Re: [PATCH v9 5/6] cache: Add L2 cache management for Andes AX45MP RISC-V core

["Arnd Bergmann" <arnd@arndb.de>]

On Wed, Jun 14, 2023, at 12:47, Prabhakar wrote:

> +static void ax45mp_dma_cache_inv(phys_addr_t paddr, unsigned long size)
> +{
> +	unsigned long start = (unsigned long)phys_to_virt(paddr);
> +	char cache_buf[2][AX45MP_CACHE_LINE_SIZE];
> +	unsigned long end = start + size;
> +	unsigned long old_start = start;
> +	unsigned long old_end = end;
> +	unsigned long line_size;
> +	unsigned long flags;
> +
> +	if (unlikely(start == end))
> +		return;
> +
> +	line_size = ax45mp_priv.ax45mp_cache_line_size;
> +
> +	memset(&cache_buf, 0x0, sizeof(cache_buf));
> +	start = start & (~(line_size - 1));
> +	end = ((end + line_size - 1) & (~(line_size - 1)));
> +
> +	local_irq_save(flags);
> +	if (unlikely(start != old_start))
> +		memcpy(&cache_buf[0][0], (void *)start, line_size);
> +
> +	if (unlikely(end != old_end))
> +		memcpy(&cache_buf[1][0], (void *)(old_end & (~(line_size - 1))), 
> line_size);
> +
> +	ax45mp_cpu_dcache_inval_range(start, end, line_size);
> +
> +	if (unlikely(start != old_start))
> +		memcpy((void *)start, &cache_buf[0][0], (old_start & (line_size - 
> 1)));
> +
> +	local_irq_restore(flags);
> +}

I'm not quite sure what this does, maybe some comments would help.

This looks like a novel method of preserving partial cache lines
at the beginning (but not the end?) of an unaligned area.

As far as I can tell, most dma_mapping implementations don't
even try to do that at all, but the ones that do take a different
approach by calling _wback_inv() on partial cache lines instead
of calling _inv().

I'd say this does not belong into the low-level operations
here, especially since the ZICBOM variant doesn't do this either.

      Arnd