[PATCH v8 24/26] PCI: dwc: Set coherent DMA-mask on MSI-address allocation

[Serge Semin <Sergey.Semin@baikalelectronics.ru>]

The MSI target address requires to be reserved within the lowest 4GB
memory in order to support the PCIe peripherals with no 64-bit MSI TLPs
support. Since the allocation is done from the DMA-coherent memory let's
modify the allocation procedure to setting the coherent DMA-mask only and
avoiding the streaming DMA-mask modification. Thus at least the streaming
DMA operations would work with no artificial limitations. It will be
specifically useful for the eDMA-capable controllers so the corresponding
DMA-engine clients would map the DMA buffers with no need in the SWIOTLB
intervention for the buffers allocated above the 4GB memory region.

While at it let's add a brief comment about the reason of having the MSI
target address allocated from the DMA-coherent memory limited with the 4GB
upper bound.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>

---

Changelog v8:
- This is a new patch added on v8 stage of the series.
  (@Robin, @Christoph)
---
 drivers/pci/controller/dwc/pcie-designware-host.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
index 3ab6ae3712c4..e10608af39b4 100644
--- a/drivers/pci/controller/dwc/pcie-designware-host.c
+++ b/drivers/pci/controller/dwc/pcie-designware-host.c
@@ -366,7 +366,16 @@ static int dw_pcie_msi_host_init(struct dw_pcie_rp *pp)
 						    dw_chained_msi_isr, pp);
 	}
 
-	ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
+	/*
+	 * Even though the iMSI-RX Module supports 64-bit addresses some
+	 * peripheral PCIe devices may lack the 64-bit messages support. In
+	 * order not to miss MSI TLPs from those devices the MSI target address
+	 * has to be reserved within the lowest 4GB.
+	 * Note until there is a better alternative found the reservation is
+	 * done by allocating from the artificially limited DMA-coherent
+	 * memory.
+	 */
+	ret = dma_set_coherent_mask(dev, DMA_BIT_MASK(32));
 	if (ret)
 		dev_warn(dev, "Failed to set DMA mask to 32-bit. Devices with only 32-bit MSI support may not work properly\n");
 
-- 
2.38.1