Hi ronald, On Mon, Jun 10, 2019 at 11:20 AM Life is hard, and then you die <ronald@xxxxxxxxxxxxx> wrote: > > > Hi Benjamin, > > Sorry for the extremely late reply - RL etc. > > On Fri, Apr 26, 2019 at 08:26:25AM +0200, Benjamin Tissoires wrote: > > On Fri, Apr 26, 2019 at 7:56 AM Life is hard, and then you die > > <ronald@xxxxxxxxxxxxx> wrote: > > > > > > On Thu, Apr 25, 2019 at 11:39:12AM +0200, Benjamin Tissoires wrote: > > > > On Thu, Apr 25, 2019 at 10:19 AM Life is hard, and then you die > > > > <ronald@xxxxxxxxxxxxx> wrote: > > > > > > > > > > Hi Benjamin, > > > > > > > > > > Thank you for looking at this. > > > > > > > > > > On Wed, Apr 24, 2019 at 04:18:23PM +0200, Benjamin Tissoires wrote: > > > > > > On Mon, Apr 22, 2019 at 5:13 AM Ronald Tschalär <ronald@xxxxxxxxxxxxx> wrote: > > > > > > > > > > > > > > The iBridge device provides access to several devices, including: > > > > > > > - the Touch Bar > > > > > > > - the iSight webcam > > > > > > > - the light sensor > > > > > > > - the fingerprint sensor > > > > > > > > > > > > > > This driver provides the core support for managing the iBridge device > > > > > > > and the access to the underlying devices. In particular, since the > > > > > > > functionality for the touch bar and light sensor is exposed via USB HID > > > > > > > interfaces, and the same HID device is used for multiple functions, this > > > > > > > driver provides a multiplexing layer that allows multiple HID drivers to > > > > > > > be registered for a given HID device. This allows the touch bar and ALS > > > > > > > driver to be separated out into their own modules. > > > > > > > > > > > > Sorry for coming late to the party, but IMO this series is far too > > > > > > complex for what you need. > > > > > > > > > > > > As I read this and the first comment of drivers/mfd/apple-ibridge.c, > > > > > > you need to have a HID driver that multiplex 2 other sub drivers > > > > > > through one USB communication. > > > > > > For that, you are using MFD, platform driver and you own sauce instead > > > > > > of creating a bus. > > > > > > > > > > Basically correct. To be a bit more precise, there are currently two > > > > > hid-devices and two drivers (touchbar and als) involved, with > > > > > connections as follows (pardon the ugly ascii art): > > > > > > > > > > hdev1 --- tb-drv > > > > > / > > > > > / > > > > > / > > > > > hdev2 --- als-drv > > > > > > > > > > i.e. the touchbar driver talks to both hdev's, and hdev2's events > > > > > (reports) are processed by both drivers (though each handles different > > > > > reports). > > > > > > > > > > > So, how about we reuse entirely the HID subsystem which already > > > > > > provides the capability you need (assuming I am correct above). > > > > > > hid-logitech-dj already does the same kind of stuff and you could: > > > > > > - create drivers/hid/hid-ibridge.c that handles USB_ID_PRODUCT_IBRIDGE > > > > > > - hid-ibridge will then register itself to the hid subsystem with a > > > > > > call to hid_hw_start(hdev, HID_CONNECT_HIDRAW) and > > > > > > hid_device_io_start(hdev) to enable the events (so you don't create > > > > > > useless input nodes for it) > > > > > > - then you add your 2 new devices by calling hid_allocate_device() and > > > > > > then hid_add_device(). You can even create a new HID group > > > > > > APPLE_IBRIDGE and allocate 2 new PIDs for them to distinguish them > > > > > > from the actual USB device. > > > > > > - then you have 2 brand new HID devices you can create their driver as > > > > > > a regular ones. > > > > > > > > > > > > hid-ibridge.c would just need to behave like any other hid transport > > > > > > driver (see logi_dj_ll_driver in drivers/hid/hid-logitech-dj.c) and > > > > > > you can get rid of at least the MFD and the platform part of your > > > > > > drivers. > > > > > > > > > > > > Does it makes sense or am I missing something obvious in the middle? > > > > > > > > > > Yes, I think I understand, and I think this can work. Basically, > > > > > instead of demux'ing at the hid-driver level as I am doing now (i.e. > > > > > the iBridge hid-driver forwarding calls to the sub-hid-drivers), we > > > > > demux at the hid-device level (events forwarded from iBridge hdev to > > > > > all "virtual" sub-hdev's, and requests from sub-hdev's forwarded to > > > > > the original hdev via an iBridge ll_driver attached to the > > > > > sub-hdev's). > > > > > > > > > > So I would need to create 3 new "virtual" hid-devices (instances) as > > > > > follows: > > > > > > > > > > hdev1 --- vhdev1 --- tb-drv > > > > > / > > > > > -- vhdev2 -- > > > > > / > > > > > hdev2 --- vhdev3 --- als-drv > > > > > > > > > > (vhdev1 is probably not strictly necessary, but makes things more > > > > > consistent). > > > > > > > > Oh, ok. > > > > > > > > How about the following: > > > > > > > > hdev1 and hdev2 are merged together in hid-apple-ibridge.c, and then > > > > this driver creates 2 virtual hid drivers that are consistent > > > > > > > > like > > > > > > > > hdev1---ibridge-drv---vhdev1---tb-drv > > > > hdev2--/ \--vhdev2---als-drv > > > > > > I don't think this will work. The problem is when the sub-drivers need > > > to send a report or usb-command: how to they specify which hdev the > > > report/command is destined for? While we could store the original hdev > > > in each report (the hid_report's device field), that only works for > > > hid_hw_request(), but not for things like hid_hw_raw_request() or > > > hid_hw_output_report(). Now, currently I don't use the latter two; but > > > I do need to send raw usb control messages in the touchbar driver > > > (some commands are not proper hid reports), so it definitely breaks > > > down there. > > > > > > Or am I missing something? > > > > I'd need to have a deeper look at the protocol, but you can emulate > > pure HID devices by having your ibridge handling a translation from > > set/get features/input to the usb control messages. Likewise, nothing > > prevents you to