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

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Date:   Thu, 12 Nov 2020 22:32:40 +0530
From:   Vidya Sagar <vidyas@...dia.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
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 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;
>>>> +}