Re: [PATCH v7 23/25] PCI: dwc: Restore DMA-mask after MSI-data allocation

[Serge Semin]

Hi Christoph

On Wed, Dec 14, 2022 at 11:13:41PM -0800, Christoph Hellwig wrote:
> On Thu, Dec 15, 2022 at 02:53:03AM +0300, Serge Semin wrote:
> > DW PCIe Root Ports and End-points can be equipped with the DW eDMA engine.
> > In that case it is critical to have the platform device pre-initialized
> > with a valid DMA-mask so the drivers using the eDMA-engine would be able
> > to allocate the DMA-able buffers. The MSI-capable data requires to be
> > allocated from the lowest 4GB region. Since that procedure implies the
> > DMA-mask change we need to restore the mask set by the low-level drivers
> > after the MSI-data allocation is done.
> 
> You can't change the DMA mask when there are existing allocations.

Em, what do you guys suggest for the DW PCIe devices with the embedded
DMA-engine then? To live forever with the SWIOTLBs? I can't drop the
DMA-mask update due to this commit 423511ec23e2 ("PCI: dwc: Drop
dependency on ZONE_DMA32") and I can't change the mask after it's
updated. Note it's updated for the memory allocation to which actually
no DMA will be performed, see
https://lore.kernel.org/linux-pci/20220825185026.3816331-2-willmcvicker@xxxxxxxxxx/.
My patches imply adding the real DMA operations support.

We've discussed this a lot with Robin in various threads and I thought
a workable solution was found. I was going to update the mask in
another place, but basically it would still mean to have first setting
the 32-bit mask here, and then change it to 64-bit one in the
framework of the DW eDMA driver.

So to speak I don't see a proper way out from the situation. Nothing I
suggested was accepted and now we'll have to live with the SWIOTLBs
used for the memory above 4GB. So please suggest a workable solution
then. We need the next things:
1. Somehow preserve a single DWORD of the PCIe bus memory for the
iMSI-RX engine. (That's what is currently done the
dw_pcie_msi_host_init() method by allocating the coherent memory.)
2. Set the actual DMA-mask to the DW PCIe platform device so the
DMA-engine clients would be able to allocate actually DMA-able memory.

@Robin, please join the discussion.

-Serge(y)