Add documentation for the three ioctls used to attach or detach
externally-created DMABUFs, and to request transfers from/to previously
attached DMABUFs.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
---
v3: New patch
---
Documentation/usb/functionfs.rst | 36 ++++++++++++++++++++++++++++++++
1 file changed, 36 insertions(+)
diff --git a/Documentation/usb/functionfs.rst b/Documentation/usb/functionfs.rst
index a3054bea38f3..d05a775bc45b 100644
--- a/Documentation/usb/functionfs.rst
+++ b/Documentation/usb/functionfs.rst
@@ -2,6 +2,9 @@
How FunctionFS works
====================
+Overview
+========
+
From kernel point of view it is just a composite function with some
unique behaviour. It may be added to an USB configuration only after
the user space driver has registered by writing descriptors and
@@ -66,3 +69,36 @@ have been written to their ep0's.
Conversely, the gadget is unregistered after the first USB function
closes its endpoints.
+
+DMABUF interface
+================
+
+FunctionFS additionally supports a DMABUF based interface, where the
+userspace can attach DMABUF objects (externally created) to an endpoint,
+and subsequently use them for data transfers.
+
+A userspace application can then use this interface to share DMABUF
+objects between several interfaces, allowing it to transfer data in a
+zero-copy fashion, for instance between IIO and the USB stack.
+
+As part of this interface, three new IOCTLs have been added. These three
+IOCTLs have to be performed on a data endpoint (ie. not ep0). They are:
+
+ ``FUNCTIONFS_DMABUF_ATTACH(int)``
+ Attach the DMABUF object, identified by its file descriptor, to the
+ data endpoint. Returns zero on success, and a negative errno value
+ on error.
+
+ ``FUNCTIONFS_DMABUF_DETACH(int)``
+ Detach the given DMABUF object, identified by its file descriptor,
+ from the data endpoint. Returns zero on success, and a negative
+ errno value on error. Note that closing the endpoint's file
+ descriptor will automatically detach all attached DMABUFs.
+
+ ``FUNCTIONFS_DMABUF_TRANSFER(struct usb_ffs_dmabuf_transfer_req *)``
+ Enqueue the previously attached DMABUF to the transfer queue.
+ The argument is a structure that packs the DMABUF's file descriptor,
+ the size in bytes to transfer (which should generally correspond to
+ the size of the DMABUF), and a 'flags' field which is unused
+ for now. Returns zero on success, and a negative errno value on
+ error.
--
2.43.0On 08/01/2024 13:00, Paul Cercueil wrote:
> Add documentation for the three ioctls used to attach or detach
> externally-created DMABUFs, and to request transfers from/to previously
> attached DMABUFs.
>
> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
>
> ---
> v3: New patch
> ---
> Documentation/usb/functionfs.rst | 36 ++++++++++++++++++++++++++++++++
> 1 file changed, 36 insertions(+)
Hi,
I'd like to point out that this file (usb/functionfs.rst) is currently
included by Documentation/subsystem-apis.rst, the top-level file for the
"Kernel subsystem documentation" set of books, which describe internal
APIs: "These books get into the details of how specific kernel
subsystems work from the point of view of a kernel developer".
However, functionfs.rst (and especially your new additions) are
documenting a userspace API, so it really belongs somewhere in
Documentation/userspace-api/ -- that's where /proc, /sys, /dev and ioctl
descriptions for userspace programmers belong.
I'm not NAKing the patch -- I just want to draw attention to this
discrepancy. Maybe we can separate the kernel-implementation details
(stuff about __init sections and stuff) from the new ioctl() info?
Looking at <https://docs.kernel.org/usb/> I see that there are many
other adjacent documents that are also not really documenting kernel
implementation details, rough categorization as follows:
USB support
-----------
- Linux ACM driver v0.16 ==> admin/user info
- Authorizing (or not) your USB devices to connect to the system ==>
admin/user info
- ChipIdea Highspeed Dual Role Controller Driver => admin/user info
- DWC3 driver ==> driver TODOs (can be moved into source code?)
