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

[Tom Rini <trini@konsulko.com>]

On Wed, Feb 21, 2018 at 03:26:05PM +0300, Alexey Brodkin 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@synopsys.com>
> Cc: Simon Glass <sjg@chromium.org>
> Cc: Tom Rini <trini@konsulko.com>
> Cc: Stefan Roese <sr@denx.de>
> ---
>  drivers/serial/ns16550.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/serial/ns16550.c b/drivers/serial/ns16550.c
> index 6f9ce689cfff..75f342f337f1 100644
> --- a/drivers/serial/ns16550.c
> +++ b/drivers/serial/ns16550.c
> @@ -13,6 +13,9 @@
>  #include <watchdog.h>
>  #include <linux/types.h>
>  #include <asm/io.h>
> +#ifdef CONFIG_ARCH_DFSS
> +#include <asm/arcregs.h>
> +#endif
>  
>  DECLARE_GLOBAL_DATA_PTR;
>  
> @@ -53,7 +56,9 @@ DECLARE_GLOBAL_DATA_PTR;
>  
>  static inline void serial_out_shift(void *addr, int shift, int value)
>  {
> -#ifdef CONFIG_SYS_NS16550_PORT_MAPPED
> +#ifdef CONFIG_ARCH_DFSS
> +	write_aux_reg((int)addr, value);
> +#elif defined(CONFIG_SYS_NS16550_PORT_MAPPED)
>  	outb(value, (ulong)addr);
>  #elif defined(CONFIG_SYS_NS16550_MEM32) && !defined(CONFIG_SYS_BIG_ENDIAN)
>  	out_le32(addr, value);
> @@ -70,7 +75,9 @@ static inline void serial_out_shift(void *addr, int shift, int value)
>  
>  static inline int serial_in_shift(void *addr, int shift)
>  {
> -#ifdef CONFIG_SYS_NS16550_PORT_MAPPED
> +#ifdef CONFIG_ARCH_DFSS
> +	return read_aux_reg((int)addr);
> +#elif defined(CONFIG_SYS_NS16550_PORT_MAPPED)
>  	return inb((ulong)addr);
>  #elif defined(CONFIG_SYS_NS16550_MEM32) && !defined(CONFIG_SYS_BIG_ENDIAN)
>  	return in_le32(addr);

As always, thanks for the detailed explanation.  Yes, I think that of
the options, putting the details in read/write_aux_reg (and please make
sure read/write_aux_reg have a good function comment too) is the best
choice.  Thanks!

-- 
Tom
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