Subject: Re: [PATCH] powerpc/powernv: Add a debugfs file to read the firmware console

From: Michael Ellerman <[email-masked]>
Date: Wed, 9 Oct 2013 17:06:14 +1100

On Wed, Oct 09, 2013 at 03:23:21PM +1100, Benjamin Herrenschmidt wrote:
> On Wed, 2013-10-09 at 14:23 +1100, Michael Ellerman wrote:
> > On Tue, Oct 08, 2013 at 06:46:40PM +1100, Benjamin Herrenschmidt wrote:
> > > With OPALv3, the firmware can provide the address of it's internal console
> > > to Linux, which we can then display using debugfs. This is handy for
> > > diagnostics and debugging.
> > > 
> > > Signed-off-by: Benjamin Herrenschmidt <[email-masked]>
> > > ---
> > > 
> > > diff --git a/arch/powerpc/platforms/powernv/opal.c b/arch/powerpc/platforms/powernv/opal.c
> > > index 2911abe..10d7894 100644
> > > --- a/arch/powerpc/platforms/powernv/opal.c
> > > +++ b/arch/powerpc/platforms/powernv/opal.c
> > > @@ -17,6 +17,8 @@
> > >  #include <linux/interrupt.h>
> > >  #include <linux/notifier.h>
> > >  #include <linux/slab.h>
> > > +#include <linux/debugfs.h>
> > > +#include <linux/uaccess.h>
> > >  #include <asm/opal.h>
> > >  #include <asm/firmware.h>
> > >  
> > > @@ -27,6 +29,21 @@ struct opal {
> > >  	u64 entry;
> > >  } opal;
> > >  
> > > +/* OPAL in-memory console */
> > 
> > It might be nice to point out that the format of the struct is defined
> > by OPAL and must be in sync with what OPAL is using.
> 
> Yes, we could move the structure definition to opal.h...
> 
> > > +struct memcons {
> > > +	uint64_t magic;
> > 
> > u64 ?
> 
> Who cares ? 

That's what we use in the kernel. But no I don't care.

> > > +#define MEMCONS_MAGIC	0x6630696567726173
> > > +	uint64_t obuf_phys;
> > > +	uint64_t ibuf_phys;
> > > +	uint32_t obuf_size;
> > > +	uint32_t ibuf_size;
> > > +	uint32_t out_pos;
> > > +#define MEMCONS_OUT_POS_WRAP	0x80000000u
> > > +#define MEMCONS_OUT_POS_MASK	0x00ffffffu
> > 
> > Where does this come from?
> 
> My a** :-) I made it up as I wrote the OPAL side one, why ?

...
 
> > > +	uint32_t in_prod;
> > > +	uint32_t in_cons;
> > > +};
> > 
> > Should it be packed?
> 
> Nope, no need. It's all nice and naturally aligned.

Sure, that's obvious.
 
> > > @@ -369,6 +386,90 @@ static irqreturn_t opal_interrupt(int irq, void *data)
> > >  	return IRQ_HANDLED;
> > >  }
> > >  
> > > +#ifdef CONFIG_DEBUG_FS
> > > +static ssize_t opal_memcons_read(struct file *file, char __user *to,
> > > +				 size_t count, loff_t *ppos)
> > > +{
> > > +	struct memcons *mc = file->private_data;
> > > +	size_t available, ret, chunk0, chunk1, lcount;
> > > +	const char *start, *conbuf = __va(mc->obuf_phys);
> > > +	loff_t opos, pos;
> > > +
> > > +	/*
> > > +	 * Find out how much is in the buffer. If it has wrapped
> > > +	 * the whole buffer, else just the beginning. It has wrapped
> > > +	 * if the next character is not \0
> > > +	 */
> > > +	if (mc->out_pos & MEMCONS_OUT_POS_WRAP) {
> > > +		available = mc->obuf_size;
> > > +		chunk1 = mc->out_pos & MEMCONS_OUT_POS_MASK;
> > > +		start = conbuf + chunk1;
> > > +		chunk0 = mc->obuf_size - chunk1;
> > > +	} else {
> > > +		available = mc->out_pos;
> > > +		start = conbuf;
> > > +		chunk0 = available;
> > > +		chunk1 = 0;
> > > +	}
> > 
> > Surely simple_read_from_buffer() could make some of this simpler?
> 
> If you can find a way to make it deal with a ring buffer...

Call it twice.

And you wonder why no one reviews your patches?

cheers