Re: [PATCH] mtd: ubi: attach MTD partition from device-tree

[Daniel Golle]

Hi Zhihao,

thank you for taking the time to review this patch.

On Sun, Apr 23, 2023 at 04:35:28PM +0800, Zhihao Cheng wrote:
> Hi Daniel,
> > Split ubi_init() function into early function to be called by
> > device_initcall() and keep cmdline attachment in late_initcall().
> > (when building ubi as module, both is still done in a single
> > module_init() call)
> > 
> > Register MTD notifier and attach MTD devices which are marked as
> > compatible with 'linux,ubi' in OF device-tree when being added, detach
> > UBI device from MTD device when it is being removed.
> > 
> > Keep behavior regarding ubiblock creation and attaching of UBI device
> > from kernel or module cmdline unchanged.
> > 
> > For existing users this should not change anything besides automatic
> > removal of (dead) UBI devices when their underlying MTD devices are
> > already gone, e.g. in case of MTD driver module or (SPI) bus driver
> > module being removed.
> > 
> > For new users this opens up the option to attach UBI using device-tree
> > which then happens early and in parallel with other drivers being
> > probed which slightly reduces the total boot time.
> > 
> > Attachment no longer happening late is also a requirement for other
> > drivers to make use of UBI, e.g. drivers/nvmem/u-boot-env.c can now
> > be extended to support U-Boot environment stored in UBI volumes.
> > 
> 
> Let me try to understand this patch, the main purpose is to add a new way to
> load ubi according to device tree, am I right?

You are right about one aspect of this patch, yes it does add another
way to attach MTD devices, by utilizing a devicetree compatible
'linux,ubi' which can be set for MTD devices (or partitions).
The other, and maybe even more significant aspect is that it moves
attaching the MTD device from being a late_initcall() to rather register
an MTD user driver instead and hence happen as a direct consequence of
the MTD device showing up.
This will be important when using UBI e.g. as an NVMEM provider:
E.g. on Wi-Fi routers it has become common for vendors to store the
bootloader environment, Ethernet MAC addresses or Wi-Fi calibration
data inside an UBI volume (and I don't mean as a file inside a
filesystem, but really just the UBI volume exposing nvmem cells for
other drivers). See [1] for an example.

> 
> > Signed-off-by: Daniel Golle <daniel@xxxxxxxxxxxxxx>
> > ---
> >   drivers/mtd/ubi/block.c |  20 ++---
> >   drivers/mtd/ubi/build.c | 163 +++++++++++++++++++++++++++++-----------
> >   drivers/mtd/ubi/cdev.c  |   2 +-
> >   drivers/mtd/ubi/ubi.h   |   4 +-
> >   4 files changed, 132 insertions(+), 57 deletions(-)
> > 
> > diff --git a/drivers/mtd/ubi/block.c b/drivers/mtd/ubi/block.c
> > index 3711d7f746003..b24b8b8f54b77 100644
> > --- a/drivers/mtd/ubi/block.c
> > +++ b/drivers/mtd/ubi/block.c
> > @@ -65,10 +65,10 @@ struct ubiblock_pdu {
> >   };
> >   /* Numbers of elements set in the @ubiblock_param array */
> > -static int ubiblock_devs __initdata;
> > +static int ubiblock_devs;
> >   /* MTD devices specification parameters */
> > -static struct ubiblock_param ubiblock_param[UBIBLOCK_MAX_DEVICES] __initdata;
> > +static struct ubiblock_param ubiblock_param[UBIBLOCK_MAX_DEVICES];
> >   struct ubiblock {
> >   	struct ubi_volume_desc *desc;
> > @@ -582,7 +582,7 @@ open_volume_desc(const char *name, int ubi_num, int vol_id)
> >   		return ubi_open_volume(ubi_num, vol_id, UBI_READONLY);
> >   }
> > -static void __init ubiblock_create_from_param(void)
> > +void ubiblock_create_from_param(void)
> >   {
> >   	int i, ret = 0;
> >   	struct ubiblock_param *p;
> > @@ -611,9 +611,10 @@ static void __init ubiblock_create_from_param(void)
> >   		ret = ubiblock_create(&vi);
> >   		if (ret) {
> > -			pr_err(
> > -			       "UBI: block: can't add '%s' volume on ubi%d_%d, err=%d\n",
> > -			       vi.name, p->ubi_num, p->vol_id, ret);
> > +			if (ret != -EEXIST && ret != -ENOENT)
> 
> The EEXIST and ENOENT types of error messages are filtered, why? After this
> patch applied, the user won't be aware of which ubi volume failed to create
> ubiblock.

The EEXIST and ENOENT errors as filtered as they may happen now without
that actually being an error.
ubiblock_create_from_param is potentially being called multiple times,
each time a new UBI device is attached in the "new" MTD user/device-tree
way, and also once in "classic" late_initcall way.

