Re: [PATCH v5 1/2] PCI: dwc: Drop dependency on ZONE_DMA32

From: Robin Murphy
Date: Fri Sep 30 2022 - 11:39:25 EST

On 2022-09-30 13:57, Serge Semin wrote:
On Fri, Sep 30, 2022 at 12:01:58PM +0100, Robin Murphy wrote:
On 2022-09-29 20:32, Serge Semin wrote:
On Thu, Sep 29, 2022 at 07:25:03PM +0100, Robin Murphy wrote:
On 2022-09-28 12:41, Serge Semin wrote:
On Thu, Aug 25, 2022 at 06:50:24PM +0000, Will McVicker wrote:
Re-work the msi_msg DMA allocation logic to use dmam_alloc_coherent() which
uses the coherent DMA mask to try to return an allocation within the DMA
mask limits. With that, we now can drop the msi_page parameter in struct
dw_pcie_rp. This allows kernel configurations that disable ZONE_DMA32 to
continue supporting a 32-bit DMA mask. Without this patch, the PCIe host
device will fail to probe when ZONE_DMA32 is disabled.

As Rob already said here
https://lore.kernel.org/all/CAL_JsqJh=d-B51b6yPBRq0tOwbChN=AFPr-a19U1QdQZAE7c1A@xxxxxxxxxxxxxx/
and I mentioned in this thread
https://lore.kernel.org/linux-pci/20220912000211.ct6asuhhmnatje5e@mobilestation/
DW PCIe MSI doesn't cause any DMA due to the way the iMSI-RX engine is
designed. So reserving any real system memory is a waste of one in
this case. Reserving DMA-coherent even more inappropriate since it
can be expensive on some platforms (see note in Part Ia of
Documentation/core-api/dma-api.rst). For instance on MIPS32 with
non-corehent common DMA.


This has been discussed before - in general it is difficult to pick an
arbitrary MSI address that is *guaranteed* not to overlap any valid DMA
address that somebody may try to use later. However there is a very easy way
to guarantee that the DMA API won't give anyone a particular DMA address,
which is to get an address directly from the DMA API and keep it. Yes, that
can technically be done with a streaming mapping *if* you already have some
memory allocated in a suitable physical location, but coherent allocations
are even more foolproof, simpler to clean up (particularly with devres), and
unlikely to be an issue on relevant platforms (do any MIPS32 systems use
this driver?)

My patchset adds the DW PCIe RP controller support on MIPS32 arch:
https://lore.kernel.org/linux-pci/20220822184701.25246-21-Sergey.Semin@xxxxxxxxxxxxxxxxxxxx/


Fixes: 35797e672ff0 ("PCI: dwc: Fix MSI msi_msg DMA mapping")
Reported-by: Isaac J. Manjarres <isaacmanjarres@xxxxxxxxxx>
Signed-off-by: Will McVicker <willmcvicker@xxxxxxxxxx>
Acked-by: Jingoo Han <jingoohan1@xxxxxxxxx>
Reviewed-by: Rob Herring <robh@xxxxxxxxxx>
---
.../pci/controller/dwc/pcie-designware-host.c | 28 +++++--------------
drivers/pci/controller/dwc/pcie-designware.h | 1 -
2 files changed, 7 insertions(+), 22 deletions(-)

diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
index 7746f94a715f..39f3b37d4033 100644
--- a/drivers/pci/controller/dwc/pcie-designware-host.c
+++ b/drivers/pci/controller/dwc/pcie-designware-host.c
@@ -267,15 +267,6 @@ static void dw_pcie_free_msi(struct dw_pcie_rp *pp)
irq_domain_remove(pp->msi_domain);
irq_domain_remove(pp->irq_domain);
-
- if (pp->msi_data) {
- struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
- struct device *dev = pci->dev;
-
- dma_unmap_page(dev, pp->msi_data, PAGE_SIZE, DMA_FROM_DEVICE);
- if (pp->msi_page)
- __free_page(pp->msi_page);
- }
}
static void dw_pcie_msi_init(struct dw_pcie_rp *pp)
@@ -336,6 +327,7 @@ static int dw_pcie_msi_host_init(struct dw_pcie_rp *pp)
struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
struct device *dev = pci->dev;
struct platform_device *pdev = to_platform_device(dev);
+ u64 *msi_vaddr;
int ret;
u32 ctrl, num_ctrls;
@@ -375,22 +367,16 @@ static int dw_pcie_msi_host_init(struct dw_pcie_rp *pp)
dw_chained_msi_isr, pp);
}

- ret = dma_set_mask(dev, DMA_BIT_MASK(32));
+ ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));

This has been redundant in the first place since none of the DW PCIe
low-level drivers update the mask, and it's of 32-bits wide by default
anyway:
https://elixir.bootlin.com/linux/latest/source/drivers/of/platform.c#L167


No, in general drivers should always explicitly set their mask(s) and check
the return value to make sure DMA is possible at all before trying any other
DMA API calls. There's no guarantee that the default mask is usable (e.g.
some systems don't have any 32-bit addressable RAM), or that it's even
always 32 bits (due to crufty reasons of something of_dma_configure() tried
to do a long time ago).

Suppose you are right and DMA-mask should be always set before any
mapping. What do you suggest to do in this case? (1) The code above
overrides the real DMA-mask which could be set by the platform
drivers, which in its turn are normally aware of the device DMA
capabilities.

I am right. Appropriate DMA API usage as defined by the DMA API maintainers
is not a matter of supposition. I literally just explained right there why
drivers can't blindly assume the default mask is usable on modern systems
(yes, it was different 20 years ago when system topologies were simpler).


However, having now gone and looked at the whole driver rather than unclear
fragments of patch context, the code here *is* technically wrong. I've been
mistakenly thinking all along that this was operating on the PCI device
because I know that's what it *should* be doing, and seeing misleading
things like "dev = pci->dev" falsely affirmed that assumption that it would
be correct because it's been around for ages.
AFAIU the correct PCI device
won't actually exist until we've got far enough through pci_host_probe(), so
I'm not sure how to easily solve this :/

Right. The code affected by the subject patch has nothing to do with
the real PCI devices. The DMA-mask is set to the DW PCIe Host controller
platform device in order to force a page being allocated within 32-bit
address space. That's it.

Here is a log of the related changes:

0. Initially a GFP_KERNEL-based page was allocated for the MSI buffer.
It may cause having the DMA/PCIe-address above 4GB, which wouldn't work
for the PCIe peripherals with only 32-bit MSI capability. Though
nobody bothered back then.

1. 07940c369a6b ("PCI: dwc: Fix MSI page leakage in suspend/resume")
After this commit nothing really has changed, but instead of
allocating the MSI-message pseudo-buffer turned to be embedded into
the private data. It could be allocated at any base address with no
actual limitation (because private data is kmalloc'ed).

2. 660c486590aa ("PCI: dwc: Set 32-bit DMA mask for MSI target address allocation")
Someone found out that some devices failed to deliver MSI to the
address above 4GB of PCIe address space and fixed the problem by
force-setting the DMA-mask to being 32-bit before mapping the MSI
buffer. It indeed fixed the problem, but the actual buffer still left
being allocated from any address space. Instead, the mapping procedure
just bounced the buffer to 4GB space. So basically the solution was
very clumsy since turns a bounce buffer being reserved forever.

3. 35797e672ff0 PCI: dwc: Fix MSI msi_msg DMA mapping
@William basically got things back to (0) but instead of GFP_KERNEL
the page was allocated from GFP_DMA32. At this stage he should have
dropped the DMA-mask setting too since the buffer was already
guaranteed to be within 4GB space, but he didn't.

I never saw that change, but frankly the justification in the commit message is wrong. I know that there are Android systems with memory above 32 bits that run with SWIOTLB disabled because they think they know what they're doing, which are almost certainly the same ones that also want to disable ZONE_DMA32 for similar reasons. That patch is really just another hack around an unexpected configuration, but without saying so.

So now we have what we have. The DMA-mask is pointlessly changed for
something not really implying any DMA. That's why I insisted on
dropping it at the very least. Another reason I thought was also
appropriate was the default DMA-mask being set to 32bits anyway.
But you said we shouldn't rely on the default DMA-mask setting. So
ok, it doesn't count then. But it doesn't change the uselessness of the
DMA-mask change in the current driver.

As I keep saying, it *is* relevant to DMA. The MSI doorbell may not be accessing memory, but it is still a thing that occupies DMA address space like a mapping of memory does, and DMA masks are how we control how DMA address space is allocated. Unless and until we have an API for arbitrarily reserving DMA address space within a given range, the approach used here and in other drivers is the next best thing, however much you don't like it.

