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Date: Wed, 18 Dec 2013 15:44:48 +0100 From: David Herrmann <dh.herrmann@...il.com> To: Antonio Ospite <ospite@...denti.unina.it> Cc: "open list:HID CORE LAYER" <linux-input@...r.kernel.org>, Dmitry Torokhov <dmitry.torokhov@...il.com>, Jiri Kosina <jkosina@...e.cz>, Benjamin Tissoires <benjamin.tissoires@...il.com>, Peter Hutterer <peter.hutterer@...-t.net>, linux-kernel <linux-kernel@...r.kernel.org>, Input Tools <input-tools@...ts.freedesktop.org> Subject: Re: [PATCH 2/4] Input: introduce ABS_MAX2/CNT2 and friends Hi On Wed, Dec 18, 2013 at 3:27 PM, Antonio Ospite <ospite@...denti.unina.it> wrote: > Hi David, > > thanks for the update. > > On Tue, 17 Dec 2013 16:48:52 +0100 > David Herrmann <dh.herrmann@...il.com> wrote: > >> As we painfully noticed during the 3.12 merge-window our >> EVIOCGABS/EVIOCSABS API is limited to ABS_MAX<=0x3f. We tried several >> hacks to work around it but if we ever decide to increase ABS_MAX, the >> EVIOCSABS ioctl ABI might overflow into the next byte causing horrible >> misinterpretations in the kernel that we cannot catch. >> >> Therefore, we decided to go with ABS_MAX2/CNT2 and introduce two new >> ioctls to get/set abs-params. They no longer encode the ABS code in the >> ioctl number and thus allow up to 4 billion ABS codes. >> > > Just a question: did you consider the possibility of not exposing _MAX2 > and _CNT2 in a header file at all but let those be queried? Or maybe > this is not necessary if we assume that userspace programs are supposed > to know what is the MAX _they_ support? > > BTW I was thinking for instance to a program compiled with ABS_MAX2 = > 0x4f but working with future kernels with an increased value, it still > won't be able to support values greater than 0x4f. Why would a program that was compiled with ABS_MAX2=0x4f want to use ABS_* values greater than 0x4f? It doesn't know of any new values so even if it could query ABS_MAX2, it could never know anything about the new constants. The only place where it is useful is for generic programs that just foward ABS_* codes. But these can just ignore ABS_MAX2 entirely and use the whole int32_t range for codes. If there is a specific use-case that'd require a dynamic way to retrieve ABS_MAX2, I can consider changing the API. But unless there's such a use-case, I'd like to keep it the same as for KEY_*, SW_*, ... >> The new API also allows to query multiple ABS values with one call. To >> allow EVIOCSABS2(code = 0, cnt = ABS_CNT2) we need to silently ignore >> writes to ABS_MT_SLOT. Furthermore, for better compatibility with >> newer user-space, we ignore writes to unknown codes. Hence, if we ever >> increase ABS_MAX2, new user-space will work with code=0,cnt=ABS_CNT2 just >> fine even on old kernels. >> > > To my inexperienced eye it's not obvious why ABS_MT_SLOT must be handled > as a special case. Maybe because I don't even know what > code=0,cnt=ABS_CNT2 is used for. ABS_MT_SLOT describes the current MT-slot that is reported via ABS_MT_* bits. It is read-only so we cannot let user-space write to it. >> Note that we also need to increase EV_VERSION so user-space can reliably >> know whether ABS2 is supported. Unfortunately, we return EINVAL instead of >> ENOSYS for unknown evdev ioctls so it's nearly impossible to catch >> reliably without EVIOCGVERSION. >> > > I'll check how to use that in evtest. > > Just one more comment below. > >> Signed-off-by: David Herrmann <dh.herrmann@...il.com> >> --- >> drivers/hid/hid-debug.c | 2 +- >> drivers/hid/hid-input.c | 2 +- >> drivers/input/evdev.c | 95 +++++++++++++++++++++++++++++++- >> drivers/input/input.c | 14 ++--- >> drivers/input/keyboard/goldfish_events.c | 6 +- >> drivers/input/keyboard/hil_kbd.c | 2 +- >> drivers/input/misc/uinput.c | 6 +- >> include/linux/hid.h | 2 +- >> include/linux/input.h | 6 +- >> include/uapi/linux/input.h | 42 +++++++++++++- >> include/uapi/linux/uinput.h | 2 +- >> 11 files changed, 155 insertions(+), 24 deletions(-) >> > > [...] >> diff --git a/include/uapi/linux/input.h b/include/uapi/linux/input.h >> index bd24470..1856461 100644 >> --- a/include/uapi/linux/input.h >> +++ b/include/uapi/linux/input.h > [...] >> >> /* >> + * Due to API restrictions the legacy evdev API only supports ABS values up to >> + * ABS_MAX/CNT. Use the extended *ABS2 ioctls to operate on any ABS values in >> + * between ABS_MAX and ABS_MAX2. >> + */ >> +#define ABS_MAX2 0x3f >> +#define ABS_CNT2 (ABS_MAX2+1) >> + > > Maybe it's just my English, but when you say "between ABS_MAX and > ABS_MAX2" it sounds like the old protocol still _must_ be used for > values <= ABS_MAX. > > IIUC this is true "only" for compatibility with older kernels right? > If a program decides to support only newer kernels it can check the > protocol version and use only the new ioctls, right? > > Maybe you can be more explicit about that in the comment? See the comment on "struct input_absinfo2". It describes the API, this comment just describes the ABS_* values. But if anyone has a better phrase to use here, I will gladly adjust it. Thanks David -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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