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

[Simon Glass]

Hi Alexey,

On 22 February 2018 at 09:23, Alexey Brodkin
<Alexey.Brodkin at synopsys.com> wrote:
> Hi Simon,
>
> On Thu, 2018-02-22 at 09:17 -0700, Simon Glass wrote:
>> 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?
>
> But in case of ARC's AUX regs portmap won't help because those AUX regs are
> couldn't be mapped - that a completely different address space and we may
> only access them via dedicated instructions (LR vs LD and SR vs ST).

Well...

1. With a separate driver, you can do whatever you want :-) I know it
introduces code duplication though...

2. With regmap you can add your own regmap driver, and again do
whatever you want. I can help with that if it sounds attractive

Regards,
Simon