linux-kernel - Re: [PATCH v2 3/3] arm64: dts: qcom: sm8450: Use GIC-ITS for PCIe0 and PCIe1

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Message-ID: <20221223174555.GE4587@thinkpad>
Date:   Fri, 23 Dec 2022 23:15:55 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     andersson@...nel.org, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, bhelgaas@...gle.com,
        konrad.dybcio@...aro.org, linux-arm-msm@...r.kernel.org,
        linux-pci@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] arm64: dts: qcom: sm8450: Use GIC-ITS for PCIe0
 and PCIe1

On Fri, Dec 23, 2022 at 07:18:32PM +0200, Dmitry Baryshkov wrote:
> On 22/12/2022 15:31, Manivannan Sadhasivam wrote:
> > Both PCIe0 and PCIe1 controllers are capable of receiving MSIs from
> > endpoint devices using GIC-ITS MSI controller. Add support for it.
> > 
> > Currently, BDF (0:0.0) and BDF (1:0.0) are enabled and with the
> > msi-map-mask of 0xff00, all the 32 devices under these two busses can
> > share the same Device ID.
> > 
> > The GIC-ITS MSI implementation provides an advantage over internal MSI
> > implementation using Locality-specific Peripheral Interrupts (LPI) that
> > would allow MSIs to be targeted for each CPU core.
> > 
> > It should be noted that the MSIs for BDF (1:0.0) only works with Device
> > ID of 0x5980 and 0x5a00. Hence, the IDs are swapped.
> > 
> > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > ---
> >   arch/arm64/boot/dts/qcom/sm8450.dtsi | 20 ++++++++++++++------
> >   1 file changed, 14 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/arm64/boot/dts/qcom/sm8450.dtsi b/arch/arm64/boot/dts/qcom/sm8450.dtsi
> > index 570475040d95..c4dd5838fac6 100644
> > --- a/arch/arm64/boot/dts/qcom/sm8450.dtsi
> > +++ b/arch/arm64/boot/dts/qcom/sm8450.dtsi
> > @@ -1733,9 +1733,13 @@ pcie0: pci@...0000 {
> >   			ranges = <0x01000000 0x0 0x60200000 0 0x60200000 0x0 0x100000>,
> >   				 <0x02000000 0x0 0x60300000 0 0x60300000 0x0 0x3d00000>;
> > -			interrupts = <GIC_SPI 141 IRQ_TYPE_LEVEL_HIGH>;
> > -			interrupt-names = "msi";
> > -			#interrupt-cells = <1>;
> > +			/*
> > +			 * MSIs for BDF (1:0.0) only works with Device ID 0x5980.
> > +			 * Hence, the IDs are swapped.
> > +			 */
> > +			msi-map = <0x0 &gic_its 0x5981 0x1>,
> > +				  <0x100 &gic_its 0x5980 0x1>;
> 
> This definitely doesn't match what has been used in the downstream.
> 

Yes, I do not know why the downstream Device ID doesn't work. I tried finding
the answer within Qcom but didn't get any answer so far :/ So I just went with
the value that works on multiple boards.

> Also if I understand correctly this change would prevent us from using
> multiple MSI interrupts for the connected device, as the last value of the
> 0x100 mapping is 0x1, while the vendor kernel uses <0x100 &its 0x5981 0x20>.
> 

Not true. The controller can still support multiple MSIs for the endpoint
devices but the only difference is, it would use the same Device ID for all.

The Qcom GIC-ITS implementation could only support 32 Device IDs. By specifying
the size of 0x20, a separate Device ID would be used for each devices of bus 1.
But if a PCIe switch is connected and the bus count becomes > 1, then the MSI
allocation would fail because Device IDs are exhausted.

The downstream implementation just assumes that there will be only bus 1 and I
do not want to follow that assumption.

That's why I used "msi-map-mask" property of value "0xff00" here, as that will
allow all the devices under the bus 1 to share the same Device ID. For now I
only mapped bus 1, but extending that in the future for other busses is simple.

Thanks,
Mani

> Do you know by chance, why do we differ from the vendor dtsi?
> 
> > +			msi-map-mask = <0xff00>;
> >   			interrupt-map-mask = <0 0 0 0x7>;
> >   			interrupt-map = <0 0 0 1 &intc 0 0 0 149 IRQ_TYPE_LEVEL_HIGH>, /* int_a */
> >   					<0 0 0 2 &intc 0 0 0 150 IRQ_TYPE_LEVEL_HIGH>, /* int_b */
> > @@ -1842,9 +1846,13 @@ pcie1: pci@...8000 {
> >   			ranges = <0x01000000 0x0 0x40200000 0 0x40200000 0x0 0x100000>,
> >   				 <0x02000000 0x0 0x40300000 0 0x40300000 0x0 0x1fd00000>;
> > -			interrupts = <GIC_SPI 307 IRQ_TYPE_LEVEL_HIGH>;
> > -			interrupt-names = "msi";
> > -			#interrupt-cells = <1>;
> > +			/*
> > +			 * MSIs for BDF (1:0.0) only works with Device ID 0x5a00.
> > +			 * Hence, the IDs are swapped.
> > +			 */
> > +			msi-map = <0x0 &gic_its 0x5a01 0x1>,
> > +				  <0x100 &gic_its 0x5a00 0x1>;
> > +			msi-map-mask = <0xff00>;
> >   			interrupt-map-mask = <0 0 0 0x7>;
> >   			interrupt-map = <0 0 0 1 &intc 0 0 0 434 IRQ_TYPE_LEVEL_HIGH>, /* int_a */
> >   					<0 0 0 2 &intc 0 0 0 435 IRQ_TYPE_LEVEL_HIGH>, /* int_b */
> 
> -- 
> With best wishes
> Dmitry
> 

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