-----Original Message----- From: Andi Kleen [mailto:ak@suse.de] Sent: Wednesday, December 06, 2006 9:31 AM
Also for usb console keep should be made default because the output
won't
be duplicated.
Still need to tx_read to make console can take command?
Or transfer to generic usb_serial or usb_debug that Greg just added with console=ttyUSB0 to get tty? Then you still need to disable that early_console sometime later.
YH
On Wednesday 06 December 2006 21:43, Lu, Yinghai wrote:
-----Original Message----- From: Andi Kleen [mailto:ak@suse.de] Sent: Wednesday, December 06, 2006 9:31 AM
Also for usb console keep should be made default because the output
won't
be duplicated.
Still need to tx_read to make console can take command?
Or transfer to generic usb_serial
I think the protocols are incompatible?
or usb_debug that Greg just added
Ah I didn't notice that. If there is a usb_debug that works later then yes it would need to be disabled.
However I see a certain advantage to keep using the early usb console because it doesn't need any interrupts. So when the kernel is very confused after an oops it might have a higher chance to still get the oops out.
I haven't looked how the other usb_debug works -- if it's polled too then it wouldn't have much advantage.
Ok one advantage of a non early usb_debug is that it will properly use pci config space locking. The early implementation just ignores the port cf8 lock which might lead to corruption later. That's ok after an oops, but during normal output it can actually lead to data corruption if it interferes with somebody else's config write. Also on some systems cf8 is broken and doesn't work.
Disadvantage of using the locks of course is that it can deadlock if an oops happen inside the critical region. So they might need to be added to bust_spinlocks()
And it would be good if the late usb_debug still wouldn't rely on interrupts.
But I agree it's probably better to transition to another usb_debug console and not do keep by default.
-Andi
Andi Kleen ak@suse.de writes:
On Wednesday 06 December 2006 21:43, Lu, Yinghai wrote:
-----Original Message----- From: Andi Kleen [mailto:ak@suse.de] Sent: Wednesday, December 06, 2006 9:31 AM
Also for usb console keep should be made default because the output
won't
be duplicated.
Still need to tx_read to make console can take command?
Or transfer to generic usb_serial
I think the protocols are incompatible?
or usb_debug that Greg just added
Ah I didn't notice that. If there is a usb_debug that works later then yes it would need to be disabled.
However I see a certain advantage to keep using the early usb console because it doesn't need any interrupts. So when the kernel is very confused after an oops it might have a higher chance to still get the oops out.
I haven't looked how the other usb_debug works -- if it's polled too then it wouldn't have much advantage.
Ok one advantage of a non early usb_debug is that it will properly use pci config space locking. The early implementation just ignores the port cf8 lock which might lead to corruption later. That's ok after an oops, but during normal output it can actually lead to data corruption if it interferes with somebody else's config write. Also on some systems cf8 is broken and doesn't work.
Disadvantage of using the locks of course is that it can deadlock if an oops happen inside the critical region. So they might need to be added to bust_spinlocks()
And it would be good if the late usb_debug still wouldn't rely on interrupts.
But I agree it's probably better to transition to another usb_debug console and not do keep by default.
The only use of the early pci code is for finding the hardware. Everything else is through mmio.
The practical issue is that during the normal initialization of the ehci the reset of the hardware state is going to remove the delegation to the ehci debug registers.
Greg's current thing uses the hardware but through the normal interrupt driven usb methods. I think it is worth using the ehci debug registers if possible as that (except for reset) gives us independent control of what is going on.
For understanding what needs to happen except for the initialization just look at my dbgp_bulk_write routine. That and the functions it call is the only code in there that is executed. It is just a hair more complicated than other early debug port code because it has to deal with retransmits. But I think it is still under 100 lines of code.
Eric
or usb_debug that Greg just added
Ah I didn't notice that. If there is a usb_debug that works later then yes it would need to be disabled.
I detect confusion here ... remember that there are potentially two distinct Linux systems involved here:
- Target, with some kind of console hooked up to the debug device; runs this _new_ "early debug port" code.
- Host, to which that console connects (through the debug device); runs usb_debug, much like any other usb-serial device
It's analagous to debugging an embedded box using a serial console with a Linux host ... except the target here is a PC, not an ARM (or PPC, MIPS, etc) custom board.
Once the coexistence issues between the debug port and normal EHCI driver get worked, there's no reason not to keep using that debug port as a system console. Heck, being able to do that might be a huge win with some of the nasty suspend/resume problems we've got.
- Dave
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Andi Kleen -
David Brownell -
ebiederm＠xmission.com -
Lu, Yinghai