Re: [RFC PATCH 1/1] drivers: introduce ARM SBSA generic UART driver

[Andre Przywara <andre.przywara@arm.com>]

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@arm.com> 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@arm.com>
>> ---
>>  .../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.