[PATCH v4 7/7] Documentation: PCI: endpoint: Document vNTB DMA export

[Koichiro Den]

Document the dma_bar attribute and the requirements for exporting endpoint
DMA channels. Also document how the selected BAR may be shared with an MW.

Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
---
Changes in v4:
  - Document operation without DMA when the advertised type or a DMA IRQ is
    unavailable. (Sashiko)
  - List the constraints for sharing DMA and MW in one BAR.
  - Fix the misleading sentence: s/select their BAR/set dma_bar/.

 Documentation/PCI/endpoint/pci-vntb-howto.rst | 30 +++++++++++++++++--
 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/Documentation/PCI/endpoint/pci-vntb-howto.rst b/Documentation/PCI/endpoint/pci-vntb-howto.rst
index 3679f5c30254..b5f843bdae38 100644
--- a/Documentation/PCI/endpoint/pci-vntb-howto.rst
+++ b/Documentation/PCI/endpoint/pci-vntb-howto.rst
@@ -90,9 +90,9 @@ of the function device and is populated with the following NTB specific
 attributes that can be configured by the user::
 
 	# ls functions/pci_epf_vntb/func1/pci_epf_vntb.0/
-	ctrl_bar  db_count  mw1_bar  mw2_bar  mw3_bar  mw4_bar	spad_count
-	db_bar	  mw1	    mw2      mw3      mw4      num_mws	vbus_number
-	vntb_vid  vntb_pid
+	ctrl_bar  dma_bar  mw2      mw3_bar  num_mws     vntb_pid
+	db_bar    mw1      mw2_bar  mw4      spad_count  vntb_vid
+	db_count  mw1_bar  mw3      mw4_bar  vbus_number
 
 A sample configuration for NTB function is given below::
 
@@ -105,6 +105,30 @@ By default, each construct is assigned a BAR, as needed and in order.
 Should a specific BAR setup be required by the platform, BAR may be assigned
 to each construct using the related ``XYZ_bar`` entry.
 
+To export DMA channels, set dma_bar before binding the function::
+
+	# echo 5 > functions/pci_epf_vntb/func1/pci_epf_vntb.0/dma_bar
+
+This currently supports the unrolled DesignWare eDMA layout. If any DMA
+resource has no fixed BAR assignment, the EPC must support subrange and
+dynamic inbound mappings.
+
+Leaving ``dma_bar`` unassigned disables DMA export.
+With a separate DMA BAR, an older ``ntb_hw_epf`` peer can ignore the extension
+and continue without DMA. A peer that understands the extension also continues
+without DMA if the advertised type is unavailable or no DMA IRQ can be
+allocated.
+
+When ``dma_bar`` selects an MW BAR, DMA resources follow that MW. In this
+configuration:
+
+* The shared MW size must be a power of two and its translation must cover the
+  full configured size.
+* Any rounded-up BAR tail is scratch-backed.
+* An updated peer is required because older peers treat the whole BAR as an
+  MW.
+* Control and doorbell BARs cannot be shared.
+
 A sample configuration for virtual NTB driver for virtual PCI bus::
 
 	# echo 0x1957 > functions/pci_epf_vntb/func1/pci_epf_vntb.0/vntb_vid
-- 
2.51.0