Subject: Re: OF properties access ?

From: Benjamin Herrenschmidt <[email-masked]>
Date: Mon, 19 Jan 2004 20:01:33 +1100

On Mon, 2004-01-19 at 01:32, Sven Luther wrote:
> On Mon, Jan 19, 2004 at 10:21:36AM +1100, Benjamin Herrenschmidt wrote:
> >
> > On Sun, 2004-01-18 at 23:07, Sven Luther wrote:
> > > On Sat, Jan 17, 2004 at 02:48:11PM +1100, Benjamin Herrenschmidt wrote:
> > > >
> > > > >
> > > > > But this doesn't reply me on why my attempts to read back integer values
> > > > > from OF property only result in the machine hanging, and since it is
> > > > > really early on, i don't even get serial console output to have an idea
> > > > > why :/
> > > > >
> > > > > in particular i cloned the code reading l2cr-value, and changed it to
> > > > > l2cr and instead of reading the value, i got only kernel silently dying.
> > > >
> > > > Show me the code and the relevant device-tree bits.
> > >
> > > Ok, here is the code that dies :
> > >
> > > void pegasos_set_l2cr(void) {
> >
> > Grrr... please put the { on a different line
>
> Ok. (altough i think this is a subject of philosophical debate :)
>
> > >         struct device_node *root = find_path_device("/");
> > >         char *machine;
> > >         struct device_node *np;
> > >
> > >         /* On Pegasos, enable the l2 cache if needed, as the OF forgets * it */
> >
> > Check if root isn't NULL...
>
> Ok.
>
> > >         machine = get_property(root, "model", NULL);
> >
> > Check if machine isn't NULL...
>
> Ok.
>
> > >         if (strncmp(machine, "Pegasos", 7) == 0) {
> > >                 /* Enable L2 cache if needed */
> > >                 np = find_devices ("cpus");
> > >                 if (np == 0)
> > >                         np = find_type_devices("cpu");
> > >                 if (np != 0) {
> > >                         unsigned int *l2cr = (unsigned int *)
> > >                                 get_property (np, "l2cr", NULL);
> >
> > The above got you a _pointer_ to the value, the code below uses
> > that point directly as the value, which is obviously wrong
>
> Arg, i copied the wrong version.
>
> > >                         if (!(l2cr & 0x80000000)) {
> > >                                 _set_L2CR(0);
> > >                                 _set_L2CR(l2cr | 0x80000000);
> > >                         }
> >
> > The above should have been...
> >
> > 			if (l2cr && !((*l2cr) & L2CR_L2E)) {
> > 				_set_L2CR(0);
> > 				_set_L2CR((*l2cr) | L2CR_L2E);
> > 			}
>
> In reality, i had both the code using l2cr, and the one using *l2cr, and
> even a version like the pmac_setup code, which copied the content of the
> l2cr pointer to a l2cr_value int, but none worked.
>
> > Now that is said providing your algorithm is right, that is the
> > device-tree provides you with an "l2cr" property containing the
> > right settings but not the enable bit, and matching what your CPU
> > is doing. If the device-tree has L2E set but not the CPU, you
> > may want to do things differently...
>
> Well, i have bugged the OF people, and they have indeed set the L2E bit
> in future versions of the OF. Mostly what happens right now, is that L2E
> is not set (i get a something around 30% decrease in compilation time
> when doing the l2cr=0x80000000 thingy) and the l2cr property returns 0.
>
> I also have had similar troubles with the /proc/pci@C0000000/display
> bus_number property, altough it didn't die there.

Well... You can move that code to a later place where you can printk
and debug waht's up. It's also possible that the _set_L2CR routine
in the kernel you are working on isn't totally correct for your CPU
model... Do you have the 7447 recongnized at all in cputable ?

> > > $ hexdump /proc/device-tree/cpus/PowerPC,74x7/l2cr
> > > 0000000 0000 0000
> >
> > So you "l2cr" value is just 0... Setting it to 0x80000000 means enabled
> > without any setting bit... that might actually be correct for on-die L2
> > though, check the CPU spec anyway. It is correct for my 7455
>
> It is either a 7447 or a 750Cxe. It is correct for both, and we have
> been using the l2cr=0x80000000 option as kernel boot option since ages.
> It quickly becomes old to have to type this per hand each time though,
> as we have no boot loader (yet).
>
> Friendly,
>
> Sven Luther
--
Benjamin Herrenschmidt <[email-masked]>

** Sent via the linuxppc-dev mail list. See http://lists.linuxppc.org/