serial: 8250: Add new 8250-core based Broadcom STB driver

serial: 8250: Add new 8250-core based Broadcom STB driver

Add a UART driver for the new Broadcom 8250 based STB UART. The new
UART is backward compatible with the standard 8250, but has some
additional features. The new features include a high accuracy baud
rate clock system and DMA support.

The driver will use the new optional BAUD MUX clock to select the best
one of the four master clocks (81MHz, 108MHz, 64MHz and 48MHz) to feed
the baud rate selection logic for any requested baud rate.  This allows
for more accurate BAUD rates when high speed baud rates are selected.

The driver will use the new UART DMA hardware if the UART DMA registers
are specified in Device Tree "reg" property.

The driver also sets the UPSTAT_AUTOCTS flag when hardware flow control
is enabled. This flag is needed for UARTs that don't assert a CTS
changed interrupt when CTS changes and AFE (Hardware Flow Control) is
enabled.

The driver also contains a workaround for a bug in the Synopsis 8250
core. The problem is that at high baud rates, the RX partial FIFO
timeout interrupt can occur but there is no RX data (DR not set in
the LSR register). In this case the driver will not read the Receive
Buffer Register, which clears the interrupt, and the system will get
continuous UART interrupts until the next RX character arrives. The
fix originally suggested by Synopsis was to read the Receive Buffer
Register and discard the character when the DR bit in the LSR was
not set, to clear the interrupt. The problem was that occasionally
a character would arrive just after the DR bit check and a valid
character would be discarded. The fix that was added will clear
receive interrupts to stop the interrupt, deassert RTS to insure
that no new data can arrive, wait for 1.5 character times for the
sender to react to RTS and then check for data and either do a dummy
read or a valid read. Debugfs error counters were also added and were
used to help create test software that would cause the error condition.
The counters can be found at:
/sys/kernel/debug/bcm7271-uart/<device-name>/stats

This also includes a few fixes for build warnings reported by
the kernel test robot.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Al Cooper <alcooperx@gmail.com>
Link: https://lore.kernel.org/r/20210325185256.16156-3-alcooperx@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>