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

[Tom Rini]

On Sat, Jul 14, 2018 at 12:47:21PM +0200, 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?

To be clear, it's the other way around.  We can't use the Linux dtb/dts
files as they've dropped (and in other cases, aren't adding) these
properties as it's handled differently.

> 
> > +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.

To try and help clarify, the property in question means "quantity to
shift the register offsets by."  It should be clear in our Kconfig help
entry as well that this is what we're looking for.

-- 
Tom
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