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

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Date:   Fri, 3 Nov 2023 13:28:49 +0530
From:   Manivannan Sadhasivam <mani@...nel.org>
To:     Konrad Dybcio <konrad.dybcio@...aro.org>
Cc:     Manivannan Sadhasivam <mani@...nel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Mrinmay Sarkar <quic_msarkar@...cinc.com>, 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 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?

> 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

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