Date: Fri, 28 Aug 2009 08:32:19 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-hackers@freebsd.org Cc: remodeler <remodeler@alentogroup.org> Subject: Re: MBR hack for serial console Message-ID: <200908280832.19782.jhb@freebsd.org> In-Reply-To: <20090828033609.M34898@alentogroup.org> References: <20090826204513.M28384@alentogroup.org> <200908270808.09790.jhb@freebsd.org> <20090828033609.M34898@alentogroup.org>
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On Thursday 27 August 2009 11:36:09 pm remodeler wrote:
> This is in response to John Baldwin's response to my question about using=
a
> non-legacy IO port for an i386 serial console.
>=20
> I notice that src/sys/boot/i386/boot0/boot0.S uses a call to the PC BIOS =
int
> 14h routine to access the serial console, if SIO is defined. I believe SI=
O is
> defined if make.conf specifies a port for BOOT_COMCONSOLE_PORT (though I'm
> probably wrong), in which case the boot0 executable would be made from
> src/sys/boot/i386/boot0/boot0sio when recompiling the boot blocks with:
>=20
> # cd /sys/boot
> # make clean
> # make
> # make install
>=20
> The 14h pc bios routine is definitely limited to 0x3F8, 0x2F8, 0x3E8, or =
0x2E8
> because it is called with the DX register set to specify COM1-COM4 (dx se=
t to
> 0, 1, 2, or 3).
That's probably true that the BIOS routines likely do not handle a serial p=
ort
in an add-on card. However, just setting the I/O address to 03xf8 may not =
make
the BIOS routines work either since the BIOS may know that there isn't a CO=
M1
on the motherboard so it may just fail I/O w/o trying.
> boot2 looks like I might be able to specify the 0xe800 address for the se=
rial
> console. src/sys/boot/i386/boot2/sio.S has this definition: .set
> SIO_PRT,SIOPRT but I am not sure where SIOPRT is set. I don't see if in a=
ny
> makefile. I would guess it is being set by BOOT_COMCONSOLE_PORT also?
Correct. It's set via 'CFLAGS' in boot2's Makefile.
> > if the device is behind a PCI-PCI bridge (which it probably is=20
> > if it is on bus 3) you have the problem that 0x3f8 is probably not
> > in the IO range decoded by the parent bridge
>=20
> The device is behind a bridge: pcib0 --> pci0 --> pcib3 --> pci3, but AMD
> doesn't have a data sheet available on the 'net to tell me what IO range =
is
> decoded by the parent bridge (750SB southbridge chip).
A verbose dmesg will tell you as it isn't a fixed range but it assigned by =
the
BIOS just like a BAR.
> I also notice in the PCI specification this ("I/O Space Address Decoding =
for
> Legacy Devices"):
>=20
> A function that supports a PC legacy function (IDE, VGA, etc.) is allowed=
to
> claim those addresses associated with the specific function when the I/O =
Space
> (see Figure 6-2) [the configuration space's Command Register layout, bit =
0]
> enable bit is set.
>=20
> These addresses are not requested using a Base Address register but are
> assigned by initialization software. If a device identifies itself as a l=
egacy
> function (class code), the initialization software grants the device
> permission to claim the I/O legacy addresses by setting the device=92s I/=
O Space
> enable bit.
Note the 'not requested using a BAR' part of the quote. :-/ The way these
transactions are handled by PCI-PCI bridges is there is a bit that effectiv=
ely
instructs the bridge to claim legacy I/O or VGA ranges. However, the PCI-I=
SA/LPC
bridge device probably is already setup that way so that I/O accesses to th=
e RTC,
keyboard controller, etc. all work.
> > read the current value of the BAR instead and use that address?
>=20
> If I can set the serial console to a non-legacy port in boot2, that would=
be
> ideal. I don't think the boot0 routine would cause problems; it is just
> looking for keystrokes AFAIK. In this approach, I would guess it would ma=
ke
> sense to read the value and set SIOPRT to it in boot2? I think from this =
point
> on, the serial console calls use sio.S, and these functions (sio_init, et=
c.)
> access the hardware directly with assembler out commands rather than usin=
g the
> PC BIOS routines like boot0. Would something as simple as:
>=20
> .set SIO_PRT,0xe800
>=20
> in src/sys/boot/i386/boot2/sio.S work? If it does, I can develop code to =
read
> and set it to the current port instead of hard-coding it.
Yes, I would try that. You can also just do 'BOOT_COMCONSOLE_PORT=3D0xe800=
' to
see if that will work as a test w/o having to modify any of the boot2 code
directly.
=2D-=20
John Baldwin
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