Re: [PATCH] arm64: dts: qcom: sc8280xp: fix PCIe DMA coherency

[Manivannan Sadhasivam]

On Thu, Nov 24, 2022 at 03:25:01PM +0100, Johan Hovold wrote:
> The devices on the SC8280XP PCIe buses are cache coherent and must be
> marked as such to avoid data corruption.
> 
> A coherent device can, for example, end up snooping stale data from the
> caches instead of using data written by the CPU through the
> non-cacheable mapping which is used for consistent DMA buffers for
> non-coherent devices.
> 

Also, the device may write into the L2 cache (or whatever cache that is
accessible) if there is an entry and the CPU may invalidate it before reading
from the DMA buffer. This will end up in a data loss.

> Note that this is much more likely to happen since commit c44094eee32f
> ("arm64: dma: Drop cache invalidation from arch_dma_prep_coherent()")
> that was added in 6.1 and which removed the cache invalidation when
> setting up the non-cacheable mapping.
> 
> Marking the PCIe devices as coherent specifically fixes the intermittent
> NVMe probe failures observed on the Thinkpad X13s, which was due to
> corruption of the submission and completion queues. This was typically
> observed as corruption of the admin submission queue (with well-formed
> completion):
> 
> 	could not locate request for tag 0x0
> 	nvme nvme0: invalid id 0 completed on queue 0
> 
> or corruption of the admin or I/O completion queues (malformed
> completion):
> 
> 	could not locate request for tag 0x45f
> 	nvme nvme0: invalid id 25695 completed on queue 25965
> 
> presumably as these queues are small enough to not be allocated using
> CMA which in turn make them more likely to be cached (e.g. due to
> accesses to nearby pages through the cacheable linear map). Increasing
> the buffer sizes to two pages to force CMA allocation also appears to
> make the problem go away.
> 

I don't think the problem will go away if the allocation happens from CMA
region. It may just decrease the chances of cache hit but it could always
happen due to the existence of linear mapping with cacheable attribute.

> Fixes: 813e83157001 ("arm64: dts: qcom: sc8280xp/sa8540p: add PCIe2-4 nodes")
> Signed-off-by: Johan Hovold <johan+linaro@xxxxxxxxxx>

Anyway, this is a really good find!

Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxx>

Thanks,
Mani

> ---
>  arch/arm64/boot/dts/qcom/sc8280xp.dtsi | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/arm64/boot/dts/qcom/sc8280xp.dtsi b/arch/arm64/boot/dts/qcom/sc8280xp.dtsi
> index 27f5c2f82338..7748cd29276d 100644
> --- a/arch/arm64/boot/dts/qcom/sc8280xp.dtsi
> +++ b/arch/arm64/boot/dts/qcom/sc8280xp.dtsi
> @@ -854,6 +854,8 @@ pcie4: pcie@1c00000 {
>  				 <0x02000000 0x0 0x30300000 0x0 0x30300000 0x0 0x1d00000>;
>  			bus-range = <0x00 0xff>;
>  
> +			dma-coherent;
> +
>  			linux,pci-domain = <6>;
>  			num-lanes = <1>;
>  
> @@ -951,6 +953,8 @@ pcie3b: pcie@1c08000 {
>  				 <0x02000000 0x0 0x32300000 0x0 0x32300000 0x0 0x1d00000>;
>  			bus-range = <0x00 0xff>;
>  
> +			dma-coherent;
> +
>  			linux,pci-domain = <5>;
>  			num-lanes = <2>;
>  
> @@ -1046,6 +1050,8 @@ pcie3a: pcie@1c10000 {
>  				 <0x02000000 0x0 0x34300000 0x0 0x34300000 0x0 0x1d00000>;
>  			bus-range = <0x00 0xff>;
>  
> +			dma-coherent;
> +
>  			linux,pci-domain = <4>;
>  			num-lanes = <4>;
>  
> @@ -1144,6 +1150,8 @@ pcie2b: pcie@1c18000 {
>  				 <0x02000000 0x0 0x38300000 0x0 0x38300000 0x0 0x1d00000>;
>  			bus-range = <0x00 0xff>;
>  
> +			dma-coherent;
> +
>  			linux,pci-domain = <3>;
>  			num-lanes = <2>;
>  
> @@ -1239,6 +1247,8 @@ pcie2a: pcie@1c20000 {
>  				 <0x02000000 0x0 0x3c300000 0x0 0x3c300000 0x0 0x1d00000>;
>  			bus-range = <0x00 0xff>;
>  
> +			dma-coherent;
> +
>  			linux,pci-domain = <2>;
>  			num-lanes = <4>;
>  
> -- 
> 2.37.4
> 

-- 
மணிவண்ணன் சதாசிவம்