[U-Boot] [RFC] ns16550: Add support for AUX regs usage on some ARC SoCs

[Simon Glass]

Hi,

On 21 February 2018 at 05:26, Alexey Brodkin
<Alexey.Brodkin at synopsys.com> wrote:
> Synopsys Data Fusion subsystem (DFSS) is targeted to deeply built-in
> use-cases and so to save some silicon area decision was made to
> escape usage of any busses and use instead directly wired to CPU
> peripherals. And one of those is DW APB UART.
>
> Later DFSS became a part of larger and more complicated SoCs with
> some other peripherals connected via common buses but default UART
> is still used via ARC core's auxulary registers which are not mapped
> to "normal" address space and we use very special instructions to access
> them, thus we cannot simply reuse whatever accessor we have in 16550
> UART as of now.
>
> Also we cannot just switch inb()/outb() to access ARC AUX regs always
> for DFSS because other peripherals have normal memory-mapped control
> registers and we need to use normal accessors for them.
>
> Frankly I don't like a lot what I did here but otherwise if I create
> a special driver for this I'll need to reimplement
> ns16550_serial_ops.putc()/getc() which will be pure copy-paseted from
> ns16550.c because the only difference is only in
> ns16550_{read|write}b().
>
> As mentioned above we cannot remap those auxiliary registers to
> normal memory address-space and thus we have to use very special
> accessors write_aux_reg()/read_aux_reg() that directly use special
> CPU intructions (namely LR/SR) for dealing with ARC AUX regs.
>
> Also note here I just use a check for a particular SoC being selected
> (CONFIG_ARCH_DFSS will be introduced shortly) but that is done just for
> simplicity, otherwise it might be a slecial Kconfig option for NS16550
> or anything else.
>
> I'd like to know what people think about possible colutions here.
> And as always any comments are much appreciated!
>
> Signed-off-by: Alexey Brodkin <abrodkin at synopsys.com>
> Cc: Simon Glass <sjg at chromium.org>
> Cc: Tom Rini <trini at konsulko.com>
> Cc: Stefan Roese <sr at denx.de>
> ---
>  drivers/serial/ns16550.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)

I think a separate driver might be better, unless we want to make the
read/write interface go through regmap or similar?

Regards,
Simon