RE: [EXT] Re: [PATCH v2] PCI: aardvark: Implement workaround for PCIe Completion Timeout

From: Elad Nachman
Date: Sun Oct 23 2022 - 05:22:29 EST

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Hi,

DIS_ORD_CHK - Disable Ordering Checks.
Setting this bit to 1 disables the ordering check in the core
between Completions and Posted requests received from the
link

Testing - I think Pali has 3700 boards, I don't have any.

The memory barrier is delaying the ability of the host to access the memory before the update is completed. This is not a bullet proof fix, it is just meant as a way to reduce the probability of this happening.
Hence it is optional; I think it is better to have it, but it is not part of the original workaround.
Once again - if the above explanation is not satisfactory, the patch can be reposted without the memory barrier.

FYI,

Elad.

-----Original Message-----
From: Lorenzo Pieralisi <lpieralisi@xxxxxxxxxx>
Sent: Friday, October 21, 2022 11:26 AM
To: Elad Nachman <enachman@xxxxxxxxxxx>
Cc: Pali Rohár <pali@xxxxxxxxxx>; Bjorn Helgaas <bhelgaas@xxxxxxxxxx>; Krzysztof Wilczyński <kw@xxxxxxxxx>; linux-pci@xxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
Subject: Re: [EXT] Re: [PATCH v2] PCI: aardvark: Implement workaround for PCIe Completion Timeout

On Thu, Oct 06, 2022 at 08:14:48AM +0000, Elad Nachman wrote:
> Hi,
>
> Regarding number #1 :
> The H/W erratum means strong ordering mode is not properly implemented
> in the H/W. The workaround is to set DIS_ORD_CHK to disable the
> ordered check, to prevent the issues Pali mentioned. Leaving it
> without the workaround means encountering the crashes Pali mentioned.
> The H/W was originally designed to have the strong ordering mode, per
> what is expected of PCI Express, but due to this erratum, it cannot be
> supported by the taped-out H/W. There is nothing to do about it now
> in the H/W, as this is a years old SOC. Software wise the only option
> is the workaround above, or to accept sporadic crashes of the kernel.

It can fix a driver, it can break another one - the PCI ordering model is broken one way or another.

I have no way to determine what setting DIS_ORD_CHK does and what it implies.

What I know is that we are sending this patch to stable kernels and I want to make sure a) it was thoroughly tested on several PCI endpoints and b) it is properly documented.

> The impact of the workaround is that the DMA done status could be set
> before the last data is written to the host memory, causing the host
> to read incomplete data. This is the reason why I recommended to add a
> memory barrier. If this is bothering, the memory barrier can be
> removed from the patch, although I suggest to thoroughly test the data
> integrity if implementing the patch without the memory barrier.

It is not bothering, it is that every mb() must be explained in depth.

We are adding an mb() in the *controller* interrupt handler.

I assume this is because that's where the DMA completion IRQ is raised and there we issue an mb() to make sure that endpoint writes into memory completed.

If that's correct, I have no idea how the barrier plays together with DIS_ORD_CHK above. The mb() has no effect on PCI writes queued in the PCI bus, so it may be a plaster but not a fix.

I would like to have a full description of what that mb() is supposed to fix - in details.

When we agree it is fixing something we can add a descriptive comment with the mb() and merge the code.

