On Thu, Nov 20, 2008 at 01:07:11PM -0600, Milton Miller wrote:

[I'm going to reply to several points in this thread in one reply.  I  
have restored context that was trimmed in later replys when I wanted to  
speak to.]

David Gibson wrote at 2008-11-18 00:28:28:

quoted

On Mon, Nov 17, 2008 at 01:41:24PM -0600, Timur Tabi wrote:

quoted

On Thu, Oct 23, 2008 at 9:54 PM, David Gibson
[off-list ref] wrote:

quoted

This patch adds a new backend for the hvc console based on the
low-level udbg callbacks.  This effectively implements a working
runtime console in terms of the simple udbg primitives.  This is kind
of a hack - since udbg isn't something you really want to be using
routinely - but it's really useful during bringup.

Why is udbg hvc something you want to not use routinely?

Well.. because the udbg methods themselves are not sometimes (not
always, but reasonably often), something you don't really want to use
except during bringup.  A number of them use various hacks that lets
them work very early, and very reliably even when much of the kernel
is borked, but aren't the sort of thing you want to do except during
bringup.  e.g. being mapped by special bolted TLB entries established
very early, rather than using ioremap(); polled access for interrupt
capable devices; hard-coded device addresses, meaning the kernel will
only work on one precise type of system.

Stated differently, if your routine (1) fundamently works one character  
at a time and (2) is not interrupt driven, and (3) only supports one  
channel, what avantage is there to an explicit hvc driver?

Using standard kernel interfaces, instead of random early debug hacks.

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This can be used to quickly implement a userspace usable console  
while
you're working on a proper driver for whatever console I/O device the
hardware has.  Or, it can be used to avoid writing a full blown
tty/console driver entirely for quick-and-dirty I/O hardware that  
will
later be replaced by something else.

Actually it looks remarkably similar to a cleaned up version of a patch  
i've been using since hvc_console was split to be a hookable shell.  Or  
was it the motivation for adding the hooks?  The code is different in  
how it structures the error checks and that it is implemented in  
drivers/char where it belongs (hence the cleaned up comment) but the  
fact that David recreated what I did does speak to its general
utility.

Heh, ok.

quoted

quoted

David, I have a stupid question.  I already have an HVC console driver
that works.  Do I need to do anything, other than enable HVC_UDBG, to
get my hvc console drive to use udbg?

As was pointed out elsewhere, this is hvc calling udbg not udbg hooks  
calling hvc_cosole methods, which might need a different context.  But I 
wanted to comment on David's reply:

quoted

Um.. well.. if you have both activated, I think you can select which  
HVC console backend will be used by using console=/dev/hvcNN on the  
commandline, where values of NN correspond to different backends, in  
order depending on link order in some complex fashion.

It doesn't just work that way because of the way the hvc_console shell  
implements the console hooks.

If the drivers request different hvc channel numbers, then yes, you can  
select which hvc backend is used by choosing console=hvcNN.  If all  
request 0, then which ever one registers with hvc_console first wins the 
slot, and the others get a busy error return.   They can still register 
their tty later, at which point they get a dynamically determined 
/dev/hvcN slot, above all registered console slots.

Ah, ok, my mistake.

There are multiple reasons for this design.  The original user, pseries  
vterm, wants to assign the channel number based on what the hypervisor  
tells it the channel should be.  There are two loops though the device  
tree, searching for different protocols (and consequently different  
backends), so they need to be able to specify the number and not just  
use the order they are found.  The platform describes not only the  
protocol, but which instance that is, and the backend currently  
registers with that number.  Another reason is its a bad idea to have  
console=/dev/hvc0 depend on link order instead of knowing which driver  
is selected.  If a user is specifing what the console is, it should not  
depend on other linked drivers.

Until this point, all mainline drivers have been exclusive in that only  
one will actually register with hvc_console midlayer on any given  
platform.  This is the first backend that is not exclusive, and  
therefore its coexistence needs special attention.

