[SeaBIOS] Re: [PATCH] Fix serial port detection on systems with Blizzard Chrome EC

[Kevin O'Connor]

On Mon, Mar 16, 2020 at 05:39:45PM +0100, Paul Menzel wrote:
> From: "Ronald G. Minnich" 
> Date: Thu, 24 May 2012 11:27:26 -0700
>
> Serial port detection in SeaBIOS tests for bit 2 in the IER. That bit is
> never set, and SeaBIOS mistakenly reports 0 serial ports. As it happens,
> just testing for IER to be not-all-ones is good enough.
>
> This happens on all systems where the AP serial port uses the Blizzard
> EC (Ivy Bridge, Haswell). Does not affect Bay Trail, since we use the
> native Intel serial port.
>
> It’s a hardware limitation of which registers the Blizzard COMx port
> implementation provides. Nothing we can do in EC software.

Thanks.  At a high-level, the change looks fine to me.  However, I
don't think we should add dprintf() statements here - I fear they'll
just flood the log with uninteresting information when trying to debug
other issues.

-Kevin
> BUG=chrome-os-partner:9946
> TEST=build SeaBIOS with serial port enabled, and note that SeaBIOS
> says
>     init serial
>     Found 1 serial ports
> BRANCH=none
>
> Signed-off-by: Ronald G. Minnich
>
> Change-Id: I6bee73788ca6533091d85e54d05b37a564f910a7
> Reviewed-on: https://chromium-review.googlesource.com/184421
> Reviewed-by: Stefan Reinauer 
> Commit-Queue: Stefan Reinauer 
> Tested-by: Stefan Reinauer 
> Reviewed-on: https://chromium-review.googlesource.com/227500
> Signed-off-by: Paul Menzel
>
> src/serial.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/src/serial.c b/src/serial.c
> index 88349c8..e67ac77 100644
> --- a/src/serial.c
> +++ b/src/serial.c
> @@ -26,10 +26,12 @@ detect_serial(u16 port, u8 timeout, u8 count)
>          return 0;
>      outb(0x02, port+SEROFF_IER);
>      u8 ier = inb(port+SEROFF_IER);
> if (ier != 0x02)
> dprintf(8, "IER is 0x%02x (Expected 0x02 or 0x00)\n", ier);
> if (ier != 0x02 && ier != 0)
>     return 0;
> u8 iir = inb(port+SEROFF_IIR);
> if ((iir & 0x3f) != 0x02)
>
> dprintf(8, "IIR is 0x%02x (Expected != 0x3f)\n", iir & 0x3f);
>
> if ((iir & 0x3f) == 0x3f)
>     return 0;
>
> outb(0x00, port+SEROFF_IER);