[PATCH 3/4] riscv: Implement Zicbom-based cache management operations

[Christoph Hellwig]

On Sun, Jun 19, 2022 at 10:32:11PM +0200, Heiko Stuebner wrote:
> +#ifdef CONFIG_RISCV_DMA_NONCOHERENT
> +#define ARCH_DMA_MINALIGN L1_CACHE_BYTES
> +#endif

This needs to be greater or equal to riscv_cbom_block_size, but the
core code requires a compile time constant here.  So we'll need a big
fat comment here, and panic if riscv_cbom_block_size is >
L1_CACHE_BYTES/ARCH_DMA_MINALIGN in the code that queries
riscv_cbom_block_size.

Note that the arm64 folks are looking into making this variable or
killing it off in this current form, so things might be getting better
soon.

> +void arch_sync_dma_for_device(phys_addr_t paddr, size_t size,
> +			      enum dma_data_direction dir)
> +{
> +	void *vaddr = phys_to_virt(paddr);
> +
> +	switch (dir) {
> +	case DMA_TO_DEVICE:
> +		ALT_CMO_OP(clean, vaddr, size, riscv_cbom_block_size);
> +		break;
> +	case DMA_FROM_DEVICE:
> +		ALT_CMO_OP(inval, vaddr, size, riscv_cbom_block_size);
> +		break;

For this also see:

https://lore.kernel.org/all/20220606152150.GA31568@willie-the-truck/

and

https://lore.kernel.org/linux-arm-kernel/20220610151228.4562-1-will@kernel.org/T/

> +void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> +		const struct iommu_ops *iommu, bool coherent)
> +{
> +	dev->dma_coherent = coherent;
> +}

This probably wants a sanity check warn if coherent if false without
any support for cache flushing as that will cause data corruption.