[PATCH v4 2/4] serial: 8250_dw: build Renesas RZN1 CPR value from DW_UART_CPR_* definitions

[Andy Shevchenko]

On Fri, Apr 24, 2026 at 05:09:46PM +0300, Ilpo Järvinen wrote:
> On Fri, 24 Apr 2026, Andy Shevchenko wrote:
> > On Fri, Apr 24, 2026 at 02:38:56PM +0300, Ilpo Järvinen wrote:
> > > On Fri, 24 Apr 2026, Jia Wang wrote:

...

> > > Hmm, maybe there should also be macro also for this one which takes the 
> > > fifosize as input and converts it to CPR field vlaue (effectively, the 
> > > macro is an inverse of DW_UART_CPR_FIFO_SIZE()). It would be more readable 
> > > than the literal.
> > 
> > But this can be done separately, right?
> 
> It's logically part of the cpr_value literal to defines conversion (of 
> course one could introduce it in own patch without users but I don't see 
> much benefit from that).

No objections.

> > > Also include BUILD_BUG_ON(!IS_ALIGNED(fifosize, 16) + bounds checks) 
> > > inside that macro to catch invalid fifo sizes (+ don't forget the 
> > > necessary headers for those two new things).
> > 
> > Hmm... Some devices may have FIFO = 8 or 4 bytes (Intel Quark IIRC has less
> > than 16 and it's DW IP).
> 
> Perhaps but according the DW databook I've at hand, there are no values in 
> FIFO_MODE field for such FIFO sizes. So what would CPR contain in those
> cases for FIFO_MODE field?

On real HW it returns 0s for all registers above 0x07 (multiplied by 4 as
a stride). But I also checked the values of xmit_fifo_size and it shows 16.
What 8 is it is the DMA max_burst. Hence the proposed assertion should work.

-- 
With Best Regards,
Andy Shevchenko