
 static void mmc_pwrseq_emmc_reset(struct mmc_host *host)
 {
 	struct mmc_pwrseq_emmc *pwrseq =  to_pwrseq_emmc(host->pwrseq);
 
-	__mmc_pwrseq_emmc_reset(pwrseq);
+	gpiod_set_value_cansleep(pwrseq->reset_gpio, 1);
+	udelay(1);
+	gpiod_set_value_cansleep(pwrseq->reset_gpio, 0);
+	udelay(200);
 }
 
 static int mmc_pwrseq_emmc_reset_nb(struct notifier_block *this,
@@ -50,8 +45,11 @@ static int mmc_pwrseq_emmc_reset_nb(struct notifier_block *this,
 {
 	struct mmc_pwrseq_emmc *pwrseq = container_of(this,
 					struct mmc_pwrseq_emmc, reset_nb);
+	gpiod_set_value(pwrseq->reset_gpio, 1);
+	udelay(1);
+	gpiod_set_value(pwrseq->reset_gpio, 0);
+	udelay(200);
 
-	__mmc_pwrseq_emmc_reset(pwrseq);
 	return NOTIFY_DONE;
 }
 
@@ -72,14 +70,18 @@ static int mmc_pwrseq_emmc_probe(struct platform_device *pdev)
 	if (IS_ERR(pwrseq->reset_gpio))
 		return PTR_ERR(pwrseq->reset_gpio);
 
-	/*
-	 * register reset handler to ensure emmc reset also from
-	 * emergency_reboot(), priority 255 is the highest priority
-	 * so it will be executed before any system reboot handler.
-	 */
-	pwrseq->reset_nb.notifier_call = mmc_pwrseq_emmc_reset_nb;
-	pwrseq->reset_nb.priority = 255;
-	register_restart_handler(&pwrseq->reset_nb);
+	if (!gpiod_cansleep(pwrseq->reset_gpio)) {
+		/*
+		 * register reset handler to ensure emmc reset also from
+		 * emergency_reboot(), priority 255 is the highest priority
+		 * so it will be executed before any system reboot handler.
+		 */
+		pwrseq->reset_nb.notifier_call = mmc_pwrseq_emmc_reset_nb;
+		pwrseq->reset_nb.priority = 255;
+		register_restart_handler(&pwrseq->reset_nb);
+	} else {
+		dev_notice(dev, "EMMC reset pin tied to a sleepy GPIO driver; reset on emergency-reboot disabled\n");
+	}
 
 	pwrseq->pwrseq.ops = &mmc_pwrseq_emmc_ops;
 	pwrseq->pwrseq.dev = dev;