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

[Brian King <brking@linux.vnet.ibm.com>]

On 10/22/21 7:24 AM, Alexey Kardashevskiy wrote:
> 
> 
> 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?

This patch should simply do what the comment in this commit mentioned below suggests, which says that
ibm,pmemory can appear anywhere in the address space. If the DMA window is large enough
to map all of MAX_PHYSMEM_BITS, we will indeed simply do direct DMA for everything,
including the pmem. If we do not have a big enough window to do that, we will do
direct DMA for DRAM and dynamic mapping for pmem.


https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/arch/powerpc/platforms/pseries/iommu.c?id=bf6e2d562bbc4d115cf322b0bca57fe5bbd26f48


Thanks,

Brian


> 
> 
>>
>> 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;
>>
> 


-- 
Brian King
Power Linux I/O
IBM Linux Technology Center