Re: [PATCH 3/3] state: support backend-diskuuid / backend-offset

From: Michael Olbrich <m.olbrich@pengutronix.de>
To: Sascha Hauer <sha@pengutronix.de>
Cc: barebox@lists.infradead.org
Subject: Re: [PATCH 3/3] state: support backend-diskuuid / backend-offset
Date: Wed, 26 Jan 2022 10:35:13 +0100	[thread overview]
Message-ID: <20220126093513.GI11273@pengutronix.de> (raw)
In-Reply-To: <20220126075727.GD23490@pengutronix.de>

On Wed, Jan 26, 2022 at 08:57:27AM +0100, Sascha Hauer wrote:
> On Mon, Jan 24, 2022 at 11:04:58AM +0100, Michael Olbrich wrote:
> > On some platforms (e.g. EFI on x86_64) the state backend can only be
> > selected by a partiton UUID. On existing devices with a DOS partition
> > table, there may be no spare partition available for state.
> > 
> > This makes it possible to select the disk via UUID. The exact position is
> > defined by an explicitly specified offset.
> > 
> > Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
> > ---
> > 
> > I wasn't sure where to add the helper function. Is include/fs.h ok or
> > should I put it somewhere else?
> > 
> > I'll implement the same helper for dt-utils, so we can avoid additional
> > #ifdef here.
> > 
> >  common/state/state.c | 55 +++++++++++++++++++++++++++++---------------
> >  include/fs.h         | 12 ++++++++++
> >  2 files changed, 49 insertions(+), 18 deletions(-)
> > 
> > diff --git a/common/state/state.c b/common/state/state.c
> > index 8c34ae83e52b..2a8b12d20c5a 100644
> > --- a/common/state/state.c
> > +++ b/common/state/state.c
> > @@ -592,6 +592,7 @@ struct state *state_new_from_node(struct device_node *node, bool readonly)
> >  	const char *backend_type;
> >  	const char *storage_type = NULL;
> >  	const char *alias;
> > +	const char *diskuuid;
> >  	uint32_t stridesize;
> >  	struct device_node *partition_node;
> >  	off_t offset = 0;
> > @@ -607,30 +608,48 @@ struct state *state_new_from_node(struct device_node *node, bool readonly)
> >  	if (IS_ERR(state))
> >  		return state;
> >  
> > -	partition_node = of_parse_phandle(node, "backend", 0);
> > -	if (!partition_node) {
> > -		dev_err(&state->dev, "Cannot resolve \"backend\" phandle\n");
> > -		ret = -EINVAL;
> > -		goto out_release_state;
> > -	}
> > +	ret = of_property_read_string(node, "backend-diskuuid", &diskuuid);
> 
> This needs some documentation in
> Documentation/devicetree/bindings/barebox/barebox,state.rst.

I can do that.

> > +	if (ret == 0) {
> > +		u64 off;
> > +
> > +		ret = devpath_from_diskuuid(diskuuid, &state->backend_path);
> > +		if (ret) {
> > +			dev_err(&state->dev, "state failed find backend device for diskuuid='%s'\n",
> > +				diskuuid);
> > +			goto out_release_state;
> > +		}
> > +		ret = of_property_read_u64(node, "backend-offset", &off);
> 
> I stumbled upon this because you have to use a 64bit type here instead
> of using 'offset' directly. I think 'offset' should be 64bit instead so
> that larger offsets can be used.

It's not that simple. 'offset' used as a 'off_t' and 'ssize_t' all over the
place in the state framework. On 32bit architecture both are defined as
'long' or 'int'. Both are 32 bit types so changing 'offset' to a 64bit type
here doesn't really help.
On 64bit architectures 'off_t' and 'ssize_t' are both 'long' which is a
64bit type. We still need to cast from unsigned to signed, but I don't
think that's really a problem here.

So it works for large offsets on 64bit architectures as it is, and it can
be extended to work in 32bit architectures without changing the binding.
But doing that requires larger code changes. And I have no way to test
those.

> > +		if (ret) {
> > +			dev_err(&state->dev, "'backend-offset' property undefined\n");
> > +			goto out_release_state;
> > +		}
> > +		offset = off;
> 
> What about the size of the state partition? This is not set anywhere in
> this case so it's still zero. It should be specified the in device tree
> as well. At the same time I'm a bit nervous that it apparently still
> works with size zero.

The code explicitly checks if the size is specified and skips any range
checks if it's not. From what I can tell, it has been like that from the
beginning.

If that's not ok, then I could add a 'backend-size' property, or do you
have something else in mind?

Regards,
Michael

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