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

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 23/10/2021 07:18, Brian King wrote:
> 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.


Right, and this is what we do already, do not we? I missing something here.

> 
> 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;
>>>
>>
> 
> 

-- 
Alexey