- EHCI driver ==> technical info + driver details
- How FunctionFS works
- Linux USB gadget configured through configfs ==> userspace API +
implementation
- Linux USB HID gadget driver ==> implementation + userspace API
- Multifunction Composite Gadget ==> technical + user info
- Linux USB Printer Gadget Driver ==> userspace API
- Linux Gadget Serial Driver v2.0 ==> user/admin + userspace API
- Linux UVC Gadget Driver ==> user/admin + userspace API
- Gadget Testing ==> user/admin + userspace API
- Infinity Usb Unlimited Readme ==> user/admin
- Mass Storage Gadget (MSG) ==> user/admin
- USB 7-Segment Numeric Display ==> user/admin
- mtouchusb driver ==> user/admin
- OHCI ==> technical info
- USB Raw Gadget ==> userspace API
- USB/IP protocol ==> technical info
- usbmon ==> user/admin + userspace API
- USB serial ==> user/admin + technical info
- USB references
- Linux CDC ACM inf
- Linux inf
- USB devfs drop permissions source
- Credits
By "admin/user info", I mean things that a user would have to do or run
(e.g. modprobe + flags) to make use of a driver; "technical info" is
more like device specifications (transfer speeds, modes of operation,
etc.); "userspace API" is stuff like configfs and ioctls; "driver
details" is really implementation details and internal considerations.
The last ones I don't even really know how to categorize.
I'm guessing nobody is really enthralled by the idea of splitting
Documentation/usb/ up like this?
Documentation/admin-guide/usb/
Documentation/driver-api/usb/ (this one actually exists already)
Documentation/userspace-api/usb/
For the stuff that is _actually_ internal to a specific driver (so not
useful for end users, not useful for admins, not generic USB info, and
not useful for userspace programmers), I would honestly propose to just
move it directly into the driver's source code, or, if the text is
obsolete, just get rid of it completely.
The distinction between user/admin and userspace API is pretty clear
(one is for end users, the other is for userspace _programmers_), but it
can sometimes be hard to determine whether something falls in one or the
other category.
In any case -- it looks like almost all of the usb/ directory does not
document "how specific kernel subsystems work from the point of view of
a kernel developer" so maybe we should just move the include to
userspace-api/ for now as an obvious improvement (if still not 100%
correct):
diff --git a/Documentation/subsystem-apis.rst
b/Documentation/subsystem-apis.rst
index 2d353fb8ea26..fe972f57bf4c 100644
--- a/Documentation/subsystem-apis.rst
+++ b/Documentation/subsystem-apis.rst
@@ -81,7 +81,6 @@ Storage interfaces
security/index
crypto/index
bpf/index
- usb/index
PCI/index
misc-devices/index
peci/index
diff --git a/Documentation/userspace-api/index.rst
b/Documentation/userspace-api/index.rst
index 82f9dbd228f5..e60cd9174ada 100644
--- a/Documentation/userspace-api/index.rst
+++ b/Documentation/userspace-api/index.rst
@@ -41,6 +41,7 @@ Subsystem-specific documentation:
tee
isapnp
dcdbas
+ ../usb/index
Kernel ABIs: These documents describe the the ABI between the Linux
kernel and userspace, and the relative stability of these interfaces.
Thoughts?
