Re: [PATCH v3 2/7] iommu/riscv: Add RISC-V IOMMU platform device driver

[Baolu Lu]

On 2024/5/1 4:01, Tomasz Jeznach wrote:
> +static int riscv_iommu_init_check(struct riscv_iommu_device *iommu)
> +{
> +	u64 ddtp;
> +
> +	/*
> +	 * Make sure the IOMMU is switched off or in pass-through mode during regular
> +	 * boot flow and disable translation when we boot into a kexec kernel and the
> +	 * previous kernel left them enabled.
> +	 */
> +	ddtp = riscv_iommu_readq(iommu, RISCV_IOMMU_REG_DDTP);
> +	if (ddtp & RISCV_IOMMU_DDTP_BUSY)
> +		return -EBUSY;
> +
> +	if (FIELD_GET(RISCV_IOMMU_DDTP_MODE, ddtp) > RISCV_IOMMU_DDTP_MODE_BARE) {
> +		if (!is_kdump_kernel())

Is kdump supported for RISC-V architectures?  If so, the documentation
in Documentation/admin-guide/kdump/kdump.rst might need an update.

There is a possibility of ongoing DMAs during the boot process of the
kdump capture kernel because there's a small chance of legacy DMA setups
targeting any memory location. Kdump typically allows these ongoing DMA
transfers to complete, assuming they were intended for valid memory
regions.

The IOMMU subsystem implements a default domain deferred attachment
mechanism for this. In the kdump capture kernel, the whole device
context tables are copied from the original kernel and will be
overridden once the device driver calls the kernel DMA interface for the
first time. This assumes that all old DMA transfers are completed after
the driver's takeover.

Will you consider this for RISC-V architecture as well?

> +			return -EBUSY;
> +		riscv_iommu_disable(iommu);
> +	}
> +
> +	/* Configure accesses to in-memory data structures for CPU-native byte order. */
> +	if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN) != !!(iommu->fctl & RISCV_IOMMU_FCTL_BE)) {
> +		if (!(iommu->caps & RISCV_IOMMU_CAP_END))
> +			return -EINVAL;
> +		riscv_iommu_writel(iommu, RISCV_IOMMU_REG_FCTL,
> +				   iommu->fctl ^ RISCV_IOMMU_FCTL_BE);
> +		iommu->fctl = riscv_iommu_readl(iommu, RISCV_IOMMU_REG_FCTL);
> +		if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN) != !!(iommu->fctl & RISCV_IOMMU_FCTL_BE))
> +			return -EINVAL;
> +	}
> +
> +	return 0;
> +}

Best regards,
baolu