[v1] ACPI: SPCR: Support UART clock frequency field

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

Posted by Markus Probst 1 month, 1 week ago

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@posteo.de>
---
 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);

---
base-commit: 9e0898f1c0f134c6bad146ca8578f73c3e40ac0a
change-id: 20260430-acpi_spcr-61902fd923f2

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

Posted by Rafael J. Wysocki 1 month ago

On Tue, May 5, 2026 at 9:36 PM Markus Probst <markus.probst@posteo.de> 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@posteo.de>
> ---
>  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.

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

Posted by Markus Probst 1 month ago

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@posteo.de> 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@posteo.de>
> > ---
> >  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

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

Posted by Markus Probst 1 month ago

On Sat, 2026-05-09 at 08:13 +0200, Markus Probst wrote:
> 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@posteo.de> 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@posteo.de>
> > > ---
> > >  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].
I might have misinterpreted the documentation here. That uartclk only
existing on earlycon seems to be intended. Feel free to ignore the last
paragraph of my reply.

Thanks
- Markus Probst