| Message ID | 20240108120056.22165-5-paul@crapouillou.net (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | usb: gadget: functionfs: DMABUF import interface | expand |
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. 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
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.
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(+)