Re: [PATCH v5 2/4] serial: 8250_dw: build Renesas RZN1 CPR value from DW_UART_CPR_* definitions

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

On Tue, 28 Apr 2026, Andy Shevchenko wrote:

> On Tue, Apr 28, 2026 at 01:26:27PM +0800, Jia Wang wrote:
> > Replace the magic CPR value for Renesas RZ/N1 with a composition using
> > DW_UART_CPR_* bit/field definitions and FIELD_PREP_CONST().
> > 
> > Introduce a helper macro to convert a FIFO size (bytes) into the CPR
> > FIFO_MODE field value, with BUILD_BUG_ON_ZERO() checks for alignment and
> > bounds. Use it to replace the literal FIFO_MODE values in the RZN1.
> 
> A couple of nit-picks below. After addressing them you can add
> 
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> 
> ...
> 
> >  #include <linux/bitfield.h>
> >  #include <linux/bits.h>
> > +#include <linux/build_bug.h>
> > +#include <linux/align.h>
> 
> Preserve order, 'a' goes before 'b'.
> 
> >  #include <linux/io.h>
> >  #include <linux/types.h>
> 
> ...
> 
> >  /* Helper for FIFO size calculation */
> >  #define DW_UART_CPR_FIFO_SIZE(a)	(FIELD_GET(DW_UART_CPR_FIFO_MODE, (a)) * 16)
> 
> > +#define DW_UART_CPR_FIFO_MODE_MAX	0x80
> 
> You used decimal values elsewhere (id est 16), use upper limit in decimal
> as well.
> 
> > +#define DW_UART_CPR_FIFO_MODE_FROM_SIZE(size)				\
> > +	(BUILD_BUG_ON_ZERO(!IS_ALIGNED((size), 16)) +			\
> > +	 BUILD_BUG_ON_ZERO(((size) / 16) > DW_UART_CPR_FIFO_MODE_MAX) +	\
> > +	 ((size) / 16))
> 
> I don't see the need in having that maximum being defined separately (we don't
> have that for 16, no need to have it for 128.
> 
> Since some ISA:s have one assembly instruction to get both / and % divisions,
> it's better to use that instead of IS_ALIGNED(). Can you check code generation
> for x86_64 / x86?

Do those BUILD_BUGs even generate code, especially when they are expected 
to only appear in a struct initializer?

> #define DW_UART_CPR_FIFO_MODE_FROM_SIZE(size)				\
> 	(BUILD_BUG_ON_ZERO((size) > 2048) + BUILD_BUG_ON_ZERO((size) % 16) + ((size) / 16))
> 
> Note, I dropped first division in order to show the upper limit in a plain
> number since 16 is also FIFO size in bytes.
> 
> Also note, this evaluates (size) three times, which might be problematic,
> but I think we can leave with that for now.

I'd put also FIELD_PREP_CONST() into the macro itself as I don't see much 
value for this macro outside of those .cpr_value initializations.

IMO, the entire macro would be cleaner looking as a truly multi-line 
construct. Can we use static_assert()s in struct field initialization 
(I'm not sure), something along these lines:

#define DW_UART_CPR_FIFO_MODE_FROM_SIZE(size)			\
({								\
	typeof (size) __size = size;				\
								\
	static_assert(IS_ALIGNED((__size), 16));		\
	static_assert(__size <= DW_UART_CPR_FIFO_MODE_MAX);	\
								\
	FIELD_PREP_CONST(DW_UART_CPR_FIFO_MODE, __size / 16);	\
})

-- 
 i.