Re: [PATCH] xload: be more flexible when searching for second stage bootloader.

[Sascha Hauer <s.hauer@pengutronix.de>]

On Tue, Sep 29, 2015 at 08:06:35PM +0100, vj wrote:
> On Tue, Sep 29, 2015 at 8:05 AM, Sascha Hauer <s.hauer@pengutronix.de> wrote:
> > On Sun, Sep 27, 2015 at 03:08:51PM +0100, Vicente Bergas wrote:
> >> A first stage bootloader can read fat and ext4 filesystems, and
> >> even both can be compiled-in at the same time.
> >> But then xload has a hardcoded fat filesystem mount option which
> >> renders ext4 unusable.
> >> This patch tries to mount it as ext4 if the fat attempt fails.
> >>
> >> Then, a typical use case of an ext4 formatted filesystem
> >> is it to be a standard linux filesystem, which contains
> >> boot-related files in /boot. So, when searching for the second stage
> >> bootloader, try /boot/barebox.bin after not finding it in /barebox.bin
> >>
> >> Signed-off-by: Vicente Bergas <vicencb@gmail.com>
> >> ---
> >>  arch/arm/mach-omap/xload.c | 4 ++++
> >>  1 file changed, 4 insertions(+)
> >>
> >> diff --git a/arch/arm/mach-omap/xload.c b/arch/arm/mach-omap/xload.c
> >> index ebcbcbc..c5e29f8 100644
> >> --- a/arch/arm/mach-omap/xload.c
> >> +++ b/arch/arm/mach-omap/xload.c
> >> @@ -118,6 +118,8 @@ static void *omap_xload_boot_mmc(void)
> >>       partname = asprintf("%s.0", diskdev);
> >>
> >>       ret = mount(partname, "fat", "/", NULL);
> >> +     if (ret)
> >> +             ret = mount(partname, "ext4", "/", NULL);
> >
> > It should also be possible to pass NULL as fs type in which case we
> > automatically detect the fs type. Could you check that?
> 
> I didn't know that. This way it would be nicer and cleaner!
> So it has been tested but it didn't work :(
> mount calls detect_fs, detect_fs calls file_name_detect_type and
> file_name_detect_type calls open.
> open fails to open the "mmc0.0" file name

Ok, I see. This indeed can't work as it depends on devfs being mounted
on /dev/. For this to work we would need a cdev_detect_filetype(const
char *name) which uses the cdev_* operations to read a buffer and pass
that to file_detect_type(). You're invited to implement that ;)

> So, the first 128KB block can hold an MBR plus partition table, then a
> raw image of the MLO file can be present at offset 128KB followed by
> the disk partitions.
> 
> In section 26.1.7.5.3 page 4939 figure 26-17 MMC/SD Booting
> states that raw mode detection is performed before searching for the
> MLO file in a filesystem.
> 
> All in all, it is possible to have a "fat-free" SD-card.

Now that you say it I remember having read about this before.

Does this work well with the rest of barebox? In omap_env_init() we
mount the first mmc partition to /boot and put the environment to
/boot/barebox.env. That doesn't work when the first partition doesn't
contain a FAT. Do you put your environment somewhere else?

Sascha

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