Vegard
Hi Vegard, Le mardi 09 janvier 2024 à 14:08 +0100, Vegard Nossum a écrit : > On 08/01/2024 13:00, Paul Cercueil wrote: > > Add documentation for the three ioctls used to attach or detach > > externally-created DMABUFs, and to request transfers from/to > > previously > > attached DMABUFs. > > > > Signed-off-by: Paul Cercueil <paul@crapouillou.net> > > > > --- > > v3: New patch > > --- > > Documentation/usb/functionfs.rst | 36 > > ++++++++++++++++++++++++++++++++ > > 1 file changed, 36 insertions(+) > > Hi, > > I'd like to point out that this file (usb/functionfs.rst) is > currently > included by Documentation/subsystem-apis.rst, the top-level file for > the > "Kernel subsystem documentation" set of books, which describe > internal > APIs: "These books get into the details of how specific kernel > subsystems work from the point of view of a kernel developer". > > However, functionfs.rst (and especially your new additions) are > documenting a userspace API, so it really belongs somewhere in > Documentation/userspace-api/ -- that's where /proc, /sys, /dev and > ioctl > descriptions for userspace programmers belong. Agreed. Even the original content prior to my additions describe a userspace API. > > I'm not NAKing the patch -- I just want to draw attention to this > discrepancy. Maybe we can separate the kernel-implementation details > (stuff about __init sections and stuff) from the new ioctl() info? > > Looking at <https://docs.kernel.org/usb/> I see that there are many > other adjacent documents that are also not really documenting kernel > implementation details, rough categorization as follows: > > USB support > ----------- > > - Linux ACM driver v0.16 ==> admin/user info > - Authorizing (or not) your USB devices to connect to the system ==> > admin/user info > - ChipIdea Highspeed Dual Role Controller Driver => admin/user info > - DWC3 driver ==> driver TODOs (can be moved into source code?) > - EHCI driver ==> technical info + driver details > - How FunctionFS works > - Linux USB gadget configured through configfs ==> userspace API + > implementation > - Linux USB HID gadget driver ==> implementation + userspace API > - Multifunction Composite Gadget ==> technical + user info > - Linux USB Printer Gadget Driver ==> userspace API > - Linux Gadget Serial Driver v2.0 ==> user/admin + userspace API > - Linux UVC Gadget Driver ==> user/admin + userspace API > - Gadget Testing ==> user/admin + userspace API > - Infinity Usb Unlimited Readme ==> user/admin > - Mass Storage Gadget (MSG) ==> user/admin > - USB 7-Segment Numeric Display ==> user/admin > - mtouchusb driver ==> user/admin > - OHCI ==> technical info > - USB Raw Gadget ==> userspace API > - USB/IP protocol ==> technical info > - usbmon ==> user/admin + userspace API > - USB serial ==> user/admin + technical info > - USB references > - Linux CDC ACM inf > - Linux inf > - USB devfs drop permissions source > - Credits > > By "admin/user info", I mean things that a user would have to do or > run > (e.g. modprobe + flags) to make use of a driver; "technical info" is > more like device specifications (transfer speeds, modes of operation, > etc.); "userspace API" is stuff like configfs and ioctls; "driver > details" is really implementation details and internal > considerations. > > The last ones I don't even really know how to categorize. > > I'm guessing nobody is really enthralled by the idea of splitting > Documentation/usb/ up like this? > > Documentation/admin-guide/usb/ > Documentation/driver-api/usb/ (this one actually exists already) > Documentation/userspace-api/usb/ > > For the stuff that is _actually_ internal to a specific driver (so > not > useful for end users, not useful for admins, not generic USB info, > and > not useful for userspace programmers), I would honestly propose to > just > move it directly into the driver's source code, or, if the text is > obsolete, just get rid of it completely. > > The distinction between user/admin and userspace API is pretty clear > (one is for end users, the other is for userspace _programmers_), but > it > can sometimes be hard to determine whether something falls in one or > the > other category. > > In any case -- it looks like almost all of the usb/ directory does > not > document "how specific kernel subsystems work from the point of view > of > a kernel developer" so maybe we should just move the include to > userspace-api/ for now as an obvious improvement (if still not 100% > correct): > > diff --git a/Documentation/subsystem-apis.rst > b/Documentation/subsystem-apis.rst > index 2d353fb8ea26..fe972f57bf4c 100644 > --- a/Documentation/subsystem-apis.rst > +++ b/Documentation/subsystem-apis.rst > @@ -81,7 +81,6 @@ Storage interfaces > security/index > crypto/index > bpf/index > - usb/index > PCI/index > misc-devices/index > peci/index > diff --git a/Documentation/userspace-api/index.rst > b/Documentation/userspace-api/index.rst > index 82f9dbd228f5..e60cd9174ada 100644 > --- a/Documentation/userspace-api/index.rst > +++ b/Documentation/userspace-api/index.rst > @@ -41,6 +41,7 @@ Subsystem-specific documentation: > tee > isapnp > dcdbas > + ../usb/index > > Kernel ABIs: These documents describe the the ABI between the Linux > kernel and userspace, and the relative stability of these > interfaces. > > > Thoughts? Makes sense to me. There's definitely some cleanup to be done in the USB documentation. > Vegard Cheers, -Paul
© 2016 - 2025 Red Hat, Inc.