Drop redundant URB transfer-buffer casts.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

Add inline sysrq break-handler dummy to allow the compiler to eliminate
further code when either console or sysrq support isn't enabled and to
clearly mark the two sysrq functions as belonging together.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

Inline the dummy sysrq character handling when either console support or
magic-sysrq support isn't enabled to allow the compiler to eliminate
unused code.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

Do not set the sysrq timestamp unless CONFIG_MAGIC_SYSRQ is enabled to
avoid unnecessary per-character processing for consoles.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

Only set the sysrq timestamp for console ports to avoid having every
driver also check the console flag when processing incoming data.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

Drop the struct tty_port pointer and rename the struct usb_serial_port
pointer "port", which is the named used throughout the subsystem and
incidentally also matches the kernel-doc comment.

Signed-off-by: Johan Hovold <johan@kernel.org>

Commit a8d78d9f3856 ("USB: serial: clean up throttle handling")
converted the throttle handling to use atomic bitops. This means that we
can relax the smp_mb() in unthrottle() to smp_mb__after_atomic(), which
for example is a no-op on architectures like x86 that provide fully
ordered atomics.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Johan Hovold <johan@kernel.org>

Drop an unnecessary goto from a write-urb completion error path.

Signed-off-by: Johan Hovold <johan@kernel.org>

Clean up the throttle implementation by dropping the redundant
throttle_req flag which was a remnant from back when there was only a
single read URB.

Also convert the throttled flag to an atomic bit flag.

Signed-off-by: Johan Hovold <johan@kernel.org>

Fix two long-standing bugs which could potentially lead to memory
corruption or leave the port throttled until it is reopened (on weakly
ordered systems), respectively, when read-URB completion races with
unthrottle().

First, the URB must not be marked as free before processing is complete
to prevent it from being submitted by unthrottle() on another CPU.

	CPU 1				CPU 2
	================		================
	complete()			unthrottle()
	  process_urb();
	  smp_mb__before_atomic();
	  set_bit(i, free);		  if (test_and_clear_bit(i, free))
	  					  submit_urb();

Second, the URB must be marked as free before checking the throttled
flag to prevent unthrottle() on another CPU from failing to observe that
the URB needs to be submitted if complete() sees that the throttled flag
is set.

	CPU 1				CPU 2
	================		================
	complete()			unthrottle()
	  set_bit(i, free);		  throttled = 0;
	  smp_mb__after_atomic();	  smp_mb();
	  if (throttled)		  if (test_and_clear_bit(i, free))
	  	  return;			  submit_urb();

Note that test_and_clear_bit() only implies barriers when the test is
successful. To handle the case where the URB is still in use an explicit
barrier needs to be added to unthrottle() for the second race condition.

Fixes: d83b405383c9 ("USB: serial: add support for multiple read urbs")
Signed-off-by: Johan Hovold <johan@kernel.org>