linux-kernel - Re: [PATCH v4 1/3] PCI: qcom: Enable cache coherency for SA8775P RC

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Date:   Thu, 30 Nov 2023 11:09:59 +0100
From:   Konrad Dybcio <konrad.dybcio@...aro.org>
To:     Manivannan Sadhasivam <mani@...nel.org>,
        Mrinmay Sarkar <quic_msarkar@...cinc.com>
Cc:     agross@...nel.org, andersson@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org,
        robh+dt@...nel.org, quic_shazhuss@...cinc.com,
        quic_nitegupt@...cinc.com, quic_ramkri@...cinc.com,
        quic_nayiluri@...cinc.com, dmitry.baryshkov@...aro.org,
        robh@...nel.org, quic_krichai@...cinc.com,
        quic_vbadigan@...cinc.com, quic_parass@...cinc.com,
        quic_schintav@...cinc.com, quic_shijjose@...cinc.com,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH v4 1/3] PCI: qcom: Enable cache coherency for SA8775P RC

On 30.11.2023 06:21, Manivannan Sadhasivam wrote:
> On Tue, Nov 21, 2023 at 08:08:11PM +0530, Mrinmay Sarkar wrote:
>> In a multiprocessor system cache snooping maintains the consistency
>> of caches. Snooping logic is disabled from HW on this platform.
>> Cache coherency doesn’t work without enabling this logic.
>>
>> 8775 has IP version 1.34.0 so intruduce a new cfg(cfg_1_34_0) for this
>> platform. Assign no_snoop_override flag into struct qcom_pcie_cfg and
>> set it true in cfg_1_34_0 and enable cache snooping if this particular
>> flag is true.
>>
> 
> I just happen to check the internal register details of other platforms and I
> see this PCIE_PARF_NO_SNOOP_OVERIDE register with the reset value of 0x0. So
> going by the logic of this patch, this register needs to be configured for other
> platforms as well to enable cache coherency, but it seems like not the case as
> we never did and all are working fine (so far no issues reported).

Guess we know that already [1]

The question is whether this override is necessary, or the default
internal state is OK on other platforms

Konrad

[1] https://lore.kernel.org/linux-arm-msm/cb4324aa-8035-ce6e-94ef-a31ed070225c@quicinc.com/