Thanks,
Lorenzo

> FYI,
>
> Elad.
>
> -----Original Message-----
> From: Pali Rohár <pali@xxxxxxxxxx>
> Sent: Tuesday, October 4, 2022 11:40 AM
> To: Elad Nachman <enachman@xxxxxxxxxxx>; Lorenzo Pieralisi
> <lpieralisi@xxxxxxxxxx>
> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>; Krzysztof Wilczyński
> <kw@xxxxxxxxx>; linux-pci@xxxxxxxxxxxxxxx;
> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [EXT] Re: [PATCH v2] PCI: aardvark: Implement workaround
> for PCIe Completion Timeout
>
> + Elad
>
> Could you please look at Lorenzo's comments and help with this fix?
>
> On Tuesday 04 October 2022 10:02:32 Lorenzo Pieralisi wrote:
> > On Mon, Oct 03, 2022 at 11:14:12PM +0200, Pali Rohár wrote:
> > > Lorenzo, is something more needed for this patch? As it
> > > workarounds crashing it is really needed to have it in mainline and backports.
> >
> > Yes, a clear explanation from Marvell about what this is actually
> > fixing - it took me a while to go through the whole thread but I
> > still don't understand what this patch actually does and why.
> >
> > An Erratum workaround (if there is any) should define and explain a
> > SW workaround.
> >
> > (1) Bjorn's concerns in relation to PCI memory model weren't
> > addressed
> > (2) We don't add undocumented memory barriers to the kernel to "minimize
> > risks". Either we fix a bug or we don't. If we do, write that down
> > and document why the barrier is there and the issue it solves.
> >
> > I understand that basically you are reverse engineering a HW bug but
> > I am afraid we can't fix the kernel this way - more so with patches
> > going to be backported to stable kernels.
> >
> > Lorenzo
> >
> > > On Wednesday 28 September 2022 14:05:10 Elad Nachman wrote:
> > > > Reviewed-by: Elad Nachman <enachman@xxxxxxxxxxx>
> > > >
> > > > Thanks,
> > > >
> > > > Elad.
> > > >
> > > > -----Original Message-----
> > > > From: Pali Rohár <pali@xxxxxxxxxx>
> > > > Sent: Monday, September 26, 2022 3:35 PM
> > > > To: Elad Nachman <enachman@xxxxxxxxxxx>
> > > > Cc: Thomas Petazzoni <thomas.petazzoni@xxxxxxxxxxx>; Lorenzo
> > > > Pieralisi <lpieralisi@xxxxxxxxxx>; Bjorn Helgaas
> > > > <bhelgaas@xxxxxxxxxx>; Krzysztof Wilczyński <kw@xxxxxxxxx>; Rob
> > > > Herring <robh@xxxxxxxxxx>; linux-pci@xxxxxxxxxxxxxxx;
> > > > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> > > > linux-kernel@xxxxxxxxxxxxxxx; Gregory Clement
> > > > <gregory.clement@xxxxxxxxxxx>; Marek Behún <kabel@xxxxxxxxxx>;
> > > > Remi Pommarel <repk@xxxxxxxxxxxx>; Xogium <contact@xxxxxxxxx>;
> > > > Tomasz Maciej Nowak <tmn505@xxxxxxxxx>
> > > > Subject: [EXT] Re: [PATCH v2] PCI: aardvark: Implement
> > > > workaround for PCIe Completion Timeout
> > > >
> > > > External Email
> > > >
> > > > ----------------------------------------------------------------
> > > > --
> > > > ---- Hello Elad, could you please review this patch? I have
> > > > implemented it according your instructions, including that full memory barrier as you described.
> > > >
> > > > On Tuesday 02 August 2022 14:38:16 Pali Rohár wrote:
> > > > > Marvell Armada 3700 Functional Errata, Guidelines, and
> > > > > Restrictions document describes in erratum 3.12 PCIe Completion Timeout (Ref #:
> > > > > 251), that PCIe IP does not support a strong-ordered model for inbound posted vs.
> > > > > outbound completion.
> > > > >
> > > > > As a workaround for this erratum, DIS_ORD_CHK flag in Debug
> > > > > Mux Control register must be set. It disables the ordering
> > > > > check in the core between Completions and Posted requests received from the link.
> > > > >
> > > > > Marvell also suggests to do full memory barrier at the
> > > > > beginning of aardvark summary interrupt handler before calling
> > > > > interrupt handlers of endpoint drivers in order to minimize
