sh_eth: Make PHY access aware of Runtime PM to fix reboot crash

Wolfram reports that his R-Car H2-based Lager board can no longer be
rebooted in v5.11-rc1, as it crashes with an imprecise external abort.
The issue can be reproduced on other boards (e.g. Koelsch with R-Car
M2-W) too, if CONFIG_IP_PNP is disabled, and the Ethernet interface is
down at reboot time:

    Unhandled fault: imprecise external abort (0x1406) at 0x00000000
    pgd = (ptrval)
    [00000000] *pgd=422b6835, *pte=00000000, *ppte=00000000
    Internal error: : 1406 [#1] ARM
    Modules linked in:
    CPU: 0 PID: 1105 Comm: init Tainted: G        W         5.10.0-rc1-00402-ge2f016cf #1048
    Hardware name: Generic R-Car Gen2 (Flattened Device Tree)
    PC is at sh_mdio_ctrl+0x44/0x60
    LR is at sh_mmd_ctrl+0x20/0x24
    ...
    Backtrace:
    [<c0451f30>] (sh_mdio_ctrl) from [<c0451fd4>] (sh_mmd_ctrl+0x20/0x24)
     r7:0000001f r6:00000020 r5:00000002 r4:c22a1dc4
    [<c0451fb4>] (sh_mmd_ctrl) from [<c044fc18>] (mdiobb_cmd+0x38/0xa8)
    [<c044fbe0>] (mdiobb_cmd) from [<c044feb8>] (mdiobb_read+0x58/0xdc)
     r9:c229f844 r8:c0c329dc r7:c221e000 r6:00000001 r5:c22a1dc4 r4:00000001
    [<c044fe60>] (mdiobb_read) from [<c044c854>] (__mdiobus_read+0x74/0xe0)
     r7:0000001f r6:00000001 r5:c221e000 r4:c221e000
    [<c044c7e0>] (__mdiobus_read) from [<c044c9d8>] (mdiobus_read+0x40/0x54)
     r7:0000001f r6:00000001 r5:c221e000 r4:c221e458
    [<c044c998>] (mdiobus_read) from [<c044d678>] (phy_read+0x1c/0x20)
     r7:ffffe000 r6:c221e470 r5:00000200 r4:c229f800
    [<c044d65c>] (phy_read) from [<c044d94c>] (kszphy_config_intr+0x44/0x80)
    [<c044d908>] (kszphy_config_intr) from [<c044694c>] (phy_disable_interrupts+0x44/0x50)
     r5:c229f800 r4:c229f800
    [<c0446908>] (phy_disable_interrupts) from [<c0449370>] (phy_shutdown+0x18/0x1c)
     r5:c229f800 r4:c229f804
    [<c0449358>] (phy_shutdown) from [<c040066c>] (device_shutdown+0x168/0x1f8)
    [<c0400504>] (device_shutdown) from [<c013de44>] (kernel_restart_prepare+0x3c/0x48)
     r9:c22d2000 r8:c0100264 r7:c0b0d034 r6:00000000 r5:4321fedc r4:00000000
    [<c013de08>] (kernel_restart_prepare) from [<c013dee0>] (kernel_restart+0x1c/0x60)
    [<c013dec4>] (kernel_restart) from [<c013e1d8>] (__do_sys_reboot+0x168/0x208)
     r5:4321fedc r4:01234567
    [<c013e070>] (__do_sys_reboot) from [<c013e2e8>] (sys_reboot+0x18/0x1c)
     r7:00000058 r6:00000000 r5:00000000 r4:00000000
    [<c013e2d0>] (sys_reboot) from [<c0100060>] (ret_fast_syscall+0x0/0x54)

As of commit e2f016cf ("net: phy: add a shutdown procedure"),
system reboot calls phy_disable_interrupts() during shutdown.  As this
happens unconditionally, the PHY registers may be accessed while the
device is suspended, causing undefined behavior, which may crash the
system.

Fix this by wrapping the PHY bitbang accessors in the sh_eth driver by
wrappers that take care of Runtime PM, to resume the device when needed.
Reported-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/renesas/sh_eth.c

@@ -3034,6 +3034,28 @@ static int sh_mdio_release(struct sh_eth_private *mdp)
 	return 0;
 }
 
+static int sh_mdiobb_read(struct mii_bus *bus, int phy, int reg)
+{
+	int res;
+
+	pm_runtime_get_sync(bus->parent);
+	res = mdiobb_read(bus, phy, reg);
+	pm_runtime_put(bus->parent);
+
+	return res;
+}
+
+static int sh_mdiobb_write(struct mii_bus *bus, int phy, int reg, u16 val)
+{
+	int res;
+
+	pm_runtime_get_sync(bus->parent);
+	res = mdiobb_write(bus, phy, reg, val);
+	pm_runtime_put(bus->parent);
+
+	return res;
+}
+
 /* MDIO bus init function */
 static int sh_mdio_init(struct sh_eth_private *mdp,
 			struct sh_eth_plat_data *pd)
@@ -3058,6 +3080,10 @@ static int sh_mdio_init(struct sh_eth_private *mdp,
 	if (!mdp->mii_bus)
 		return -ENOMEM;
 
+	/* Wrap accessors with Runtime PM-aware ops */
+	mdp->mii_bus->read = sh_mdiobb_read;
+	mdp->mii_bus->write = sh_mdiobb_write;
+
 	/* Hook up MII support for ethtool */
 	mdp->mii_bus->name = "sh_mii";
 	mdp->mii_bus->parent = dev;