On 11/23/2015 01:27 PM, Russell King - ARM Linux wrote:

On Mon, Nov 23, 2015 at 11:28:59AM +0200, Stanimir Varbanov wrote:

quoted

Add 'write memory' barrier after enable region in PCIE_ATU_CR2
register. The barrier is needed to ensure that the region enable
request has been reached it's destination at time when we
read/write to PCI configuration space.

Without this barrier PCI device enumeration during kernel boot
is not reliable, and reading configuration space for particular
PCI device on the bus returns zero aka no device.

Signed-off-by: Stanimir Varbanov <redacted>
---
 drivers/pci/host/pcie-designware.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/pci/host/pcie-designware.c b/drivers/pci/host/pcie-designware.c
index 02a7452bdf23..e15a2ae1583f 100644
--- a/drivers/pci/host/pcie-designware.c
+++ b/drivers/pci/host/pcie-designware.c

@@ -164,6 +164,11 @@ static void dw_pcie_prog_outbound_atu(struct pcie_port *pp, int index,
 	dw_pcie_writel_rc(pp, upper_32_bits(pci_addr), PCIE_ATU_UPPER_TARGET);
 	dw_pcie_writel_rc(pp, type, PCIE_ATU_CR1);
 	dw_pcie_writel_rc(pp, PCIE_ATU_ENABLE, PCIE_ATU_CR2);
+	/*
+	 * ensure that the ATU enable has been happaned before accessing
+	 * pci configuration/io spaces through dw_pcie_cfg_[read|write].
+	 */
+	smp_wmb();

So, why is this a SMP barrier?

I wrongly assumed that smp_wmb will come down to wmb(), I have no excuse.

But despite my ignorant, as I noted in cover letter I want this patch to
be treated as an RFC (maybe I had to add it in the subject).

Do we really need a memory barrier after enabling ATU in PCI_ATU_CR2
register and before first access to pci configuration space using
read[lwb] (see dw_pcie_rd_other_conf)? Or I have made totally wrong
assumption.

-- 
regards,
Stan