[PATCH v4 4/5] PCI: brcmstb: Don't assume 2711 bootloader leaves PERST# asserted

[Jim Quinlan <jim2101024@gmail.com>]

The current PCIe driver assumes PERST# is asserted when probe() is invoked.
The reasons are as follows:

(1) One Broadcom SOC (7278) causes a panic if the PERST# register is
    written during this time window.

(2) If PERST# is deasserted at Linux probe() time, experience and QA
    suspend/resume tests have shown that some endpoint devices fail if the
    PERST# is pulsed (deasserted => asserted => deasserted) quickly in this
    fashion, even though the timing is in accordance with their datasheets.

(3) Keeping things in reset tends to save power, if for some reason the
    PCIe driver is not yet present.

Broadcom STB and CM SOCs bootloaders always have PERST# asserted at probe()
time.  This is not necessarily the case for the 2711/RPi bootloader.  In
addition, there is a failing test case [1] that may be caused by a
deasserted PERST#.  Finally, Raspian version of Linux does assert PERST# at
probe() time.

So, for 2711/RPi SOCs, do what Raspian does and assert PERST#.

[1] https://lore.kernel.org/linux-pci/20230411165919.23955-1-jim2101024@gmail.com/T/#m39ebab8bc2827b2304aeeff470a6c6a58f46f987

Signed-off-by: Jim Quinlan <jim2101024@gmail.com>
---
 drivers/pci/controller/pcie-brcmstb.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/pci/controller/pcie-brcmstb.c b/drivers/pci/controller/pcie-brcmstb.c
index c2cb683447ac..c486f4b979cc 100644
--- a/drivers/pci/controller/pcie-brcmstb.c
+++ b/drivers/pci/controller/pcie-brcmstb.c
@@ -884,6 +884,11 @@ static int brcm_pcie_setup(struct brcm_pcie *pcie)
 
 	/* Reset the bridge */
 	pcie->bridge_sw_init_set(pcie, 1);
+
+	/* Ensure that PERST# is asserted; some bootloaders may deassert it. */
+	if (pcie->type == BCM2711)
+		pcie->perst_set(pcie, 1);
+
 	usleep_range(100, 200);
 
 	/* Take the bridge out of reset */
-- 
2.17.1