[PATCH v3 2/2] PCI: rcar: Return all Fs from read which triggered an exception

[marek.vasut@gmail.com]

From: Marek Vasut <marek.vasut+renesas@gmail.com>

In case the controller is transitioning to L1 in rcar_pcie_config_access(),
any read/write access to PCIECDR triggers asynchronous external abort. This
is because the transition to L1 link state must be manually finished by the
driver. The PCIe IP can transition back from L1 state to L0 on its own.

The current asynchronous external abort hook implementation restarts
the instruction which finally triggered the fault, which can be a
different instruction than the read/write instruction which started
the faulting access. Usually the instruction which finally triggers
the fault is one which has some data dependency on the result of the
read/write. In case of read, the read value after fixup is undefined,
while a read value of faulting read should be all Fs.

It is possible to enforce the fault using 'isb' instruction placed
right after the read/write instruction which started the faulting
access. Add custom register accessors which perform the read/write
followed immediately by 'isb'.

This way, the fault always happens on the 'isb' and in case of read,
which is located one instruction before the 'isb', it is now possible
to fix up the return value of the read in the asynchronous external
abort hook and make that read return all Fs.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Krzysztof Wilczyński <kw@linux.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Wolfram Sang <wsa@the-dreams.de>
Cc: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Cc: linux-renesas-soc@vger.kernel.org
---
V2: Rebase on 1/2
V3: - Add .text.fixup on all three ldr/str/isb instructions and call
      fixup_exception() in the abort handler to trigger the fixup.
    - Propagate return value from read/write accessors, in case the
      access fails, return PCIBIOS_SET_FAILED, else PCIBIOS_SUCCESSFUL.
---
 drivers/pci/controller/pcie-rcar-host.c | 53 +++++++++++++++++++++++--
 1 file changed, 49 insertions(+), 4 deletions(-)

diff --git a/drivers/pci/controller/pcie-rcar-host.c b/drivers/pci/controller/pcie-rcar-host.c
index 7d38a9c50093..b2e521ee95eb 100644
--- a/drivers/pci/controller/pcie-rcar-host.c
+++ b/drivers/pci/controller/pcie-rcar-host.c
@@ -114,6 +114,51 @@ static u32 rcar_read_conf(struct rcar_pcie *pcie, int where)
 	return val >> shift;
 }
 
+#ifdef CONFIG_ARM
+#define __rcar_pci_rw_reg_workaround(instr)				\
+		"1:	" instr " %1, [%2]\n"				\
+		"2:	isb\n"						\
+		"3:	.pushsection .text.fixup,\"ax\"\n"		\
+		"	.align	2\n"					\
+		"4:	mov	%0, #" __stringify(PCIBIOS_SET_FAILED) "\n" \
+		"	b	3b\n"					\
+		"	.popsection\n"					\
+		"	.pushsection __ex_table,\"a\"\n"		\
+		"	.align	3\n"					\
+		"	.long	1b, 4b\n"				\
+		"	.long	1b, 4b\n"				\
+		"	.popsection\n"
+#endif
+
+int rcar_pci_write_reg_workaround(struct rcar_pcie *pcie, u32 val, unsigned int reg)
+{
+	int error = PCIBIOS_SUCCESSFUL;
+#ifdef CONFIG_ARM
+	asm volatile(
+		__rcar_pci_rw_reg_workaround("str")
+	: "+r"(error):"r"(val), "r"(pcie->base + reg) : "memory");
+#else
+	rcar_pci_write_reg(pcie, val, reg);
+#endif
+	return error;
+}
+
+int rcar_pci_read_reg_workaround(struct rcar_pcie *pcie, u32 *val, unsigned int reg)
+{
+	int error = PCIBIOS_SUCCESSFUL;
+#ifdef CONFIG_ARM
+	asm volatile(
+		__rcar_pci_rw_reg_workaround("ldr")
+	: "+r"(error), "=r"(*val) : "r"(pcie->base + reg) : "memory");
+
+	if (error != PCIBIOS_SUCCESSFUL)
+		*val = 0xffffffff;
+#else
+	*val = rcar_pci_read_reg(pcie, reg);
+#endif
+	return error;
+}
+
 /* Serialization is provided by 'pci_lock' in drivers/pci/access.c */
 static int rcar_pcie_config_access(struct rcar_pcie_host *host,
 		unsigned char access_type, struct pci_bus *bus,
@@ -185,14 +230,14 @@ static int rcar_pcie_config_access(struct rcar_pcie_host *host,
 		return PCIBIOS_DEVICE_NOT_FOUND;
 
 	if (access_type == RCAR_PCI_ACCESS_READ)
-		*data = rcar_pci_read_reg(pcie, PCIECDR);
+		ret = rcar_pci_read_reg_workaround(pcie, data, PCIECDR);
 	else
-		rcar_pci_write_reg(pcie, *data, PCIECDR);
+		ret = rcar_pci_write_reg_workaround(pcie, *data, PCIECDR);
 
 	/* Disable the configuration access */
 	rcar_pci_write_reg(pcie, 0, PCIECCTLR);
 
-	return PCIBIOS_SUCCESSFUL;
+	return ret;
 }
 
 static int rcar_pcie_read_conf(struct pci_bus *bus, unsigned int devfn,
@@ -1097,7 +1142,7 @@ static struct platform_driver rcar_pcie_driver = {
 static int rcar_pcie_aarch32_abort_handler(unsigned long addr,
 		unsigned int fsr, struct pt_regs *regs)
 {
-	return !!rcar_pcie_wakeup(pcie_dev, pcie_base);
+	return !fixup_exception(regs);
 }
 
 static const struct of_device_id rcar_pcie_abort_handler_of_match[] __initconst = {
-- 
2.34.1