Re: [PATCH v12 06/22] gpu: nova-core: Blackwell: use correct sysmem flush registers

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Hardware match requirement violation during sysmem flush configuration on GB10x.
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commit 4c62a61ec72a7cc29b5d20a82279ea5490f2e759
Author: John Hubbard <jhubbard@xxxxxxxxxx>

gpu: nova-core: Blackwell: use correct sysmem flush registers

This commit updates the Blackwell GPUs to route the sysmem flush page
registers to their proper locations. It defines the registers relative to
HSHUB0 for GB10x and FBHUB0 for GB20x, and implements the flush paths in
their respective HALs.

> diff --git a/drivers/gpu/nova-core/fb/hal/gb100.rs b/drivers/gpu/nova-core/fb/hal/gb100.rs
> index 8d63350abf8aa..ecea4ff446fff 100644
> --- a/drivers/gpu/nova-core/fb/hal/gb100.rs
> +++ b/drivers/gpu/nova-core/fb/hal/gb100.rs

[ ... ]

> +/// Write the sysmem flush page address through the GB10x HSHUB0 registers.
> +///
> +/// Both the primary and EG (egress) register pairs must be programmed to the same address,
> +/// as required by hardware.
> +fn write_sysmem_flush_page_gb100(bar: &Bar0, addr: Bounded<u64, 52>) {
> +    // CAST: lower 32 bits. Hardware ignores bits 7:0.
> +    let addr_lo = *addr as u32;
> +    let addr_hi = addr.shr::<32, 20>().cast::<u32>();
> +
> +    // Write HI first. The hardware will trigger the flush on the LO write.
> +
> +    // Primary HSHUB pair.
> +    bar.write(
> +        regs::NV_PFB_HSHUB_PCIE_FLUSH_SYSMEM_ADDR_HI::of::<Gb100>(),
> +        regs::NV_PFB_HSHUB_PCIE_FLUSH_SYSMEM_ADDR_HI::zeroed().with_adr(addr_hi),
> +    );
> +    bar.write(
> +        regs::NV_PFB_HSHUB_PCIE_FLUSH_SYSMEM_ADDR_LO::of::<Gb100>(),
> +        regs::NV_PFB_HSHUB_PCIE_FLUSH_SYSMEM_ADDR_LO::zeroed().with_adr(addr_lo),
> +    );

[Severity: High]
Does writing the primary LO register here trigger the flush before the EG
registers are updated to match?

The comments state that the hardware triggers the flush on the LO write, and
that both the primary and EG pairs must be programmed to the same address.
Because the EG pair is written after this primary LO register write, could
the hardware perform the flush while the EG registers still contain their old,
mismatched address?

> +
> +    // EG (egress) pair -- must match the primary pair.
> +    bar.write(
> +        regs::NV_PFB_HSHUB_EG_PCIE_FLUSH_SYSMEM_ADDR_HI::of::<Gb100>(),
> +        regs::NV_PFB_HSHUB_EG_PCIE_FLUSH_SYSMEM_ADDR_HI::zeroed().with_adr(addr_hi),
> +    );
> +    bar.write(
> +        regs::NV_PFB_HSHUB_EG_PCIE_FLUSH_SYSMEM_ADDR_LO::of::<Gb100>(),
> +        regs::NV_PFB_HSHUB_EG_PCIE_FLUSH_SYSMEM_ADDR_LO::zeroed().with_adr(addr_lo),
> +    );
> +}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260602032111.224790-1-jhubbard@xxxxxxxxxx?part=6