On Wed, Jun 28, 2017 at 07:11:00AM +0200, Greg KH wrote: > On Tue, Jun 27, 2017 at 06:32:07PM -0600, Philip Prindeville wrote: > > What’s odd is that when I look in /dev/ I see ttyS0…S3, even though > > there are only 2 hardware ports (a single DUART). > > > > [root@kvm2 logwatch]# setserial -G /dev/ttyS0 > > /dev/ttyS0 uart 16550A port 0x03f8 irq 4 baud_base 115200 spd_normal skip_test > > [root@kvm2 logwatch]# setserial -G /dev/ttyS1 > > /dev/ttyS1 uart 16550A port 0x02f8 irq 3 baud_base 115200 spd_normal skip_test > > [root@kvm2 logwatch]# setserial -G /dev/ttyS2 > > /dev/ttyS2 uart unknown port 0x03e8 irq 4 baud_base 115200 spd_normal skip_test > > [root@kvm2 logwatch]# setserial -G /dev/ttyS3 > > /dev/ttyS3 uart unknown port 0x02e8 irq 3 baud_base 115200 spd_normal The "uart unknown port" for /dev/ttyS2 and /dev/ttyS3 means that the driver knows that nothing is there. If you try reading or writing to those devices you will get an EIO error. > > > > [ 3.173481] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled > > [ 3.194077] 00:03: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A > > [ 3.214710] 00:04: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A Yep, working as designed. > > I was trying to figure out where this happens, and why, so maybe I > > could submit a patch to allow one to suppress this behavior (say with > > “8250.only_real_hw=1” or some other module parameter). The fact that you can set and get the settings of /dev/ttyS2 and /dev/ttyS3 is a feature, not a bug. This is so if you have a system with an old-style bus that doesn't allow you to enumerate what devices are attached to which I/O ports, you can manually configure those ports using the setserial command. It isn't necessary with modern buses (e.g., PCI or USB), for legacy hardware (e.g., ISA buses) this is necessary. > > Anyone know the backstory on why this happens, and how to > > (conditionally) change this behavior? > > Why do you need to change this? What is wrong with the existing way > this works? As for the code that implements it, it's the platform > driver for your system that thinks the serial ports are there to remain > compatible with the old PC standard. Philip, The current behaviour is not a bug, and should NOT be changed. The legacy COM1..COM4 ports are not on an enumeratable bus (since they are essentially emulated ISA bus devices). So the only way we can tell whether or not they are there by probing to see if the UART is there. That's why ttyS2 and ttyS3 exist inside the kernel. If you are mortally offended that /dev/ttyS2 and /dev/ttyS3 as device nodes exist, I suppose it could be possible to hack up some udev rules that check to see if the UART is unknown, and then delete the device nodes, but is it really worth the effort? - Ted -- 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
