linux-kernel - Re: [PATCH v1 1/3] PCI: qcom: Enable cache coherency for SA8775P RC

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Message-ID: <cb4324aa-8035-ce6e-94ef-a31ed070225c@quicinc.com>
Date:   Mon, 6 Nov 2023 12:49:51 +0530
From:   Mrinmay Sarkar <quic_msarkar@...cinc.com>
To:     Manivannan Sadhasivam <mani@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>
CC:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        <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>, <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 v1 1/3] PCI: qcom: Enable cache coherency for SA8775P RC


On 11/3/2023 1:28 PM, Manivannan Sadhasivam wrote:
> On Thu, Nov 02, 2023 at 11:25:36PM +0100, Konrad Dybcio wrote:
>>
>> On 02/11/2023 17:36, Manivannan Sadhasivam wrote:
>>> On Thu, Nov 02, 2023 at 05:34:24PM +0200, Dmitry Baryshkov wrote:
>>>> On Tue, 31 Oct 2023 at 17:46, Mrinmay Sarkar <quic_msarkar@...cinc.com> wrote:
>>>>> This change will enable cache snooping logic to support
>>>>> cache coherency for SA8755P RC platform.
>>>>>
>>>>> Signed-off-by: Mrinmay Sarkar <quic_msarkar@...cinc.com>
>>>>> ---
>>>>>    drivers/pci/controller/dwc/pcie-qcom.c | 11 +++++++++++
>>>>>    1 file changed, 11 insertions(+)
>>>>>
>>>>> diff --git a/drivers/pci/controller/dwc/pcie-qcom.c b/drivers/pci/controller/dwc/pcie-qcom.c
>>>>> index 6902e97..6f240fc 100644
>>>>> --- a/drivers/pci/controller/dwc/pcie-qcom.c
>>>>> +++ b/drivers/pci/controller/dwc/pcie-qcom.c
>>>>> @@ -51,6 +51,7 @@
>>>>>    #define PARF_SID_OFFSET                                0x234
>>>>>    #define PARF_BDF_TRANSLATE_CFG                 0x24c
>>>>>    #define PARF_SLV_ADDR_SPACE_SIZE               0x358
>>>>> +#define PCIE_PARF_NO_SNOOP_OVERIDE             0x3d4
>>>>>    #define PARF_DEVICE_TYPE                       0x1000
>>>>>    #define PARF_BDF_TO_SID_TABLE_N                        0x2000
>>>>>
>>>>> @@ -117,6 +118,9 @@
>>>>>    /* PARF_LTSSM register fields */
>>>>>    #define LTSSM_EN                               BIT(8)
>>>>>
>>>>> +/* PARF_NO_SNOOP_OVERIDE register value */
>>>>> +#define NO_SNOOP_OVERIDE_EN                    0xa
>>>>> +
>>>>>    /* PARF_DEVICE_TYPE register fields */
>>>>>    #define DEVICE_TYPE_RC                         0x4
>>>>>
>>>>> @@ -961,6 +965,13 @@ static int qcom_pcie_init_2_7_0(struct qcom_pcie *pcie)
>>>>>
>>>>>    static int qcom_pcie_post_init_2_7_0(struct qcom_pcie *pcie)
>>>>>    {
>>>>> +       struct dw_pcie *pci = pcie->pci;
>>>>> +       struct device *dev = pci->dev;
>>>>> +
>>>>> +       /* Enable cache snooping for SA8775P */
>>>>> +       if (of_device_is_compatible(dev->of_node, "qcom,pcie-sa8775p"))
>>>> Obviously: please populate a flag in the data structures instead of
>>>> doing of_device_is_compatible(). Same applies to the patch 2.
>>>>
>>> Not necessary at this point. For some unknown reasons, the HW team ended up
>>> disabling cache snooping on this specific platform. Whereas on other platforms,
>>> it is enabled by default. So I have low expectations that we would need this
>>> setting on other platforms in the future.
>>>
>>> My concern with the usage of flag is that it warrants a new "qcom_pcie_cfg"
>>> instance just for this quirk and it looks overkill to me.
>>>
>>> So if we endup seeing this behavior on other platforms as well (unlikely) then
>>> we can switch to the flag approach.
>> This register reads zeroes on 8250, can we confirm it works as
>> expected there?
> I don't know if this register is even implemented in 8250. Mrinmay, can you
> check?
Yes we have this register in 8250 platform as well
and I can see the default value is 0x0.

--Mrinmay
>> I guess some benchmarks with and without
>> 'dma-coherent'?
>>
> The performance benefit can be measured by saturating the link. But it is
> obvious that snooping the cache will give better performance (plus it avoids
> cache flush in kernel).
>
> - Mani
>