pci: Work around PCIe link training failures

Attempt to handle cases with a downstream port of a PCIe switch where
link training never completes and the link continues switching between
speeds indefinitely with the data link layer never reaching the active
state.

It has been observed with a downstream port of the ASMedia ASM2824 Gen 3
switch wired to the upstream port of the Pericom PI7C9X2G304 Gen 2
switch, using a Delock Riser Card PCI Express x1 > 2 x PCIe x1 device,
P/N 41433, wired to a SiFive HiFive Unmatched board.  In this setup the
switches are supposed to negotiate the link speed of preferably 5.0GT/s,
falling back to 2.5GT/s.

However the link continues oscillating between the two speeds, at the
rate of 34-35 times per second, with link training reported repeatedly
active ~84% of the time, e.g.:

02:03.0 PCI bridge [0604]: ASMedia Technology Inc. ASM2824 PCIe Gen3 Packet Switch [1b21:2824] (rev 01) (prog-if 00 [Normal decode])
[...]
	Bus: primary=02, secondary=05, subordinate=05, sec-latency=0
[...]
	Capabilities: [80] Express (v2) Downstream Port (Slot+), MSI 00
[...]
		LnkSta:	Speed 5GT/s (downgraded), Width x1 (ok)
			TrErr- Train+ SlotClk+ DLActive- BWMgmt+ ABWMgmt-
[...]
		LnkCtl2: Target Link Speed: 8GT/s, EnterCompliance- SpeedDis+, Selectable De-emphasis: -3.5dB
			 Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS-
			 Compliance De-emphasis: -6dB
[...]

Forcibly limiting the target link speed to 2.5GT/s with the upstream
ASM2824 device makes the two switches communicate correctly however:

02:03.0 PCI bridge [0604]: ASMedia Technology Inc. ASM2824 PCIe Gen3 Packet Switch [1b21:2824] (rev 01) (prog-if 00 [Normal decode])
[...]
	Bus: primary=02, secondary=05, subordinate=09, sec-latency=0
[...]
	Capabilities: [80] Express (v2) Downstream Port (Slot+), MSI 00
[...]
		LnkSta:	Speed 2.5GT/s (downgraded), Width x1 (ok)
			TrErr- Train- SlotClk+ DLActive+ BWMgmt- ABWMgmt-
[...]
		LnkCtl2: Target Link Speed: 2.5GT/s, EnterCompliance- SpeedDis+, Selectable De-emphasis: -3.5dB
			 Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS-
			 Compliance De-emphasis: -6dB
[...]

and then:

05:00.0 PCI bridge [0604]: Pericom Semiconductor PI7C9X2G304 EL/SL PCIe2 3-Port/4-Lane Packet Switch [12d8:2304] (rev 05) (prog-if 00 [Normal decode])
[...]
	Bus: primary=05, secondary=06, subordinate=09, sec-latency=0
[...]
	Capabilities: [c0] Express (v2) Upstream Port, MSI 00
[...]
		LnkSta:	Speed 2.5GT/s (downgraded), Width x1 (downgraded)
			TrErr- Train- SlotClk+ DLActive- BWMgmt- ABWMgmt-
[...]
		LnkCtl2: Target Link Speed: 5GT/s, EnterCompliance- SpeedDis-
			 Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS-
			 Compliance De-emphasis: -6dB
[...]

Make use of this observation then and attempt to detect the inability to
negotiate the link speed automatically, and then handle it by hand.  Use
the Data Link Layer Link Active status flag as the primary indicator of
successful link speed negotiation, but given that the flag is optional
by hardware to implement (the ASM2824 does have it though), resort to
checking for the mandatory Link Bandwidth Management Status flag showing
that the link speed or width has been changed in an attempt to correct
unreliable link operation (the ASM2824 does set it too).

If these checks indicate that link may not operate correctly, then poll
the Data Link Layer Link Active status flag along with the Link Training
flag for the duration of 200ms to see if the link has stabilised, that
is either that the Data Link Layer Link Active status flag has been set
or that Link Training has been inactive during at least the second half
of the interval.

If that has indicated failure, restrict the target speed to 2.5GT/s,
request a link retrain and check again if the link has stabilised.  If
that does not work either, then restore the original speed setting and
claim defeat, otherwise we are done.

NB interestingly enough with the ASM2824 vs PI7C9X2G304 configuration
referred above asking the ASM2824 to retrain with a higher target link
speed once the 2.5GT/s speed has been negotiated makes the two devices
successfully negotiate 5.0GT/s.  Lifting the 2.5GT/s speed restriction
would however prevent our workaround from working with an OS that issues
a reset and that is unaware of the problem.  This is because the devices
would then try to negotiate a higher link speed from scratch and fail,
while the sticky property of the Target Link Speed setting will keep the
2.5GT/s speed restriction across a reset.

