[PATCH v9 2/2] Documentation: fpga: dfl: Add description for DFL UIO support

[Xu Yilun <yilun.xu@intel.com>]

This patch adds description for UIO support for dfl devices on DFL
bus.

Signed-off-by: Xu Yilun <yilun.xu@intel.com>
---
v2: no doc in v1, add it for v2.
v3: some documentation fixes.
v4: documentation change since the driver matching is changed.
v5: no change.
v6: improve the title of the userspace driver support section.
    some word improvement.
v7: rebased to next-20210119
v8: some doc fixes.
v9: some doc change since we switch to the driver in drivers/uio.
---
 Documentation/fpga/dfl.rst | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/Documentation/fpga/dfl.rst b/Documentation/fpga/dfl.rst
index c41ac76..e35cf87 100644
--- a/Documentation/fpga/dfl.rst
+++ b/Documentation/fpga/dfl.rst
@@ -7,6 +7,7 @@ Authors:
 - Enno Luebbers <enno.luebbers@intel.com>
 - Xiao Guangrong <guangrong.xiao@linux.intel.com>
 - Wu Hao <hao.wu@intel.com>
+- Xu Yilun <yilun.xu@intel.com>
 
 The Device Feature List (DFL) FPGA framework (and drivers according to
 this framework) hides the very details of low layer hardwares and provides
@@ -530,6 +531,28 @@ Being able to specify more than one DFL per BAR has been considered, but it
 was determined the use case did not provide value.  Specifying a single DFL
 per BAR simplifies the implementation and allows for extra error checking.
 
+
+Userspace driver support for DFL devices
+========================================
+The purpose of an FPGA is to be reprogrammed with newly developed hardware
+components. New hardware can instantiate a new private feature in the DFL, and
+then present a DFL device in the system. In some cases users may need a
+userspace driver for the DFL device:
+
+* Users may need to run some diagnostic test for their hardware.
+* Users may prototype the kernel driver in user space.
+* Some hardware is designed for specific purposes and does not fit into one of
+  the standard kernel subsystems.
+
+This requires direct access to MMIO space and interrupt handling from
+userspace. The uio_dfl module exposes the UIO device interfaces for this
+purpose.
+
+UIO_DFL should be selected to enable the uio_dfl module driver. To support a
+new DFL feature via UIO direct access, its feature id should be added to the
+driver's id_table.
+
+
 Open discussion
 ===============
 FME driver exports one ioctl (DFL_FPGA_FME_PORT_PR) for partial reconfiguration
-- 
2.7.4