On 11/21/21 00:03, Maciej W. Rozycki wrote:

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

Hi,

I believe this version has addressed all concerns raised in the review thus far. With the nature of a problem better understood now I'm sending a corresponding update for Linux as well.

One case that has been nurturing me however is the reverse scenario, that is where the Pericom PI7C9X2G304 switch is upstream while the ASMedia ASM2824 switch is downstream. Presumably the same situation will happen, so it's clear that this generic approach I chose for U-Boot is going to handle both cases. For Linux, where I chose to match on the ASM2824 ID it would be a problem, though our conservative approach will make both cases work in the OS.

Option hardware with M.2 slots is commercially available with the ASM2824 onboard, so a test environment can be in principle arranged, though I'm not sure if just for the sake of such an experiment I'm willing to spend money that will ultimately go to a manufacturer that cannot be bothered to take responsibility for their faults and at the very least respond to a problem report.

Thank you, Stefan, in particular for mentioning the de-emphasis setting; while not directly relevant it has prompted me to further experiment with the hardware involved and consequently discover that the 5GT/s speed does actually work with these devices if arranged in a particular if not peculiar way.

Further questions, comments or concerns? Otherwise please apply.

Maciej

Changes from v2:

• only restrict the link speed to 2.5GT/s and keep the restriction; do not try other speeds; update the comments and change description accordingly,

• print informational messages where a problematic link has been found,

• keep the names of macros in include/pci.h consistent with Linux (NB it may make sense to synchronise the file on a routine basis, just as many free software projects do that reuse code maintained in other projects),

• rephrase comments and the change description here and there for style and clarity.

Changes from v1:

• also handle root complex and PCI/PCI-X to PCIe bridge devices,

• update comments throughout accordingly,

• likewise the change description; mention the rate of oscillations observed with the ASM2824 device.

---

drivers/pci/pci_auto.c | 170 +++++++++++++++++++++++++++++++++++++++++++++++++ include/pci.h | 21 +++++- 2 files changed, 189 insertions(+), 2 deletions(-)

Reviewed-by: Stefan Roese sr@denx.de

Thanks, Stefan

u-boot-pcie-manual-retrain.diff Index: u-boot/drivers/pci/pci_auto.c =================================================================== --- u-boot.orig/drivers/pci/pci_auto.c +++ u-boot/drivers/pci/pci_auto.c @@ -5,6 +5,7 @@

• Author: Matt Porter mporter@mvista.com
• Copyright 2000 MontaVista Software Inc.

• • Copyright (c) 2021 Maciej W. Rozycki macro@orcam.me.uk

  */

  #include <common.h>

@@ -12,6 +13,7 @@ #include <errno.h> #include <log.h> #include <pci.h> +#include <time.h> #include "pci_internal.h"

/* the user can define CONFIG_SYS_PCI_CACHE_LINE_SIZE to avoid problems */ @@ -180,6 +182,168 @@ static void dm_pciauto_setup_device(stru dm_pci_write_config8(dev, PCI_LATENCY_TIMER, 0x80); }

+/*

• • Check if the link of a downstream PCIe port operates correctly.
• • For that check if the optional Data Link Layer Link Active status gets
• • on within a 200ms period or failing that wait until the completion of
• • that period and check if link training has shown the completed status
• • continuously throughout the second half of that period.
• • Observation with the ASMedia ASM2824 Gen 3 switch indicates it takes
• • 11-44ms to indicate the Data Link Layer Link Active status at 2.5GT/s,
• • though it may take a couple of link training iterations.
• */

+static bool dm_pciauto_exp_link_stable(struct udevice *dev, int pcie_off) +{

• u64 loops = 0, trcount = 0, ntrcount = 0, flips = 0;
• bool dllla, lnktr, plnktr;
• u16 exp_lnksta;
• pci_dev_t bdf;
• u64 end;
• dm_pci_read_config16(dev, pcie_off + PCI_EXP_LNKSTA, &exp_lnksta);
• plnktr = !!(exp_lnksta & PCI_EXP_LNKSTA_LT);
• end = get_ticks() + usec_to_tick(200000);
• do {