slightly rewrite the report descriptors you present to > > the als and touchbar to have a clear separation with the report ID. > > > > For example, if both hdev1 and hdev2 use a report ID of 0x01, you > > could rewrite the report descriptor so that when you receive a report > > with an id of 0x01 you send this to hdev1, but you can also translate > > 0x11 to a report ID 0x01 to hdev2. > > Likewise, report ID 0x42 could be a raw USB control message to the USB > > under hdev2. > > > > Note that you will have to write 2 report descriptors for your new > > devices, but you can take what makes sense from the original ones, and > > just add a new collection with a vendor application with with an > > opaque meaning (for the USB control messages). > > A couple things here. First of all, I went and rewrote the mfd driver > with the hid-driver demultiplexer as a straight hid driver with 3 > (well, 4 actually) virtual hid devices, as first discussed above. > This overall led to some simplifications, with only smaller > adjustments in the Touch Bar and ALS drivers (the diff stat shows 468 > insertions, 825 deletions), so this looks good. Importantly (IMO), > this leaves the whole awareness of the fact that the Touch Bar driver > is talking to multiple usb interfaces and needs to coordinate > appropriately between them (including things like which order they are > accessed, sleep times between those accesses, and different power > management) clearly in the Touch Bar driver, and the ibridge driver is > still fairly generic unaware of any of the details that the > sub-drivers need to worry about. > > Then I started looking more closely at your last suggestion above of > creating only 2 virtual hid devices, with report descriptor > merging/mangling and the addition of 3 custom reports (for the > set-power, io-wait, and usb-control functionality), and I'm having > trouble seeing the justification for it. AFAICT, the only advantage of > this approach is that there are fewer virtual hid devices. But the > disadvantages are significantly more code (especially in the ibridge > driver) and more leakage of knowledge from the Touch Bar driver into > the ibridge driver. In particular: > > * this leads to additional work, synchronization, and state management > in the ibridge driver to deal with the fact that we have to wait for > all (real) hid devices to be probed before we can start creating the > virtual hid devices (and visa versa on removal). > * merging and mangling those report descriptors requires re-parsing > of the descriptors and dealing with various corner cases, which adds > a bunch of code. > * while the custom set-power and io-wait reports are simple, the > custom usb-control report is ugly, because it actually ends up > needing to compute some of the parameters and rewrite the data, as > both those have values that require knowledge of the real underlying > reports and usb interfaces (i.e. it's not a report for a generic > usb-control call, but a very Touch Bar specific one, i.e. leaking > particular Touch Bar driver knowledge into the ibridge driver). > * In addition, while creating especially the custom usb-control > report, I started to wonder if it's really worth serializing the > parameters for the custom functions/report into an actual report > buffer, just to be deserialized again two function calls down the > stack. This became obvious when I was adding a helper function to > the ibridge driver to serialize the function parameters, and at the > same time writing a function right below it to deserialize those > parameters again, all so we can call hid_hw_request() to pass them > from the Touch Bar driver to the ibridge driver - but since the > Touch Bar driver is already calling a custom function in the ibridge > driver to serialize the parameters, it seems like that function > might just as well make the desired underlying function call > directly in the first place. Alternatively, the serializing > functionality can be put in the Touch Bar driver instead, with the > disadvantage that the implicit knowledge of the structure of the > custom report is now spread over two modules. All of this seems > somewhat ugly to me. > > Lastly, as hinted earlier, while this tries to hide the fact that > there are actually multiple hid devices (aka usb interfaces) that are > being driven by the Touch Bar driver, the Touch Bar driver still needs > to be acutely aware of that fact because it cannot treat them equally. > So now instead it clearly dealing with two different devices, it now > has to do so indirectly by figuring out which reports (in the same > virtual hid device) belong to which underlying real hid devices so it > can treat those reports accordingly (e.g. when creating the reports to > trigger set-power or usb-ctrl, instead of the desired device/interface > being targeted directly, that info has to instead be passed somewhat > opaquely via a report-id of a report that it happens to know is mapped > to the desired device/interface). > > Sorry for the long-winded response. I hope it isn't too cryptic. But > basically it boils down to: going for single virtual hid-device per > real device adds a bunch of complexity and knowledge leaking from the > Touch Bar driver the ibridge driver with AFAICT only a small advantage > (namely fewer virtual devices). > I must confess that understanding all the details above without seeing the code is rather hard. However, if you have a simple and elegant solution right now that doesn't imply the MFD driver, how about posting it now so we can discuss it by looking at the code? I am fine putting the above explanation in a commit message to justify the current approach, but we are already talking about revision 3 when I haven't seen revision 2. Anyway, I can be convinced a design is better than the one I suggested. And sometime it's better to not abstract too much if the overall gets a little bit too complex. So can you post your current WIP? Cheers, Benjamin
