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pciexp_device: Prevent race condition with retrain link 

The PCIe specification[1] describes a race condition that
can occur when using the Retrain Link bit in the Link
Control Register.

The race condition is avoided by checking the retrain link
bit in the link status register and waiting until it is
set to 0, before initiating a new link retraining.

[1] PCI Express Base Specification Revision 3.0
    Page 633

Change-Id: I9d5840fb9a6e63838b5a4084d3bbe483f1d870ed
Signed-off-by: Youness Alaoui <youness.alaoui@puri.sm>
Reviewed-on: https://review.coreboot.org/19556
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Martin Roth <martinroth@google.com>
This commit is contained in:
Youness Alaoui 2017-05-03 17:57:13 -04:00 committed by Martin Roth
parent 3c0d7d21ef
commit bb5fb64e11
1 changed files with 20 additions and 2 deletions
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22
src/device/pciexp_device.c
View File

@ -50,16 +50,34 @@ unsigned int pciexp_find_extended_cap(device_t dev, unsigned int cap)
#define PCIE_TRAIN_RETRY 10000
static int pciexp_retrain_link(device_t dev, unsigned cap)
{
unsigned try = PCIE_TRAIN_RETRY;
unsigned int try;
u16 lnk;
/*
* Implementation note (page 633) in PCIe Specification 3.0 suggests
* polling the Link Training bit in the Link Status register until the
* value returned is 0 before setting the Retrain Link bit to 1.
* This is meant to avoid a race condition when using the
* Retrain Link mechanism.
*/
for (try = PCIE_TRAIN_RETRY; try > 0; try--) {
lnk = pci_read_config16(dev, cap + PCI_EXP_LNKSTA);
if (!(lnk & PCI_EXP_LNKSTA_LT))
break;
udelay(100);
}
if (try == 0) {
printk(BIOS_ERR, "%s: Link Retrain timeout\n", dev_path(dev));
return -1;
}
/* Start link retraining */
lnk = pci_read_config16(dev, cap + PCI_EXP_LNKCTL);
lnk |= PCI_EXP_LNKCTL_RL;
pci_write_config16(dev, cap + PCI_EXP_LNKCTL, lnk);
/* Wait for training to complete */
while (try--) {
for (try = PCIE_TRAIN_RETRY; try > 0; try--) {
lnk = pci_read_config16(dev, cap + PCI_EXP_LNKSTA);
if (!(lnk & PCI_EXP_LNKSTA_LT))
return 0;