• dm_pci_read_config16(dev, pcie_off + PCI_EXP_LNKSTA,
  

• 		     &exp_lnksta);
  

• dllla = !!(exp_lnksta & PCI_EXP_LNKSTA_DLLLA);
  

• lnktr = !!(exp_lnksta & PCI_EXP_LNKSTA_LT);
  

• flips += plnktr ^ lnktr;
  

• if (lnktr) {
  

• 	ntrcount = 0;
  

• 	trcount++;
  

• } else {
  

• 	ntrcount++;
  

• }
  

• loops++;
  

• plnktr = lnktr;
  

• } while (!dllla && get_ticks() < end);
• bdf = dm_pci_get_bdf(dev);
• debug("PCI Autoconfig: %02x.%02x.%02x: Fixup link: DL active: %u; "

•      "%3llu flips, %6llu loops of which %6llu while training, "
  

•      "final %6llu stable\n",
  

•      PCI_BUS(bdf), PCI_DEV(bdf), PCI_FUNC(bdf),
  

•      (unsigned int)dllla,
  

•      (unsigned long long)flips, (unsigned long long)loops,
  

•      (unsigned long long)trcount, (unsigned long long)ntrcount);
  

• return dllla || ntrcount >= loops / 2;

+}

+/*

• • Retrain the link of a downstream PCIe port by hand if necessary.
• • This is needed at least where a downstream port of the ASMedia ASM2824
• • Gen 3 switch is wired to the upstream port of the Pericom PI7C9X2G304
• • Gen 2 switch, and observed with the Delock Riser Card PCI Express x1 >
• • 2 x PCIe x1 device, P/N 41433, plugged into the SiFive HiFive Unmatched
• • board.
• • In such a configuration the switches are supposed to negotiate the link
• • speed of preferably 5.0GT/s, falling back to 2.5GT/s. However the link
• • continues switching between the two speeds indefinitely and the data
• • link layer never reaches the active state, with link training reported
• • repeatedly active ~84% of the time. Forcing the target link speed to
• • 2.5GT/s with the upstream ASM2824 device makes the two switches talk to
• • each other correctly however. And more interestingly retraining with a
• • higher target link speed afterwards lets the two successfully negotiate
• • 5.0GT/s.
• • As this can potentially happen with any device and is cheap in the case
• • of correctly operating hardware, let's do it for all downstream ports,
• • for root complexes, PCIe switches and PCI/PCI-X to PCIe bridges.
• • First check if automatic link training may have failed to complete, as
• • indicated by the optional Data Link Layer Link Active status being off
• • and the Link Bandwidth Management Status indicating that hardware has
• • changed the link speed or width in an attempt to correct unreliable
• • link operation. If this is the case, then check if the link operates
• • correctly by seeing whether it is being trained excessively. If it is,
• • then conclude the link is broken.
• • In that case restrict the speed to 2.5GT/s, observing that the Target
• • Link Speed field is sticky and therefore the link will stay restricted
• • even after a device reset is later made by an OS that is unaware of the
• • problem. With the speed restricted request that the link be retrained
• • and check again if the link operates correctly. If not, then set the
• • Target Link Speed back to the original value.
• • This requires the presence of the Link Control 2 register, so make sure
• • the PCI Express Capability Version is at least 2. Also don't try, for
• • obvious reasons, to limit the speed if 2.5GT/s is the only link speed
• • supported.
• */

+static void dm_pciauto_exp_fixup_link(struct udevice *dev, int pcie_off) +{

• u16 exp_lnksta, exp_lnkctl, exp_lnkctl2;
• u16 exp_flags, exp_type, exp_version;
• u32 exp_lnkcap;
• pci_dev_t bdf;
• dm_pci_read_config16(dev, pcie_off + PCI_EXP_FLAGS, &exp_flags);
• exp_version = exp_flags & PCI_EXP_FLAGS_VERS;
• if (exp_version < 2)

• return;
  

• exp_type = (exp_flags & PCI_EXP_FLAGS_TYPE) >> 4;
• switch (exp_type) {
• case PCI_EXP_TYPE_ROOT_PORT:
• case PCI_EXP_TYPE_DOWNSTREAM:
• case PCI_EXP_TYPE_PCIE_BRIDGE:

