[U-Boot] [PATCH] serial: ns16550: Add register shift variable

[Felix Brack]

Hello Wolfgang,

On 14.07.2018 12:47, Wolfgang Denk wrote:
> Dear Felix,
> 
> In message <1531492980-16543-1-git-send-email-fb at ltec.ch> you wrote:
>>
>> The motivation for writing this patch originates in the
>> effort of synchronizing U-Boot DT to Linux DT for am33xx SOCs.
>> The current am33xx.dtsi file from U-Boot defines the <reg-shift>
>> property for all UART nodes. The actual (4.18+) am33xx.dtsi
>> file from Linux does not define <reg-shift> anymore. To prevent
>> (probably difficult) changes in many .dts and .dtsi files once
>> the synchronization is done, one can use this new variable. For
>> the pdu001 board, for example, SYS_NS16550_REG_SHIFT is set
>> to 2; no need to clutter U-Boot and board specific dts files
>> with <reg-shift> properties.
> 
> Does this mean that U-Boot will not be able to use the same DTB as
> Linux?
>
This has already been answered very accurately by Tom.
I have also written an RFC dealing with this: "[RFC] arm: dts: am33xx:
Sync DTS with Linux 4.16.11", posted May 24.

>> +config SYS_NS16550_REG_SHIFT
>> +	int "Number of bytes to shift register offsets"
>> +	default 0
>> +	depends on SYS_NS16550
>> +	help
>> +	  Use this to specify the amount of bytes between discrete
>> +	  device registers. If, for example, the device registers are
>> +	  located at 0x00, 0x04, 0x08, 0x0C and so forth than set
>> +	  this to 2. The default value is 0.
> 
> This description is inconsistent or misleading.  How do you define
> "space between registers"?  The unused gaps?  In the example, the
> registers are spaced at 4 bytes intervals, so a value of 2 would
> only make sense of we have 16 bit registters and you count the gap
> bytes.
> 
> But this is a very strange and uncommon way to describe such a
> situation, especially when you write that you "shift register
> offsets".  Here I think about something like a LSL operation, so
> shifing by 2 bits would result in a 2^2 = 4 byte spacing.
> 
> This needs to be rewritten.
> 
You are right, sorry. It seems I used my brain to mix up two different
explanations and that's the result -  will be fixed.
> 
> Best regards,
> 
> Wolfgang Denk
> 
regards, Felix