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Date:   Tue, 4 Oct 2022 10:02:32 +0200
From:   Lorenzo Pieralisi <lpieralisi@kernel.org>
To:     Pali =?iso-8859-1?Q?Roh=E1r?= <pali@kernel.org>
Cc:     Bjorn Helgaas <bhelgaas@google.com>,
        Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= <kw@linux.com>,
        linux-pci@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
Subject: Re: [EXT] Re: [PATCH v2] PCI: aardvark: Implement workaround for
 PCIe Completion Timeout
Message-ID: <YzvomObCatuKMujz@lpieralisi>
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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@marvell.com>
> > 
> > Thanks,
> > 
> > Elad.
> > 
> > -----Original Message-----
> > From: Pali Rohár <pali@kernel.org> 
> > Sent: Monday, September 26, 2022 3:35 PM
> > To: Elad Nachman <enachman@marvell.com>
> > Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>; Lorenzo Pieralisi <lpieralisi@kernel.org>; Bjorn Helgaas <bhelgaas@google.com>; Krzysztof Wilczyński <kw@linux.com>; Rob Herring <robh@kernel.org>; linux-pci@vger.kernel.org; linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org; Gregory Clement <gregory.clement@bootlin.com>; Marek Behún <kabel@kernel.org>; Remi Pommarel <repk@triplefau.lt>; Xogium <contact@xogium.me>; Tomasz Maciej Nowak <tmn505@gmail.com>
> > 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_BN9PR18MB425154FE5019DCAF2028A1D5DB8D9-40BN9PR18MB4251.namp
> > > rd18.prod.outlook.com_t_-23u&d=DwIDaQ&c=nKjWec2b6R0mOyPaz7xtfQ&r=eTeNT
> > > LEK5-TxXczjOcKPhANIFtlB9pP4lq9qhdlFrwQ&m=bjgkhgPgOjqCEsbHYHONCZMiFDX72
> > > MztWaE0AvWBktQVn3zKEDtUdn02Kx_KJ14B&s=SToGsDGEObwbZGilVtVZPyME8jNiRgrq
> > > 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@bootlin.com>
> > > Signed-off-by: Pali Rohár <pali@kernel.org>
> > > Fixes: 8c39d710363c ("PCI: aardvark: Add Aardvark PCI host controller 
> > > driver")
> > > Cc: stable@vger.kernel.org
> > > ---
> > >  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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