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

[Lorenzo Pieralisi]

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.kernel
> > > > > .org_l 
> > > > > inux-2Dpci_BN9PR18MB425154FE5019DCAF2028A1D5DB8D9-40BN9PR18MB425
> > > > > 1.namp 
> > > > > rd18.prod.outlook.com_t_-23u&d=DwIDaQ&c=nKjWec2b6R0mOyPaz7xtfQ&r
> > > > > =eTeNT
> > > > > LEK5-TxXczjOcKPhANIFtlB9pP4lq9qhdlFrwQ&m=bjgkhgPgOjqCEsbHYHONCZM
> > > > > iFDX72 
> > > > > MztWaE0AvWBktQVn3zKEDtUdn02Kx_KJ14B&s=SToGsDGEObwbZGilVtVZPyME8j
> > > > > 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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