AFAIU the correct PCI device
won't actually exist until we've got far enough through pci_host_probe(), so
I'm not sure how to easily solve this :/

Walk over all PCIe devices detected on the PCIe-bus. Check their
MSI-capability flags. If any of them have no 64-bit MSI flag set, then
make sure the MSI-base address is allocated within 4GB memory region.
It isn't that easy to implement though...

It has nothing to do with capabilities (but also: consider hotplug). We simply need the host bridge PCI device to pass to the DMA API to ensure that the mapping/allocation is relative to PCI Mem space rather than system physical address space, because the two don't have to be identical. The challenge is how to reliably pick up that device and set up the doorbell *before* any other PCI devices also discovered by pci_host_probe() have a chance to start binding drivers and trying to request MSIs.

Of course *this* patch doesn't change any of that either, so it's no worse
than the existing code and I don't see that dropping it helps you at all;
the current driver is already trampling your 64-bit mask back to 32 bits

Yes, and by this pathset @William intend to fix the DMA-mask-override
behaviour by using the dma_alloc_coherent() method.

No, that is effectively unchanged. Whether it's a streaming mapping with dma_mask or a coherent allocation with coherent_dma_mask, masks are getting set way, it's the fact that it's on the wrong device that's the real problem.

If you expose the eDMA as a generic dmaengine device then there's every chance some consumer would make a streaming mapping with it, so even though the current code happens to not override the coherent mask it's still biting you in the streaming mask.

So any
platform-specific DMA-mask setting will be overwritten, and the
DMA-mask setting won't be able to be moved/dropped due to the
dma_alloc_coherent() method usage.

I have added a DW eDMA-engine support to the DW PCIe driver (you've
already seen my patches) and the engine initialization is supposed to
be performed after any basic initializations like CSRs mapping, data
allocations, MSI, etc. Since DMA is performed by the controller itself
it's required to have a correct DMA-mask set to the DW PCIe platform
device otherwise any consequent mapping will be bounce buffered to the
lowest 4GB even though the corresponding platform can support more
than 4GB of memory (even our MIPS32 can) with DW eDMA easily reaching
that memory. What would help me in this case if the MSI-buffer
allocation procedure wouldn't change the device DMA-mask. As an
alternative to completely dropping the DMA-mask setting, the DMA-mask
setup process could be moved to the low-level platform device drivers.
It would be even more justified since the platform-specific device
capabilities (like DW PCIe AXI-interface address-bus width) are
unknown in the generic code.

As another alternative I could override the DMA-mask within the DW
eDMA probe procedure. But that would make things more complicated than
relying on the low-level platform drivers doing that.

and
reserving the doorbell address in the wrong DMA address space (modulo the
other dma-ranges bug which also took far too long to figure out).

Actually current driver (without William patch) reserve the doorbell
address in the correct DMA address space (if we don't take the
dma-ranges settings into account).

No it does not. With or without this patch it is still passing the *platform device* to the DMA API, which means the mapping is relative to the platform address space, not PCI Mem space on the other side of the iATU. The fact that the iATU's dma-ranges translation is erroneously applied to the platform device at the moment is, as I have said, a bug.

It works as expected in case if the
PCIe<->CPU space has one-on-one mapping (which is true in the most of
the cases). The only thing which is wrong is the pointless DMA-mask
update. I could have easily dropped it in my patchset. But after the
@William patchset is applied I won't be able to do that due to using
the dma_alloc_coherent() here.

Once again, it is not pointless. There is no guarantee that __GFP_DMA32 does anything, since ZONE_DMA32 may not exist (per this patch), and the zones may not be as expected anyway (look at what arm64 currently does if all RAM is above 32 bits, but save those complaints for another thread).

At this
point I'd rather keep it since getting rid of the __GFP_DMA32 abuse is
objectively good. If losing one page of coherent memory is a measurably
significant problem for T1 once the other issues are worked out and that
series lands, then you're welcome to propose a change on top (but I would
prefer that all the drivers using this trick are changed consistently).

Regarding DMA-coherent allocation. I am not happy with losing a whole
page of the dma-coherent memory, but we can live with that. What give
additional difficulty for our eDMA-patches is the DMA-mask override.
If you still insist on preserving the @William patchset as it is,
where do you suggest for me to update the DMA-mask if the low-level
driver won't be suitable for that anymore?

I'm not insisting anything, it's just that this patch is already reviewed and queued, is a small step towards being less wrong overall, and dropping it wouldn't actually solve any of your problems anyway, so I just feel that being obstructive because it falls short of perfection isn't helpful.

Robin.