Re: [PATCH] ACPI: SPCR: Support UART clock frequency field

[Markus Probst]

On Fri, 2026-05-08 at 21:19 +0200, Rafael J. Wysocki wrote:
> On Tue, May 5, 2026 at 9:36 PM Markus Probst <markus.probst@xxxxxxxxx> wrote:
> > 
> > The Microsoft Serial Port Console Redirection (SPCR) specification
> > revision 1.08 comprises additional field: UART Clock Frequency [1].
> > 
> > It contains a non-zero value indicating the UART clock frequency in Hz.
> > 
> > Link: https://learn.microsoft.com/en-us/windows-hardware/drivers/serports/serial-port-console-redirection-table [1]
> > Signed-off-by: Markus Probst <markus.probst@xxxxxxxxx>
> > ---
> >  drivers/acpi/spcr.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/acpi/spcr.c b/drivers/acpi/spcr.c
> > index 73cb933fdc89..d80ad87efae0 100644
> > --- a/drivers/acpi/spcr.c
> > +++ b/drivers/acpi/spcr.c
> > @@ -220,6 +220,9 @@ int __init acpi_parse_spcr(bool enable_earlycon, bool enable_console)
> >         if (!baud_rate) {
> >                 snprintf(opts, sizeof(opts), "%s,%s,0x%llx", uart, iotype,
> >                          table->serial_port.address);
> > +       } else if (table->header.revision >= 3 && table->uart_clk_freq) {
> > +               snprintf(opts, sizeof(opts), "%s,%s,0x%llx,%d,%u", uart, iotype,
> > +                       table->serial_port.address, baud_rate, table->uart_clk_freq);
> >         } else {
> >                 snprintf(opts, sizeof(opts), "%s,%s,0x%llx,%d", uart, iotype,
> >                          table->serial_port.address, baud_rate);
> > 
> > ---
> 
> Please have a look at the sashiko.dev feedback at
> 
> https://sashiko.dev/#/patchset/20260505-acpi_spcr-v1-1-fd4bc6f4eb53%40posteo.de
> 
> and let me know what you think.
For the first comment, I don't think thats an issue, as there is no
length check for the other fields either.


The second comment however seems true.

while earlycon parses

<baudrate>,<uartclk>

regular uart parses

<baudrate><parity><bits><flow>

This is strange, because according to kernel documentation, options for
uart should be identical for earlycon and console kernel parameters.
[1] [2].

Thanks
- Markus Probst


[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/tty/serial/serial_core.c?h=v7.1-rc1#n2097

[2]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/admin-guide/kernel-parameters.txt?h=v7.1-rc1#n1451

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