In my internal tree, I register (my version of) this not as hvc0 but as  
hvc4.  hvc0 is rtas, hvc1-3 are an internal backend, and this one is  
hvc4.  The order I chose is arbitrary, but the main point is it does't  
compete with hvc_rtas for slot 0.  (All of these drivers coexist, and I  
can choose which one I want to use based on my needs for that boot.)  We 
could make it configurable via Kconfig, or just choose a slot.  (I think 
vterm can have 2 channels on some boxes where they drive real serial 
ports on the box.)

Hrm, ok.  Not sure what the sensible way to do this for my case would
be.

quoted

But if you already have a working HVC console driver, I don't see why
you'd want to use HVC_UDBG - it's essentially a bringup hack.

As I said, why should we need a fancier hvc_console backend if you are  
polling and gain no efficency processing  multiple characters at once.

The udbg drivers know if they are sharing the interface with a  
hvc_console some other driver.  If we trigger the registration of this  
backend on the udbg hook saying its useful (eg by setting a flag word in 
the function where we assign the udbg putc and getc pointers) then this 
driver will not need to be compiled out for "production" kernels.

David Gibson wrote at 2008-11-19 00:42:20:

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On Tue, Nov 18, 2008 at 09:06:17AM -0600, Timur Tabi wrote:

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David Gibson wrote: [at 2008-11-18 05:14:08]

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Given the variety of strange I/O configurations in prototype and
embedded platforms, I can't imagine this was a unique situation.  So
I've pushed my patch out, so anyone else in a similar situation can
immediately turn their little udbg methods for whatever strange I/O
they have into a fully-functional console.  Maybe it's not something
you'd want to go to release with, but it certainly simplifies life
during bringup.

Ok, I understand now.  However, I would like to see two changes:

1) Re-arrange the Makefile as I pointed out in another post.

[the other post]
Timur Tabi - 2008-11-17 20:18:31

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One other thing ...
HVC console drivers must be compiled before hvc_console.o

see http://git.kernel.org/?p=linux/kernel/git/paulus/powerpc.git; 
a=commit;h=938473b24636d77dc5e9c3f41090d071b6cf4389

[end other post]

quoted

Um.. yeah.. I'm a bit baffled by this.. all the existing backends
are listed after hvc_console, I just added hvc_udbg to the end.  I
didn't really understand the rationale in that commit, but then I
haven't had time to look at it very much yet.

I too challanged the necessity of that change.  I carefully designed the 
hvc_console layer to find the selected console weither it was registered 
before or after the console_initcall of hvc_console.

I am very disapointed that the changelog is so sparse.

Uh... but you're the author of that commit...?

However, I think the bottom line was that add_preferred_console is  
suppsoed to be called by architecture setup code.   Doing it the  
console_initcall is almost abuse.  But if the console_initcall is going  
to call add_preferred_console, then it must link before the hvc_console  
driver.   I would have to go back and find the discussion to remember  
the exact details.  (maybe it only needed to call add_preferreed_console 
before registering itself if its the only hvc backend).

Which does bring up the point of avoinding calling add_preferred_console 
to udbg just because it is linked in.  The udbg getc routines might work 
for xmon, but one would probably not like the result of the real driver 
and udbg trying to read the same device at the same time.  Perhaps the 
same flag that says "consider me or udbg_hvc" should apply, or we do 
add_preferred_console in arch code before console_initcall.  (its ok to 
preferr a console that never registers as long as another preferred 
console does register).

Ah, yeah.  I'm not sure if my hvc_udbg really wants an
add_preferred_console.  It's there because it was in hvc_rtas or
whichever other hvc backend I based this on (forgotten which, now).
And the semantics of console selection makes my brain hurt, so I'm not
sure if it's actually needed for this case or not.

quoted

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2) Update the Kconfig help file to be very clear that this feature
is only meaningful if the platform has a udbg back-end but no other
console or TTY driver.

Alrighty...

That does bring up the point, I don't see any kconfig help for this  
user-selectable option.

Yeah, that's a good point.

And if we don't have the udbg hooks say to enable it, then we really  
need to have text discouraging its use as it might break their console.

Possibly it should just be dependent on CONFIG_PPC_EARLY_DEBUG.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson