linux-kernel - Re: [PATCH V2] PCI: dwc: Add support to configure for ECRC

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Message-ID: <20201124210228.GA589610@bjorn-Precision-5520>
Date:   Tue, 24 Nov 2020 15:02:28 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Vidya Sagar <vidyas@...dia.com>
Cc:     Jingoo Han <jingoohan1@...il.com>,
        "gustavo.pimentel@...opsys.com" <gustavo.pimentel@...opsys.com>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "amurray@...goodpenguin.co.uk" <amurray@...goodpenguin.co.uk>,
        "robh@...nel.org" <robh@...nel.org>,
        "treding@...dia.com" <treding@...dia.com>,
        "jonathanh@...dia.com" <jonathanh@...dia.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kthota@...dia.com" <kthota@...dia.com>,
        "mmaddireddy@...dia.com" <mmaddireddy@...dia.com>,
        "sagar.tv@...il.com" <sagar.tv@...il.com>
Subject: Re: [PATCH V2] PCI: dwc: Add support to configure for ECRC

On Tue, Nov 24, 2020 at 03:50:01PM +0530, Vidya Sagar wrote:
> Hi Bjorn,
> Please let me know if this patch needs any further modifications

I'm fine with it, but of course Lorenzo will take care of it.

> On 11/12/2020 10:32 PM, Vidya Sagar wrote:
> > External email: Use caution opening links or attachments
> > 
> > 
> > On 11/12/2020 3:59 AM, Bjorn Helgaas wrote:
> > > External email: Use caution opening links or attachments
> > > 
> > > 
> > > On Wed, Nov 11, 2020 at 10:21:46PM +0530, Vidya Sagar wrote:
> > > > 
> > > > 
> > > > On 11/11/2020 9:57 PM, Jingoo Han wrote:
> > > > > External email: Use caution opening links or attachments
> > > > > 
> > > > > 
> > > > > On 11/11/20, 7:12 AM, Vidya Sagar wrote:
> > > > > > 
> > > > > > DesignWare core has a TLP digest (TD) override bit in
> > > > > > one of the control
> > > > > > registers of ATU. This bit also needs to be programmed for proper ECRC
> > > > > > functionality. This is currently identified as an issue
> > > > > > with DesignWare
> > > > > > IP version 4.90a.
> > > > > > 
> > > > > > Signed-off-by: Vidya Sagar <vidyas@...dia.com>
> > > > > > Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>
> > > > > > ---
> > > > > > V2:
> > > > > > * Addressed Bjorn's comments
> > > > > > 
> > > > > >    drivers/pci/controller/dwc/pcie-designware.c | 52
> > > > > > ++++++++++++++++++--
> > > > > >    drivers/pci/controller/dwc/pcie-designware.h |  1 +
> > > > > >    2 files changed, 49 insertions(+), 4 deletions(-)
> > > > > > 
> > > > > > diff --git
> > > > > > a/drivers/pci/controller/dwc/pcie-designware.c
> > > > > > b/drivers/pci/controller/dwc/pcie-designware.c
> > > > > > index c2dea8fc97c8..ec0d13ab6bad 100644
> > > > > > --- a/drivers/pci/controller/dwc/pcie-designware.c
> > > > > > +++ b/drivers/pci/controller/dwc/pcie-designware.c
> > > > > > @@ -225,6 +225,46 @@ static void
> > > > > > dw_pcie_writel_ob_unroll(struct dw_pcie *pci, u32 index,
> > > > > > u32 reg,
> > > > > >         dw_pcie_writel_atu(pci, offset + reg, val);
> > > > > >    }
> > > > > > 
> > > > > > +static inline u32 dw_pcie_enable_ecrc(u32 val)
> > > > > 
> > > > > What is the reason to use inline here?
> > > > 
> > > > Actually, I wanted to move the programming part inside the
> > > > respective APIs
> > > > but then I wanted to give some details as well in comments so to avoid
> > > > duplication, I came up with this function. But, I'm making it inline for
> > > > better code optimization by compiler.
> > > 
> > > I don't really care either way, but I'd be surprised if the compiler
> > > didn't inline this all by itself even without the explicit "inline".
> > I just checked it and you are right that compiler is indeed inlining it
> > without explicitly mentioning 'inline'.
> > I hope it is ok to leave it that way.
> > 
> > > 
> > > > > > +{
> > > > > > +     /*
> > > > > > +      * DesignWare core version 4.90A has this strange design issue
> > > > > > +      * where the 'TD' bit in the Control register-1 of
> > > > > > the ATU outbound
> > > > > > +      * region acts like an override for the ECRC
> > > > > > setting i.e. the presence
> > > > > > +      * of TLP Digest(ECRC) in the outgoing TLPs is
> > > > > > solely determined by
> > > > > > +      * this bit. This is contrary to the PCIe spec
> > > > > > which says that the
> > > > > > +      * enablement of the ECRC is solely determined by
> > > > > > the AER registers.
> > > > > > +      *
> > > > > > +      * Because of this, even when the ECRC is enabled through AER
> > > > > > +      * registers, the transactions going through ATU
> > > > > > won't have TLP Digest
> > > > > > +      * as there is no way the AER sub-system could
> > > > > > program the TD bit which
> > > > > > +      * is specific to DesignWare core.
> > > > > > +      *
> > > > > > +      * The best way to handle this scenario is to program the TD bit
> > > > > > +      * always. It affects only the traffic from root
> > > > > > port to downstream
> > > > > > +      * devices.
> > > > > > +      *
> > > > > > +      * At this point,
> > > > > > +      * When ECRC is enabled in AER registers,
> > > > > > everything works normally
> > > > > > +      * When ECRC is NOT enabled in AER registers, then,
> > > > > > +      * on Root Port:- TLP Digest (DWord size) gets
> > > > > > appended to each packet
> > > > > > +      *                even through it is not required.
> > > > > > Since downstream
> > > > > > +      *                TLPs are mostly for
> > > > > > configuration accesses and BAR
> > > > > > +      *                accesses, they are not in
> > > > > > critical path and won't
> > > > > > +      *                have much negative effect on the performance.
> > > > > > +      * on End Point:- TLP Digest is received for
> > > > > > some/all the packets coming
> > > > > > +      *                from the root port. TLP Digest
> > > > > > is ignored because,
> > > > > > +      *                as per the PCIe Spec r5.0 v1.0 section 2.2.3
> > > > > > +      *                "TLP Digest Rules", when an
> > > > > > endpoint receives TLP
> > > > > > +      *                Digest when its ECRC check
> > > > > > functionality is disabled
> > > > > > +      *                in AER registers, received TLP
> > > > > > Digest is just ignored.
> > > > > > +      * Since there is no issue or error reported
> > > > > > either side, best way to
> > > > > > +      * handle the scenario is to program TD bit by default.
> > > > > > +      */
> > > > > > +
> > > > > > +     return val | PCIE_ATU_TD;
> > > > > > +}