On Thu, Feb 15, 2024 at 09:26:21AM +0000, Rengarajan.S@xxxxxxxxxxxxx wrote: > On Wed, 2024-02-14 at 15:50 +0200, Andy Shevchenko wrote: ... > > + /* > > + * 8250 core considers prescaller value to be always 16. > > + * The MCHP ports support downscaled mode and hence the > > + * functional UART clock can be lower, i.e. 62.5MHz, than > > + * software expects in order to support higher baud rates. > > + * Assign here 64MHz to support 4Mbps. > > + * > > + * The value itself is not really used anywhere except baud > > + * rate calculations, so we can mangle it as we wish. > > + */ > > + port->port.uartclk = 64 * HZ_PER_MHZ; > > As per internal MCHP DOS, PCI1XXXX driver uses a simple method of > converting "legacy 16 bit baud rate generator" to a "32 bit fractional > baud rate generator" which enables generation of an acceptable baud > rate from any valuable frequency. > > This is applicable only when the baud clock selected is 62.5 MHz, so > when we configure the baud clock to 64 MHz(as above) will it be > downscaled to 62.5 MHz, thus supporting the above feature? I specifically added the above comment. If you look closer, your driver does not use this value at all, the 8250 port code uses it in several places: - 8250_rsa case (not applicable to your driver) - probe_baud() call (applicable iff the kernel command line misses the baudrate, but even without this patch it's broken for your driver) - serial8250_update_uartclk() call (not applicable to your driver) - serial8250_get_baud_rate() call (only to get max and min range; my change will have an effect on min (max is exactly what your quirk is doing right no), so 62500000/16/65535 ~= 59.6, while with my change 64000000/16/65535 ~= 61.0, but standard baudrate here is 50 and 75, the former isn't supported by the existing code either - serial8250_do_get_divisor() call when magic_multiplier supplied (not applicable to your driver) - autoconfig_16550a() call (not applicable to your driver) Hope this clarifies the case. Of course if you able to test, will be even better. But wait for v2 where I update what Greg caught. ... > Acked-by: Rengarajan S <rengarajan.s@xxxxxxxxxxxxx> Thank you! -- With Best Regards, Andy Shevchenko
