Hi Rob, thanks for looking at this. On 02/09/14 04:06, Rob Herring wrote: > On Fri, Aug 29, 2014 at 11:13 AM, Andre Przywara <andre.przywara@xxxxxxx> wrote: >> The ARM Server Base System Architecture (SBSA) describes a generic >> UART which all compliant level 1 systems should implement. This is >> actually a PL011 subset, so a full PL011 implementation will satisfy >> this requirement. >> However if a system does not have a PL011, a very stripped down >> implementation complying to the SBSA defined specification will >> suffice. The Linux PL011 driver is not guaranteed to drive this >> limited device (and indeed the fast model implentation hangs the >> kernel if driven by the PL011 driver). >> So introduce a new driver just implementing the part specified by the >> SBSA (which lacks DMA, the modem control signals and many of the >> registers including baud rate control). This driver has been derived >> by the actual PL011 one, removing all unnecessary code. >> >> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx> >> --- >> .../devicetree/bindings/serial/arm_sbsa_uart.txt | 6 + >> drivers/tty/serial/Kconfig | 28 + >> drivers/tty/serial/Makefile | 1 + >> drivers/tty/serial/sbsa_uart.c | 793 ++++++++++++++++++++ >> include/uapi/linux/serial_core.h | 1 + > > Sorry, but I think this is all wrong. We've now just duplicated some > subset of the pl011 driver leaving out the parts like setting baudrate > which can never be added since those things could be different for > every vendor. > > The original intent of the SBSA uart was to provide a common early > debug uart. It was not to have a full fledged driver. I think the SBSA > has failed in this area and created the potential to create a mess of > serial drivers different for every vendor. Reality will hopefully not > be that extreme and most vendors will just use the pl011 and create > their value add somewhere besides the uart. For the purpose of debug > output, we already support that as the pl011 earlycon only touches > SBSA compatible registers. I see your point (and was actually looking for those kind of comments when posting this). I agree to that debug aspect and understand that earlycon already does this, but I think we need some support beyond earlycon, to be able to login and use it as a console (which is not possible with earlycon, right?) This is probably still for debugging or emergency access to the system only, but maybe also for logging - actually quite similar to how UARTs are used on today's x86 servers. So after having written three incarnations of this driver (goldfish based, PL010 based, PL011 based) I wonder if supporting the SBSA subset in the real PL011 driver is an option. Either this would be enabled by a new explicit DT property or preferably by a clever compatible string. Ideally we would just provide a different set of "struct uart_ops" members, with some pointing to generic PL011 routines, some to SBSA UART specific ones. Maybe we make the full featured PL011 support a config option (defaulting to y), allowing people to only use the SBSA subset in their kernel? Does that make more sense? (for a general SBSA h/w rationale see below) >> 5 files changed, 829 insertions(+) >> create mode 100644 Documentation/devicetree/bindings/serial/arm_sbsa_uart.txt >> create mode 100644 drivers/tty/serial/sbsa_uart.c >> >> diff --git a/Documentation/devicetree/bindings/serial/arm_sbsa_uart.txt b/Documentation/devicetree/bindings/serial/arm_sbsa_uart.txt >> new file mode 100644 >> index 0000000..8e2c5d6 >> --- /dev/null >> +++ b/Documentation/devicetree/bindings/serial/arm_sbsa_uart.txt >> @@ -0,0 +1,6 @@ >> +* ARM SBSA defined generic UART >> + >> +Required properties: >> +- compatible: must be "arm,sbsa-uart" > > This alone is not okay. There is no such implementation of hardware. But the SBSA explicitly allows this. I don't know of any vendor who just implements the subset, but I've been told that this has been asked for. > The DT must specify the implementation such as pl011. If it is a full featured PL011: sure. Then we don't need this driver at all and just use the SBSA UART spec as a guideline for our earlycon implementation. I will try to learn if there is someone actually implementing only the subset. Cheers, Andre. -- 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