> > > > > the risk for the race condition documented in the Erratum
> > > > > between the DMA done status reading and the completion of writing to the host memory.
> > > > >
> > > > > More details about this issue and suggested workarounds are in discussion:
> > > > > https://urldefense.proofpoint.com/v2/url?u=https-3A__lore.kern
> > > > > el
> > > > > .org_l
> > > > > inux-2Dpci_BN9PR18MB425154FE5019DCAF2028A1D5DB8D9-40BN9PR18MB4
> > > > > 25
> > > > > 1.namp
> > > > > rd18.prod.outlook.com_t_-23u&d=DwIDaQ&c=nKjWec2b6R0mOyPaz7xtfQ
> > > > > &r
> > > > > =eTeNT
> > > > > LEK5-TxXczjOcKPhANIFtlB9pP4lq9qhdlFrwQ&m=bjgkhgPgOjqCEsbHYHONC
> > > > > ZM
> > > > > iFDX72
> > > > > MztWaE0AvWBktQVn3zKEDtUdn02Kx_KJ14B&s=SToGsDGEObwbZGilVtVZPyME
> > > > > 8j
> > > > > NiRgrq
> > > > > 4SDYvqqT0TA&e=
> > > > >
> > > > > It was reported that enabling this workaround fixes
> > > > > instability issues and "Unhandled fault" errors when using 60
> > > > > GHz WiFi 802.11ad card with Qualcomm
> > > > > QCA6335 chip under significant load which were caused by
> > > > > interrupt status stuck in the outbound CMPLT queue traced back to this erratum.
> > > > >
> > > > > This workaround fixes also kernel panic triggered after some
> > > > > minutes of usage 5 GHz WiFi 802.11ax card with Mediatek MT7915 chip:
> > > > >
> > > > > Internal error: synchronous external abort: 96000210 [#1] SMP
> > > > > Kernel panic - not syncing: Fatal exception in interrupt
> > > > >
> > > > > Signed-off-by: Thomas Petazzoni <thomas.petazzoni@xxxxxxxxxxx>
> > > > > Signed-off-by: Pali Rohár <pali@xxxxxxxxxx>
> > > > > Fixes: 8c39d710363c ("PCI: aardvark: Add Aardvark PCI host
> > > > > controller
> > > > > driver")
> > > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > > ---
> > > > > drivers/pci/controller/pci-aardvark.c | 10 ++++++++++
> > > > > 1 file changed, 10 insertions(+)
> > > > >
> > > > > diff --git a/drivers/pci/controller/pci-aardvark.c
> > > > > b/drivers/pci/controller/pci-aardvark.c
> > > > > index 060936ef01fe..3ae8a85ec72e 100644
> > > > > --- a/drivers/pci/controller/pci-aardvark.c
> > > > > +++ b/drivers/pci/controller/pci-aardvark.c
> > > > > @@ -210,6 +210,8 @@ enum {
> > > > > };
> > > > >
> > > > > #define VENDOR_ID_REG (LMI_BASE_ADDR + 0x44)
> > > > > +#define DEBUG_MUX_CTRL_REG (LMI_BASE_ADDR + 0x208)
> > > > > +#define DIS_ORD_CHK BIT(30)
> > > > >
> > > > > /* PCIe core controller registers */
> > > > > #define CTRL_CORE_BASE_ADDR 0x18000
> > > > > @@ -558,6 +560,11 @@ static void advk_pcie_setup_hw(struct advk_pcie *pcie)
> > > > > PCIE_CORE_CTRL2_TD_ENABLE;
> > > > > advk_writel(pcie, reg, PCIE_CORE_CTRL2_REG);
> > > > >
> > > > > + /* Disable ordering checks, workaround for erratum 3.12 "PCIe completion timeout" */
> > > > > + reg = advk_readl(pcie, DEBUG_MUX_CTRL_REG);
> > > > > + reg |= DIS_ORD_CHK;
> > > > > + advk_writel(pcie, reg, DEBUG_MUX_CTRL_REG);
> > > > > +
> > > > > /* Set lane X1 */
> > > > > reg = advk_readl(pcie, PCIE_CORE_CTRL0_REG);
> > > > > reg &= ~LANE_CNT_MSK;
> > > > > @@ -1581,6 +1588,9 @@ static irqreturn_t advk_pcie_irq_handler(int irq, void *arg)
> > > > > struct advk_pcie *pcie = arg;
> > > > > u32 status;
> > > > >
> > > > > + /* Full memory barrier (ARM dsb sy), workaround for erratum 3.12 "PCIe completion timeout" */
> > > > > + mb();
> > > > > +
> > > > > status = advk_readl(pcie, HOST_CTRL_INT_STATUS_REG);
> > > > > if (!(status & PCIE_IRQ_CORE_INT))
> > > > > return IRQ_NONE;
> > > > > --
> > > > > 2.20.1
> > > > >
> > >
> > > _______________________________________________
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In reply to: Lorenzo Pieralisi: "Re: [EXT] Re: [PATCH v2] PCI: aardvark: Implement workaround for PCIe Completion Timeout"
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