linux-kernel - Re: [PATCH 09/12] i386/x86_64: EHCI usb debug port early printk support.

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Date:	Mon, 30 Apr 2007 14:54:18 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andi Kleen <ak@...e.de>
Cc:	virtualization <virtualization@...ts.linux-foundation.org>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Dave Jones <davej@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: [PATCH 09/12] i386/x86_64: EHCI usb debug port early printk support.

Andi Kleen <ak@...e.de> writes:

> Thanks for writing that code. It should be an interesting alternative
> on boxes where firescope doesn't work.
>
> I hope I can eventually merge early firewire support code too.
>
> On Mon, Apr 30, 2007 at 10:32:02AM -0600, Eric W. Biederman wrote:
>> 
>> With legacy free systems serial ports have stopped being an option
>> to get early boot traces and other debug information out of a machine.
>
> This needs a CONFIG_* at least. And some documentation on how to set it
> up on both sides.

Besides CONFIG_EARLY_PRINTK I assume.  It is enough code so there is a
case for it.

>> This debug device can be used to replace serial ports for
>> kgdb, kdb, and console support.  And gregkh has a simple usb
>> serial driver for it so user space applications that control
>> serial ports should work unmodified.
>
> But not merged yet, right? I was hoping it could be done from
> user space anyways.

Sorry old comment, that piece has been merged for a while.
It is the usb_debug module.  It creates a tty device that you
can just cat to get the usb debug output.

>> For users the hard part looks like it will be finding cables and
>> finding which is usb debug port 1 and realizing that there is
>> flow control so the kernel boot will not happen if someone is not
>> reading the serial console data.
>
> That's nasty. Any way to work around that?

Maybe.  It has been long enough since I wrote the code
I need to go back and look.

>> index 92213d2..dc097aa 100644
>> --- a/arch/x86_64/kernel/early_printk.c
>> +++ b/arch/x86_64/kernel/early_printk.c
>> @@ -3,9 +3,19 @@
>>  #include <linux/init.h>
>>  #include <linux/string.h>
>>  #include <linux/screen_info.h>
>> +#include <linux/usb/ch9.h>
>> +#include <linux/pci_regs.h>
>> +#include <linux/pci_ids.h>
>> +#include <linux/errno.h>
>
> Can you put it in a separate file please?
> Perhaps with a little abstraction in drivers/usb ? 
>
>> +static void dbgp_breath(void)
>> +{
>> +	/* Sleep to give the debug port a chance to breathe */
>
> But you don't?

Good point.  At least this early I'm not certain there is any
way I can productively do that.  This is before we have calibrated
the tsc's and the like so timeouts are difficult, and as I
recall our default guess isn't.

This lack of a good timeout looks to be the reason ehci_wait_for_port
doesn't timeout in a timely fashion because I don't timeout until
I have wrapped a 32bit number.

>> +static __u32 __init find_dbgp(int ehci_num, unsigned *rbus, unsigned *rslot,
> unsigned *rfunc)
>
> This should be probably merged into the early quirks loop
>
>>   		early_console = &simnow_console;
>>   		keep_early = 1;
>> +	} else if (!strncmp(buf, "dbgp", 4)) {
>
> usb would seem to be more intuitive

Could be.  I was thinking usb debug port. dbgp is at least unique,
and unfortunately it doesn't look like any old usb cable will
do, so a straight usb I expect would be very misleading.

The truth is I don't have a big need for this.  I put it together as
a proof of concept to see how hard it would be, etc.  I can clean
it up a little but I'm really hoping I can get this into one of
the development trees and people who have more use for it then I
do can play with it and improve things.

One persons experience on two machines probably isn't quite enough
of a sample to Document how to use this.  At least beyond what I
did in my changelog.

Eric
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