Keep the 2.5GT/s speed restriction then, conservatively, if functional
once applied.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Reviewed-by: Stefan Roese <sr@denx.de>

1 files changed, 19 insertions, 2 deletions

diff --git a/include/pci.h b/include/pci.h
index 0ea41a7e1b..ad1171b0a2 100644
--- a/include/pci.h
+++ b/include/pci.h
@@ -5,6 +5,7 @@
  *
  * (C) Copyright 2002
  * Wolfgang Denk, DENX Software Engineering, wd@denx.de.
+ * Copyright (c) 2021  Maciej W. Rozycki <macro@orcam.me.uk>
  */
 
 #ifndef _PCI_H
@@ -475,16 +476,24 @@
 
 /* PCI Express capabilities */
 #define PCI_EXP_FLAGS		2	/* Capabilities register */
+#define  PCI_EXP_FLAGS_VERS	0x000f	/* Capability Version */
 #define  PCI_EXP_FLAGS_TYPE	0x00f0	/* Device/Port type */
-#define  PCI_EXP_TYPE_ROOT_PORT 0x4	/* Root Port */
+#define   PCI_EXP_TYPE_ROOT_PORT   0x4	/* Root Port */
+#define   PCI_EXP_TYPE_DOWNSTREAM  0x6	/* Downstream Port */
+#define   PCI_EXP_TYPE_PCIE_BRIDGE 0x8	/* PCI/PCI-X to PCIe Bridge */
 #define PCI_EXP_DEVCAP		4	/* Device capabilities */
 #define  PCI_EXP_DEVCAP_FLR	0x10000000 /* Function Level Reset */
 #define PCI_EXP_DEVCTL		8	/* Device Control */
 #define  PCI_EXP_DEVCTL_BCR_FLR	0x8000  /* Bridge Configuration Retry / FLR */
 #define PCI_EXP_LNKCAP		12	/* Link Capabilities */
 #define  PCI_EXP_LNKCAP_SLS	0x0000000f /* Supported Link Speeds */
+#define  PCI_EXP_LNKCAP_SLS_2_5GB 0x00000001 /* LNKCAP2 SLS Vector bit 0 */
+#define  PCI_EXP_LNKCAP_SLS_5_0GB 0x00000002 /* LNKCAP2 SLS Vector bit 1 */
+#define  PCI_EXP_LNKCAP_SLS_8_0GB 0x00000003 /* LNKCAP2 SLS Vector bit 2 */
 #define  PCI_EXP_LNKCAP_MLW	0x000003f0 /* Maximum Link Width */
 #define  PCI_EXP_LNKCAP_DLLLARC	0x00100000 /* Data Link Layer Link Active Reporting Capable */
+#define PCI_EXP_LNKCTL		16	/* Link Control */
+#define  PCI_EXP_LNKCTL_RL	0x0020	/* Retrain Link */
 #define PCI_EXP_LNKSTA		18	/* Link Status */
 #define  PCI_EXP_LNKSTA_CLS	0x000f	/* Current Link Speed */
 #define  PCI_EXP_LNKSTA_CLS_2_5GB 0x0001 /* Current Link Speed 2.5GT/s */
@@ -492,7 +501,9 @@
 #define  PCI_EXP_LNKSTA_CLS_8_0GB 0x0003 /* Current Link Speed 8.0GT/s */
 #define  PCI_EXP_LNKSTA_NLW	0x03f0	/* Negotiated Link Width */
 #define  PCI_EXP_LNKSTA_NLW_SHIFT 4	/* start of NLW mask in link status */
+#define  PCI_EXP_LNKSTA_LT	0x0800	/* Link Training */
 #define  PCI_EXP_LNKSTA_DLLLA	0x2000	/* Data Link Layer Link Active */
+#define  PCI_EXP_LNKSTA_LBMS	0x4000	/* Link Bandwidth Management Status */
 #define PCI_EXP_SLTCAP		20	/* Slot Capabilities */
 #define  PCI_EXP_SLTCAP_PSN	0xfff80000 /* Physical Slot Number */
 #define PCI_EXP_RTCTL		28	/* Root Control */
@@ -503,8 +514,14 @@
 #define  PCI_EXP_DEVCAP2_ARI	0x00000020 /* ARI Forwarding Supported */
 #define PCI_EXP_DEVCTL2		40	/* Device Control 2 */
 #define  PCI_EXP_DEVCTL2_ARI	0x0020 /* Alternative Routing-ID */
-
+#define PCI_EXP_LNKCAP2		44	/* Link Capability 2 */
+#define  PCI_EXP_LNKCAP2_SLS	0x000000fe /* Supported Link Speeds Vector */
 #define PCI_EXP_LNKCTL2		48	/* Link Control 2 */
+#define  PCI_EXP_LNKCTL2_TLS	0x000f	/* Target Link Speed */
+#define  PCI_EXP_LNKCTL2_TLS_2_5GT 0x0001 /* Target Link Speed 2.5GT/s */
+#define  PCI_EXP_LNKCTL2_TLS_5_0GT 0x0002 /* Target Link Speed 5.0GT/s */
+#define  PCI_EXP_LNKCTL2_TLS_8_0GT 0x0003 /* Target Link Speed 8.0GT/s */
+
 /* Single Root I/O Virtualization Registers */
 #define PCI_SRIOV_CAP		0x04	/* SR-IOV Capabilities */
 #define PCI_SRIOV_CTRL		0x08	/* SR-IOV Control */