Hi Ard,
On 2020/10/16 14:54, Ard Biesheuvel wrote:
On Fri, 16 Oct 2020 at 08:51, Hanjun Guo <guohanjun@xxxxxxxxxx> 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@xxxxxxxxxx>
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
