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

From: Benjamin Herrenschmidt <[email-masked]>
Date: Wed, 09 Oct 2013 15:23:21 +1100

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 ? Especially if it goes into opal.h it should stick to the
types used in that file.

> > +#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.

> > @@ -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...

Cheers,
Ben.