Re: [PATCH v3 7/8] arm64: mm: Set ZONE_DMA size based on early IORT scan

[Hanjun Guo <guohanjun@huawei.com>]

Hi Ard,

On 2020/10/16 14:54, Ard Biesheuvel wrote:
> On Fri, 16 Oct 2020 at 08:51, Hanjun Guo <guohanjun@huawei.com> wrote:
>>
>> On 2020/10/16 2:03, Catalin Marinas wrote:
>>> On Thu, Oct 15, 2020 at 10:26:18PM +0800, Hanjun Guo wrote:
>>>> On 2020/10/15 3:12, Nicolas Saenz Julienne wrote:
>>>>> From: Ard Biesheuvel <ardb@kernel.org>
>>>>>
>>>>> We recently introduced a 1 GB sized ZONE_DMA to cater for platforms
>>>>> incorporating masters that can address less than 32 bits of DMA, in
>>>>> particular the Raspberry Pi 4, which has 4 or 8 GB of DRAM, but has
>>>>> peripherals that can only address up to 1 GB (and its PCIe host
>>>>> bridge can only access the bottom 3 GB)
>>>>>
>>>>> Instructing the DMA layer about these limitations is straight-forward,
>>>>> even though we had to fix some issues regarding memory limits set in
>>>>> the IORT for named components, and regarding the handling of ACPI _DMA
>>>>> methods. However, the DMA layer also needs to be able to allocate
>>>>> memory that is guaranteed to meet those DMA constraints, for bounce
>>>>> buffering as well as allocating the backing for consistent mappings.
>>>>>
>>>>> This is why the 1 GB ZONE_DMA was introduced recently. Unfortunately,
>>>>> it turns out the having a 1 GB ZONE_DMA as well as a ZONE_DMA32 causes
>>>>> problems with kdump, and potentially in other places where allocations
>>>>> cannot cross zone boundaries. Therefore, we should avoid having two
>>>>> separate DMA zones when possible.
>>>>>
>>>>> So let's do an early scan of the IORT, and only create the ZONE_DMA
>>>>> if we encounter any devices that need it. This puts the burden on
>>>>> the firmware to describe such limitations in the IORT, which may be
>>>>> redundant (and less precise) if _DMA methods are also being provided.
>>>>> However, it should be noted that this situation is highly unusual for
>>>>> arm64 ACPI machines. Also, the DMA subsystem still gives precedence to
>>>>> the _DMA method if implemented, and so we will not lose the ability to
>>>>> perform streaming DMA outside the ZONE_DMA if the _DMA method permits
>>>>> it.
>>>>
>>>> Sorry, I'm still a little bit confused. With this patch, if we have
>>>> a device which set the right _DMA method (DMA size >= 32), but with the
>>>> wrong DMA size in IORT, we still have the ZONE_DMA created which
>>>> is actually not needed?
>>>
>>> With the current kernel, we get a ZONE_DMA already with an arbitrary
>>> size of 1GB that matches what RPi4 needs. We are trying to eliminate
>>> such unnecessary ZONE_DMA based on some heuristics (well, something that
>>> looks "better" than a OEM ID based quirk). Now, if we learn that IORT
>>> for platforms in the field is that broken as to describe few bits-wide
>>> DMA masks, we may have to go back to the OEM ID quirk.
>>
>> Some platforms using 0 as the memory size limit, for example D05 [0] and
>> D06 [1], I think we need to go back to the OEM ID quirk.
>>
>> For D05/D06, there are multi interrupt controllers named as mbigen,
>> mbigen is using the named component to describe the mappings with
>> the ITS controller, and mbigen is using 0 as the memory size limit.
>>
>> Also since the memory size limit for PCI RC was introduced by later
>> IORT revision, so firmware people may think it's fine to set that
>> as 0 because the system works without it.
>>
> 
> Hello Hanjun,
> 
> The patch only takes the address limit field into account if its value > 0.

Sorry I missed the if (*->memory_address_limit) check, thanks
for the reminding.

> 
> Also, before commit 7fb89e1d44cb6aec ("ACPI/IORT: take _DMA methods
> into account for named components"), the _DMA method was not taken
> into account for named components at all, and only the IORT limit was
> used, so I do not anticipate any problems with that.

Then this patch is fine to me.

Thanks
Hanjun