[PATCH v2] PCI: apple: Follow the PCIe specifications when resetting the port

[Marc Zyngier <maz@kernel.org>]

While the Apple PCIe driver works correctly when directly booted
from the firmware, it fails to initialise when the kernel is booted
from a bootloader using PCIe such as u-boot.

That's beacuse we're missing a proper reset of the port (we only
clear the reset, but never assert it).

The PCIe spec requirements are two-fold:

- #PERST must be asserted before setting up the clocks, and
  stay asserted for at least 100us (Tperst-clk).

- Once #PERST is deasserted, the OS must wait for at least 100ms
  "from the end of a Conventional Reset" before we can start talking
  to the devices

Implementing this results in a booting system.

Fixes: 1e33888fbe44 ("PCI: apple: Add initial hardware bring-up")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Pali Rohár <pali@kernel.org>
---
 drivers/pci/controller/pcie-apple.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/pci/controller/pcie-apple.c b/drivers/pci/controller/pcie-apple.c
index 1bf4d75b61be..957960a733c4 100644
--- a/drivers/pci/controller/pcie-apple.c
+++ b/drivers/pci/controller/pcie-apple.c
@@ -539,13 +539,23 @@ static int apple_pcie_setup_port(struct apple_pcie *pcie,
 
 	rmw_set(PORT_APPCLK_EN, port->base + PORT_APPCLK);
 
+	/* Engage #PERST before setting up the clock */
+	gpiod_set_value(reset, 0);
+
 	ret = apple_pcie_setup_refclk(pcie, port);
 	if (ret < 0)
 		return ret;
 
+	/* The minimal Tperst-clk value is 100us (PCIe CMS r2.0, 2.6.2) */
+	usleep_range(100, 200);
+
+	/* Deassert #PERST */
 	rmw_set(PORT_PERST_OFF, port->base + PORT_PERST);
 	gpiod_set_value(reset, 1);
 
+	/* Wait for 100ms after #PERST deassertion (PCIe r2.0, 6.6.1) */
+	msleep(100);
+
 	ret = readl_relaxed_poll_timeout(port->base + PORT_STATUS, stat,
 					 stat & PORT_STATUS_READY, 100, 250000);
 	if (ret < 0) {
-- 
2.30.2