• break;
  

• default:

• return;
  

• }
• dm_pci_read_config32(dev, pcie_off + PCI_EXP_LNKCAP, &exp_lnkcap);
• if ((exp_lnkcap & PCI_EXP_LNKCAP_SLS) <= PCI_EXP_LNKCAP_SLS_2_5GB)

• return;
  

• dm_pci_read_config16(dev, pcie_off + PCI_EXP_LNKSTA, &exp_lnksta);
• if ((exp_lnksta & (PCI_EXP_LNKSTA_LBMS | PCI_EXP_LNKSTA_DLLLA)) !=

•    PCI_EXP_LNKSTA_LBMS)
  

• return;
  

• if (dm_pciauto_exp_link_stable(dev, pcie_off))

• return;
  

• bdf = dm_pci_get_bdf(dev);
• printf("PCI Autoconfig: %02x.%02x.%02x: "

•       "Downstream link non-functional\n",
  

•       PCI_BUS(bdf), PCI_DEV(bdf), PCI_FUNC(bdf));
  

• printf("PCI Autoconfig: %02x.%02x.%02x: "

•       "Retrying with speed restricted to 2.5GT/s...\n",
  

•       PCI_BUS(bdf), PCI_DEV(bdf), PCI_FUNC(bdf));
  

• dm_pci_read_config16(dev, pcie_off + PCI_EXP_LNKCTL, &exp_lnkctl);
• dm_pci_read_config16(dev, pcie_off + PCI_EXP_LNKCTL2, &exp_lnkctl2);
• dm_pci_write_config16(dev, pcie_off + PCI_EXP_LNKCTL2,

• 	      (exp_lnkctl2 & ~PCI_EXP_LNKCTL2_TLS) |
  

• 	      PCI_EXP_LNKCTL2_TLS_2_5GT);
  

• dm_pci_write_config16(dev, pcie_off + PCI_EXP_LNKCTL,

• 	      exp_lnkctl | PCI_EXP_LNKCTL_RL);
  

• if (dm_pciauto_exp_link_stable(dev, pcie_off)) {

• printf("PCI Autoconfig: %02x.%02x.%02x: Succeeded!\n",
  

•        PCI_BUS(bdf), PCI_DEV(bdf), PCI_FUNC(bdf));
  

• } else {

• printf("PCI Autoconfig: %02x.%02x.%02x: Failed!\n",
  

•        PCI_BUS(bdf), PCI_DEV(bdf), PCI_FUNC(bdf));
  

• dm_pci_write_config16(dev, pcie_off + PCI_EXP_LNKCTL2,
  

• 		      exp_lnkctl2);
  

• dm_pci_write_config16(dev, pcie_off + PCI_EXP_LNKCTL,
  

• 		      exp_lnkctl | PCI_EXP_LNKCTL_RL);
  

• }

+}

• void dm_pciauto_prescan_setup_bridge(struct udevice *dev, int sub_bus) { struct pci_region *pci_mem;

@@ -189,6 +353,7 @@ void dm_pciauto_prescan_setup_bridge(str u8 io_32; struct udevice *ctlr = pci_get_controller(dev); struct pci_controller *ctlr_hose = dev_get_uclass_priv(ctlr);

• int pcie_off;

  pci_mem = ctlr_hose->pci_mem; pci_prefetch = ctlr_hose->pci_prefetch;

@@ -267,6 +432,11 @@ void dm_pciauto_prescan_setup_bridge(str cmdstat |= PCI_COMMAND_IO; }

• /* For PCIe devices see if we need to retrain the link by hand */
• pcie_off = dm_pci_find_capability(dev, PCI_CAP_ID_EXP);
• if (pcie_off)

• dm_pciauto_exp_fixup_link(dev, pcie_off);
  

• /* Enable memory and I/O accesses, enable bus master */ dm_pci_write_config16(dev, PCI_COMMAND, cmdstat | PCI_COMMAND_MASTER); }

Index: u-boot/include/pci.h

--- u-boot.orig/include/pci.h +++ u-boot/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 */

Viele Grüße, Stefan Roese