Re: [PATCH] powerpc: Enhance pmem DMA bypass handling

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 22/10/2021 04:44, Brian King wrote:
> If ibm,pmemory is installed in the system, it can appear anywhere
> in the address space. This patch enhances how we handle DMA for devices when
> ibm,pmemory is present. In the case where we have enough DMA space to
> direct map all of RAM, but not ibm,pmemory, we use direct DMA for
> I/O to RAM and use the default window to dynamically map ibm,pmemory.
> In the case where we only have a single DMA window, this won't work, > so if the window is not big enough to map the entire address range,
> we cannot direct map.

but we want the pmem range to be mapped into the huge DMA window too if 
we can, why skip it?


> 
> Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
> ---
>   arch/powerpc/platforms/pseries/iommu.c | 19 ++++++++++---------
>   1 file changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
> index 269f61d519c2..d9ae985d10a4 100644
> --- a/arch/powerpc/platforms/pseries/iommu.c
> +++ b/arch/powerpc/platforms/pseries/iommu.c
> @@ -1092,15 +1092,6 @@ static phys_addr_t ddw_memory_hotplug_max(void)
>   	phys_addr_t max_addr = memory_hotplug_max();
>   	struct device_node *memory;
>   
> -	/*
> -	 * The "ibm,pmemory" can appear anywhere in the address space.
> -	 * Assuming it is still backed by page structs, set the upper limit
> -	 * for the huge DMA window as MAX_PHYSMEM_BITS.
> -	 */
> -	if (of_find_node_by_type(NULL, "ibm,pmemory"))
> -		return (sizeof(phys_addr_t) * 8 <= MAX_PHYSMEM_BITS) ?
> -			(phys_addr_t) -1 : (1ULL << MAX_PHYSMEM_BITS);
> -
>   	for_each_node_by_type(memory, "memory") {
>   		unsigned long start, size;
>   		int n_mem_addr_cells, n_mem_size_cells, len;
> @@ -1341,6 +1332,16 @@ static bool enable_ddw(struct pci_dev *dev, struct device_node *pdn)
>   	 */
>   	len = max_ram_len;
>   	if (pmem_present) {
> +		if (default_win_removed) {
> +			/*
> +			 * If we only have one DMA window and have pmem present,
> +			 * then we need to be able to map the entire address
> +			 * range in order to be able to do direct DMA to RAM.
> +			 */
> +			len = order_base_2((sizeof(phys_addr_t) * 8 <= MAX_PHYSMEM_BITS) ?
> +					(phys_addr_t) -1 : (1ULL << MAX_PHYSMEM_BITS));
> +		}
> +
>   		if (query.largest_available_block >=
>   		    (1ULL << (MAX_PHYSMEM_BITS - page_shift)))
>   			len = MAX_PHYSMEM_BITS;
> 

-- 
Alexey