Raspberrypi3: Mini UART driver

[Christian Mauderer]

Hello Niteesh,

thanks for doing that work.

On 22/12/2019 12:10, Niteesh wrote:
> The rpi1 and rpi2 use the PL011 UART, whereas, with RPI's equipped with
> wireless/Bluetooth module, the PL011 is connected to the Bluetooth
> module, and the mini UART is used as the primary UART.

In my opinion it would be great if you could use the FDT to distinguish
between the boards. That should allow to add raspberry 3 (and maybe 4)
support without adding another BSP. More BSPs mean a bigger maintenance
effort for the RTEMS community.

> 
> https://www.raspberrypi.org/app/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
> But from the above doc (PAGE 10), the mini uart has 16550 like registers
> and RTEMS already has the driver for it
> bsps/shared/dev/serial/ns16550.c. But I am not sure how compatible they
> are? Should a new driver be implemented from scratch or use ns16550 if
> possible?

In general it's better to re-use existing code. That has multiple
advantages:

- It reduces the maintenance effort. Fewer code means fewer work.
- If you have multiple driver for the same or similar hardware it can
happen that a bug is fixed in one but not the other.
- It's simpler to find a hardware to test changes.
- The driver becomes more universal with every new supported hardware.
That increases the chance that it fits the next new hardware.

I'm sure there are some more if you ask someone else.

> 
> Also, the core clock on which the PL011 is based on is changed in rpi3.
> Rpi1 and 2 use 250Mhz as the default clock but it was changed to 400Mhz
> in Rpi3 and newer

Again: Would be great if that could be adapted based on FDT or by
reading the right registers.

> 
> Few differences between PL011 and Mini uart
> The mini UART has smaller FIFOs. Combined with the lack of flow control,
> this makes it more prone to losing characters at higher baud rates. It
> is also generally less capable than the PL011, mainly due to its baud
> rate link to the VPU clock speed.

That shouldn't really be a problem for the system console.

> 
> The particular deficiencies of the mini UART compared to the PL011 are :
> 
> No break detection
> No framing errors detection
> No parity bit
> No receive timeout interrupt
> No DCD, DSR, DTR or RI signals
>