Hi Rob, > Am 13.01.2017 um 15:48 schrieb Rob Herring <robh@xxxxxxxxxx>: > > On Fri, Jan 13, 2017 at 5:22 AM, H. Nikolaus Schaller <hns@xxxxxxxxxxxxx> wrote: >> Hi Rob, >> was it intentional to answer privately only? > > Damn gmail. Added everyone back. No problem. Happens to everyone every now and then. > >>> Am 12.01.2017 um 23:07 schrieb Rob Herring <robh@xxxxxxxxxx>: >>> >>> On Tue, Jan 10, 2017 at 5:44 AM, H. Nikolaus Schaller <hns@xxxxxxxxxxxxx> wrote: >>>> Hi Rob, >>>> >>>>> Am 06.01.2017 um 17:26 schrieb Rob Herring <robh@xxxxxxxxxx>: >>>>> > > [...] > >>>> 2. When I try to open the tty from user space to fetch the serial data I >>>> just get >>>> >>>> root@letux:~# cat /dev/ttyO1 >>>> [ 659.290618] ttyO ttyO1: tty_open: tty->count(2) != #fd's(1) >>>> [ 665.257232] ttyO ttyO1: tty_release: tty->count(2) != #fd's(1) >>>> ^C >>>> root@letux:~# >>>> >>>> So it does not seem to be possible to read the data from the tty any more. >>>> >>>> Maybe there can be some function serdev_device_set_shared(dev, flag). >>>> If set to exclusive the /dev node would be hidden from user-space. >>> >>> I don't think sharing should be allowed. Either you have an in-kernel >>> driver or you handle it in userspace. Sharing is just asking for >>> trouble IMO. >> >> My tty-slave patch series works and has no trouble with sharing (the UART) >> because it was designed with this in mind. >> >> The only trouble is that it did not find maintainer's acceptance... >> >>> Though it could be supported later. >> >> Firstly, let me point out once again that we have a mobile device, battery >> powered and every component should be and stay turned off, if not needed by >> any consumer. > > That's every device... > >> The only component that can reliably detect if there is no consumer in user-space >> is the kernel. Hence it must be a kernel driver that powers the device on/off. >> >> On the other side, it should simply pass data unmodified to user space (because the >> chip provides cooked data), so we do not need a driver for doing any data processing. >> The data stream is provided perfectly (without proper power control) by simply >> accessing /dev/ttyO1 from user-space. >> >> So if there were no power control topic, we would not even ask for a driver. > > I think this reasoning is exactly why we have no proper subsystem > today. We *almost* don't need one. It all works fine except I have > this one GPIO to control. Oh, and a regulator and firmware and > suspend/resume control and ... In our case we have no firmware, just the gpio. > >> We only need the driver to detect the power state of the chip by *monitoring* >> the data stream that goes to user space. >> >> Of course shared writing to the chip would give trouble, but we do not need it. >> Therefore I am happy if this sharing is an option (with a big WARNING sign). >> >> If we would try to achieve the same power management in user-space we would have >> to run a power-consuming daemon and make sure that it *never* crashes (or people >> come and complain about short battery life even if they think they have GPS >> turned off). >> >> And we need to be able to maintain the daemon for many different distributions >> people want to run on our device. This takes years to get it into Debian and >> others where we are not developers at all. > > Exactly one of the problems we're trying to solve. With your desired > approach though, you are still leaving the problem of having to know > which tty device the GPS chip is connected to which varies with each > board. > Either you hardcode the tty device in userspace, provide some > sysfs file with the tty name (like TI-ST) or link, provide the tty > name in DT (which I'll NAK) or have userspace parse the DT to find the > connection. I've seen all but the last case. I want to solve this > problem, too, such that userspace just opens the BT, WiFi, GPS, etc. > device. Yes, this is indeed an issue. There are also USB or Bluetooth based external GPS devices which do not need a driver because they speak some standard profile, identify themselves as GPS and create some tty port through standard mechanisms. > >> So this data stream sharing/monitoring is the most important part for getting our >> chip supported by a kernel driver. >> >>> >>> I've updated the series to skip creating the /dev nodes. >> >> That is exactly what we do NOT need for this chip. Now I can't even access it >> any more when powered on... > > It's a minor change. Essentially, it is call tty_register_device_attr > or not. Fine! > There's the issue of the file open count warning which I don't > know how to solve. Unfortunately I am not experienced with the tty layer to help here. > >>>> 3. for completely implementing my w2sg0004 driver (and two others) it would >>>> be nice to have additional serdev_device_ops: >>>> >>>> a) to be notified about user-space clients doing open/close on the tty >>>> b) and/or to be notified about user-space tcsetattr or TIOCMSET (for DTR) >>>> >>>> There may be other means (ldisc?) to get these notifications, but that >>>> needs the serdev driver to register with two different subsystems. >>>> >>>> Another approach could be to completely rewrite the driver so that it wraps >>>> and hides the /dev/ttyO1 and registers its own /dev/gps tty port for user-space >>>> communication. Then it would be notified for all user-space and serial >>>> interface activities as a man-in-the-middle. >>> >>> This was my thinking. If we only need data read and write, I don't >>> think we gain much using a tty vs. a new char dev. >> >> We gain re-use of the existing tty for its key purpose to mediate between an uart >> device and user-space. >> >> I have looked into the chardev approach and it appears to be quite some overkill >> to copy serial data from the UART to a user space file handle. >> >> About buffering: with the chardev approach we have to implement our own buffer >> in the driver and decide what happens on overflow while the tty layer already >> efficiently handles this. > > Not exactly. There's already buffering in tty_buffer.c. That handles > overflow of the UART. Then there is a 4K circular buffer in n_tty. The > question really is how much of the per character and flag processing > of n_tty do you need as that is where the complexity is. I'd guess not > much of it. If it is needed, then perhaps n_tty.c could be refactored > to provide common functions. Ah, that is good if the serdev driver can rely on / reuse this buffer. Where I don't see immediately is how I can make the chardev read file operation block until I get some receive_buf serdev_device_op and how I manage different data size requests without introducing another driver-internal buffer. So it could be better to alloc_tty_driver another tty and copy to its read buffer. > >> And it is a little strange that the device can either be accessed through >> /dev/ttyO1 if the driver is not loaded and only through /dev/gps if it is. > > We have similar things with SPI, I2C, and USB. Either you have a > kernel driver or you have a userspace driver. The userspace drivers > have limitations and if those limitations are a problem, we right > kernel drivers instead. > >> New problems arise if there were two such chips. Then we must be prepared >> that several instances provide different /dev/gps[1-9] nodes and that they >> can be identified and are stable. Maybe we have to introduce udev-rules... > > That's a solved problem generally (though not all like the answer). Well, I have no problem doing it that way but it increases complexity of the driver and makes it more difficult to configure in user space. > >> So what seems to be an obvious and straightforward solution (from serdev perspective) >> is not, if we look into implementation details of the driver. > > I can't solve all problems for all possible drivers on day one. It > does provide a solution for drivers that are already in the kernel. Well, I hope you understand that I am a little impatient here... It is more than 2 years ago that we proposed the first driver for this chip for upstreaming (while we have something running on our own kernels much longer time): http://lkml.iu.edu/hypermail/linux/kernel/1410.2/00634.html And if I counted correctly, I am now working on the fourth completely different proposal with no finalization in sight. So this situation is a little circular: because we are not in kernel we have to wait longer until we can get in... > I'd suggest we debate adding sharing capability vs. a GPS subsystem > separately. Well, I would be perfectly happy without having to wait for a fully worked out GPS subsystem. If it arrives in my (device's) life-time we can rework the driver to fit into it. This approach seems not to be an exception to me. For example the original misc/bmp085 driver has completely gone in 4.9 and been replaced by a generic iio driver. > If there was already a GPS subsystem there would be no > debate. I don't think what's here is preventing either case. > Similarly, I don't pretend the configuration API (baud rate and > flow-control) is complete. I'm sure someone will need additional > functions, but those can all be incrementally added as needed. > >>>> But I expect that it delays the communication and is quite some overhead. >>>> >>>> >>>> 4. It seems as if I have to modprobe the driver explicitly (it is not >>>> located and loaded automatically based on the compatible string in DT >>>> like i2c clients). >>> >>> I've added what I think should be needed for that. I pushed out a new >>> branch[1]. Can you give it a try? >> >> Yes, sure. I have tried and now our driver module is loaded as expected :) >> >> But in general we are turning away from a solution for our w2sg0004 driver >> (see above). >> >> BTW: I see an issue in our kernel (config?) that the console and initd >> blocks for approx. 60 seconds during boot when serdev is merged (even >> if not configured). This issue disappears when using the omap2plus_defconfig. >> >> But I have not digged into that (because it may be spurious or EPROBE_DEFER >> related). > > I've not seen anything like that. Do you have a diff of your configs? > And what is your init system? I did use Debian with sysv init and a getty on /dev/ttyO2 (OMAP3 console UART). But let me cross-check some things before digging deeper into it. To exclude that it is our fault and indeed in the serdev patch set. BR and thanks, Nikolaus -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
