On 17/03/2019 23:36, Marek Vasut wrote:
On 3/17/19 11:29 AM, Geert Uytterhoeven wrote:
Hi Marek,


On Sun, Mar 17, 2019 at 12:04 AM Marek Vasut <marek.va...@gmail.com> wrote:
On 3/16/19 10:25 PM, Marek Vasut wrote:
On 3/13/19 7:30 PM, Christoph Hellwig wrote:
On Sat, Mar 09, 2019 at 12:23:15AM +0100, Marek Vasut wrote:
On 3/8/19 8:18 AM, Christoph Hellwig wrote:
On Thu, Mar 07, 2019 at 12:14:06PM +0100, Marek Vasut wrote:
Right, but whoever *interprets* the device masks after the driver has
overridden them should be taking the (smaller) bus mask into account as
well, so the question is where is *that* not being done correctly?

Do you have a hint where I should look for that ?

If this a 32-bit ARM platform it might the complete lack of support
for bus_dma_mask in arch/arm/mm/dma-mapping.c..

It's an ARM 64bit platform, just the PCIe controller is limited to 32bit
address range, so the devices on the PCIe bus cannot read the host's
DRAM above the 32bit limit.

arm64 should take the mask into account both for the swiotlb and
iommu case.  What are the exact symptoms you see?

With the nvme, the device is recognized, but cannot be used.
It boils down to PCI BAR access being possible, since that's all below
the 32bit boundary, but when the device tries to do any sort of DMA,
that transfer returns nonsense data.

But when I call dma_set_mask_and_coherent(dev->dev, DMA_BIT_MASK(32) in
the affected driver (thus far I tried this nvme, xhci-pci and ahci-pci
drivers), it all starts to work fine.

Could it be that the driver overwrites the (coherent_)dma_mask and
that's why the swiotlb/iommu code cannot take this into account ?

Does it involve
swiotlb not kicking in, or iommu issues?

How can I check ? I added printks into arch/arm64/mm/dma-mapping.c and
drivers/iommu/dma-iommu.c , but I suspect I need to look elsewhere.

Digging further ...

drivers/nvme/host/pci.c nvme_map_data() calls dma_map_sg_attrs() and the
resulting sglist contains entry with >32bit PA. This is because
dma_map_sg_attrs() calls dma_direct_map_sg(), which in turn calls
dma_direct_map_sg(), then dma_direct_map_page() and that's where it goes

dma_direct_map_page() does a dma_direct_possible() check before
triggering swiotlb_map(). The check succeeds, so the later isn't executed.

dma_direct_possible() calls dma_capable() with dev->dma_mask =
DMA_BIT_MASK(64) and dev->dma_bus_mask = 0, so
min_not_zero(*dev->dma_mask, dev->bus_dma_mask) returns DMA_BIT_MASK(64).

Surely enough, if I hack dma_direct_possible() to return 0,
swiotlb_map() kicks in and the nvme driver starts working fine.

I presume the question here is, why is dev->bus_dma_mask = 0 ?

Because that's the default, and almost no code overrides that?

But shouldn't drivers/of/device.c set that for the PCIe controller ?

Urgh, I really should have spotted the significance of "NVMe", but somehow it failed to click :(

Of course the existing code works fine for everything *except* PCI devices on DT-based systems... That's because of_dma_get_range() has never been made to work correctly with the trick we play of passing the host bridge of_node through of_dma_configure(). I've got at least 2 or 3 half-finished attempts at improving that, but they keep getting sidetracked into trying to clean up the various new of_dma_configure() hacks I find in drivers and/or falling down the rabbit-hole of starting to redesign the whole dma_pfn_offset machinery entirely. Let me dig one up and try to constrain it to solve just this most common "one single limited range" condition for the sake of making actual progress...


$ git grep "\<bus_dma_mask ="
arch/mips/pci/fixup-sb1250.c:           dev->dev.bus_dma_mask =
arch/x86/kernel/pci-dma.c:      pdev->dev.bus_dma_mask = DMA_BIT_MASK(32);
drivers/acpi/arm64/iort.c:              dev->bus_dma_mask = mask;
drivers/of/device.c:            dev->bus_dma_mask = mask;

dev is the nvme PCI device, I assume? So you can ignore the last match.

The first two seem to be related to platforms that cannot do >32 bit DMA
on PCI. So that's a hint on how to fix this...

That doesn't feel right, it's not a platform limitation, but a PCIe IP
limitation, so this fix should live somewhere in drivers/ I think ? >

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