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

[Hanjun Guo <guohanjun@huawei.com>]

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.

Thanks
Hanjun

[0]:
https://github.com/tianocore/edk2-platforms/blob/master/Silicon/Hisilicon/Hi1616/D05AcpiTables/D05Iort.asl
[1]:
https://github.com/tianocore/edk2-platforms/blob/master/Silicon/Hisilicon/Hi1620/Hi1620AcpiTables/Hi1620Iort.asl
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