Re: DMABOUNCE in pci-rcar

From: Ben Dooks
Date: Thu Mar 20 2014 - 13:25:41 EST

On 20/03/14 17:41, Arnd Bergmann wrote:
On Thursday 20 March 2014 17:12:43 Ben Dooks wrote:
On 20/03/14 17:09, Arnd Bergmann wrote:
On Thursday 20 March 2014 16:04:36 Ben Dooks wrote:

As a note, if we now boot a Lager with DT on 3.14-rc3 series the USB
controllers no longer work with full 2GiB RAM enabled in the device

Did it work before these patches got applied initially?

It did, but I cannot remember if we where limiting DRAM to 1GiB
or not.

I would assume you did, or you happened to never actually use
memory higher than that for DMA during tests.

Could we work around this by having 1GiB of memory defined in the
32bit memory and then add the rest of the 3GiB from the >32bit
area via LPAE? Will the kernel ever try to allocate DMA memory from
anything >32bit?

You can solve the case for dma_alloc_coherent() this way, or by
setting the mask correctly. It won't help you for dma_map_* though,
which still requires someone to add support for swiotlb or using
an IOMMU if present.

We do not have an IOMMU present at the moment. Not sure how
to go about setting a mask on a pci-probed device.

Ah, right. The mask is a problem because PCI devices assume that
they can do DMA to any 32-bit masked address without calling
dma_set_mask. Your trick to describe the system memory at a different
physical alias would solve this part. Another option might be to
add a quirk in the OHCI/EHCI drivers, if you are able to detect
this special case from the PCI IDs. Whether we can allow the PCI host
controller to just set a mask is an open question. If we decide to go
that route, you should be able to do it using the add_bus() callback
in the host controller driver. However, it would be a departure from
the normal way of doing PCI DMA, and I can't foresee what the implications
would be.

So doing:

static void pci_rcar_fixup(struct pci_dev *dev)
if (dev->bus->ops == &rcar_pci_ops) {
dev_info(&dev->dev, "applying new dma mask\n");
dev->dma_mask = DMA_BIT_MASK(31);


Did not work for me :(

Ben Dooks
Senior Engineer Codethink - Providing Genius
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