[lldb-dev] Serial port support in LLDB

[Pavel Labath via lldb-dev]

Thanks for the nice summary Michał. I've found it very helpful.

The thing I am missing from this proposal is how would those settings 
translate into actual termios calls? Like, who would be responsible 
reading those settings and acting on them? Currently we have some tty 
code in ConnectionFileDescriptorPosix (in the Host library) but I think 
a bit too low-level for that (I don't think the host library knows 
anything about "settings"). I am also not sure if a class called 
"ConnectionFileDescriptor" is really the best place for this.

On 05/10/2021 11:21, Michał Górny via lldb-dev wrote:
> Hi, everyone.
> 
> I'm working on improving LLDB's feature parity with GDB.  As part of
> this, I'm working on bettering LLDB's serial port support.  Since serial
> ports are not that common these days, I've been asked to explain a bit
> what I'd like to do.
> 
> 
> At this point, LLDB (client) has minimal support for serial port
> debugging.  You can use a command like:
> 
>      process connect file:///dev/ttyS0
> 
> to connect via the GDB Remote Protocol over a serial port.  However,
> the client hardcodes serial transmission parameters (I'll explain
> below).  I haven't been able to find an option to bind lldb-server to a
> serial port.
> 
> I'd like to fix the both limitations, i.e. make it possible to configure
> serial port parameters and add support for serial port in lldb-server.
> 
> 
> The RS-232 standard is quite open ended, so I'm going to focus on 8250-
> compatible serial port with DB-9 connector below (i.e. the kind found
> in home PCs).  However, I'm going to skip the gory details and just
> focus on the high-level problem.
> 
> The exact protocol used to transmit data over the serial port is
> configurable to some degree.  However, there is no support for
> autoconfiguration, so both ends have to be configured the same.
> The synchronization support is also minimal.
> 
> The important hardware parameters that can be configured are:
> 
> - baud rate, i.e. data transmission speed that implies the sampling
>    rate.  The higher the baud rate, the shorter individual bits are
>    in the transmitted signal.  If baud rate is missynced, then
>    the receiver will get wrong bit sequences.
> 
> - number of data bits (5-8) in the frame, lower values meaning that
>    the characters sent are being truncated.  For binary data transfers,
>    8 data bits must be used.
I believe gdb-remote protocol is compatible with 7-bit connections, 
though we would need to make sure lldb refrains from using some packets. 
Should I take it this is not an avenue you wish to pursue?

> 
> - parity used to verify frame correctness.  The parity bit is optional,
>    and can be configured to use odd or even parity.  Additionally, Linux
>    supports sending constant 0 or 1 as parity bit.
> 
> - number of stop bits (1 or 1.5/2) in the frame.  The use of more than
>    one stop bit is apparently a relict that was supposed to give
>    the receiver more time for processing.  I think this one isn't
>    strictly necessary nowadays.
Gotta love those half-bits.

> 
> - flow control (none, software, hardware).  This is basically used by
>    the receiver to inform the sender that it's got its buffer full
>    and the sender must stop transmitting.  Software flow control used
>    in-band signaling, so it's not suitable for binary protocols.
>    Hardware flow control uses control lines.
> 
> Of course, there is more to serial ports than just that but for LLDB's
> purposes, this should be sufficient.
> 
> 
> The POSIX and win32 API for serial ports are quite similar in design.
> In the POSIX API, you have to open a character device corresponding to
> the serial port, while in win32 API a special path \\.\COMn.  In both
> cases, reads and writes effect the transmission.  Both systems also have
> a dedicated API to configure the serial transmission parameters
> (ioctl/termios on POSIX [1], DCB on win32 [2]).  Note that I haven't
> tried the win32 API, just looked it up.
> 
> The POSIX serial ports are a teletype (tty) devices just like virtual
> consoles used by terminal emulators.  This makes it easy to use a serial
> port as a remote terminal for other system.  This also adds a bunch of
> configuration options related to input/output processing and special
> behavior.  When a serial port is going to be used for non-console
> purposes, these flags have to be disabled (i.e. the tty is set to 'raw'
> mode).
> 
> 
> The rough idea is that after opening the serial port, we need to set its
> parameters to match the other end.  For this to work, I need to replace
> LLDB's hardwired parameters with some way of specifying this.  I think
> the cleanest way of doing this (and following GDB's example) would be to
> add a new set of configuration variables to specify:
> 
> a. the baud rate used
> 
> b. the parity kind used
> 
> c. the number of stop bits
> 
> d. whether to use hardware flow control
> 
> I'm thinking of creating a new setting group for this, maybe
> 'host.serial'.  When connecting to a serial port, LLDB would set its
> parameters based on the settings from this group.
> 
> That said, I can't think of a really clean way of making this
> configurable on lldb-server's end but I guess adding more command-line
> parameters should suffice.
> 
> WDYT?

Let me try to make a counterproposal.

Since the serial parameters are a property of a specific connection, and 
one could theoretically have be debugging multiple processes with 
different connection parameters, having a (global) setting for them does 
not seem ideal. And since lldb already has a history of using made up 
urls (unix-connect://, tcp://), I am wondering if we couldn't/shouldn't 
invent a new url scheme for this. So like instead of file:///dev/ttyS0, 
one would use a new scheme (say serial:// to connect), and we would 
somehow encode the connection parameters into the url. For example, this 
could look something like
   serial://[PARODD,STOP1,B19200]/dev/ttyS0
This connection string could be passed to both lldb and lldb-server 
without any new arguments. Implementation-wise this url could be 
detected at a fairly high level, and would cause us to instantiate a new 
class which would handle the serial connection.

Would this be too ugly?

pl