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

[Alexey Brodkin]

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).

-Alexey