One way to partially resolve this could be to add a boolean parameter to
ubiblock_create_from_param which indicates if -ENOENT should be treated
as an error, and set that parameter only when calling from
late_initcall.

Being able to warn users if they added the same ubiblock device more
than once **on the cmdline or module parameter** is more tricky and
would require tracking why a ubiblock device has previously been
created. However, I don't think that emitting a warning in this case
is crucial at all, the user definitely wanted the ubiblock device to be
created and saying that more than once is unneccesary but yet also
doesn't leave room for any interpretation other than just that the user
*really* wants the ubiblock to be created.

> 
> > +				pr_err(
> > +				       "UBI: block: can't add '%s' volume on ubi%d_%d, err=%d\n",
> > +				       vi.name, p->ubi_num, p->vol_id, ret);
> >   			continue;
> >   		}
> >   	}
> > @@ -644,13 +645,6 @@ int __init ubiblock_init(void)
> >   	if (ubiblock_major < 0)
> >   		return ubiblock_major;
> > -	/*
> > -	 * Attach block devices from 'block=' module param.
> > -	 * Even if one block device in the param list fails to come up,
> > -	 * still allow the module to load and leave any others up.
> > -	 */
> > -	ubiblock_create_from_param();
> > -
> >   	/*
> >   	 * Block devices are only created upon user requests, so we ignore
> >   	 * existing volumes.
> > diff --git a/drivers/mtd/ubi/build.c b/drivers/mtd/ubi/build.c
> > index 9cd565daad368..a764f97eee791 100644
> > --- a/drivers/mtd/ubi/build.c
> > +++ b/drivers/mtd/ubi/build.c
> > @@ -27,6 +27,7 @@
> >   #include <linux/log2.h>
> >   #include <linux/kthread.h>
> >   #include <linux/kernel.h>
> > +#include <linux/of.h>
> >   #include <linux/slab.h>
> >   #include <linux/major.h>
> >   #include "ubi.h"
> > @@ -1065,6 +1066,7 @@ int ubi_attach_mtd_dev(struct mtd_info *mtd, int ubi_num,
> >    * ubi_detach_mtd_dev - detach an MTD device.
> >    * @ubi_num: UBI device number to detach from
> >    * @anyway: detach MTD even if device reference count is not zero
> > + * @have_lock: called by MTD notifier holding mtd_table_mutex
> >    *
> >    * This function destroys an UBI device number @ubi_num and detaches the
> >    * underlying MTD device. Returns zero in case of success and %-EBUSY if the
> > @@ -1074,7 +1076,7 @@ int ubi_attach_mtd_dev(struct mtd_info *mtd, int ubi_num,
> >    * Note, the invocations of this function has to be serialized by the
> >    * @ubi_devices_mutex.
> >    */
> > -int ubi_detach_mtd_dev(int ubi_num, int anyway)
> > +int ubi_detach_mtd_dev(int ubi_num, int anyway, bool have_lock)
> >   {
> >   	struct ubi_device *ubi;
> > @@ -1108,7 +1110,7 @@ int ubi_detach_mtd_dev(int ubi_num, int anyway)
> >   	 * EC updates that have been made since the last written fastmap.
> >   	 * In case of fastmap debugging we omit the update to simulate an
> >   	 * unclean shutdown. */
> > -	if (!ubi_dbg_chk_fastmap(ubi))
> > +	if (!have_lock && !ubi_dbg_chk_fastmap(ubi))
> >   		ubi_update_fastmap(ubi);
> 
> Why do you skip updating fastmap if ubi is detached in mtd notification way?

The reason here is simple: Once we receive a notification about the MTD
device being removed it is too late to update fastmap. The device is
gone, nothing we can do about that any more. Just like removing a
device holding a filesystem without having priorly unmounted it, e.g.
if you 'rmmod usb-storage' (just that block devices unfortunately still
lack notifications about removal or size changes...)

> 
> [...]> +static void ubi_notify_remove(struct mtd_info *mtd)
> > +{
> > +	int i;
> > -	ubi_wl_entry_slab = kmem_cache_create("ubi_wl_entry_slab",
> > -					      sizeof(struct ubi_wl_entry),
> > -					      0, 0, NULL);
> > -	if (!ubi_wl_entry_slab) {
> > -		err = -ENOMEM;
> > -		goto out_dev_unreg;
> > -	}
> > +	/* called while holding mtd_table_mutex */
> > +	mutex_lock_nested(&ubi_devices_mutex, SINGLE_DEPTH_NESTING);
> > +	for (i = 0; i < UBI_MAX_DEVICES; i++)
> > +		if (ubi_devices[i] &&
> > +		    ubi_devices[i]->mtd->index == mtd->index)
> > +			ubi_detach_mtd_dev(ubi_devices[i]->ubi_num, 1, true);
> 
> Add a 'break'?

Good point :)
Will add 'break' statement here.

[1]: https://forum.openwrt.org/t/asus-tuf-ax4200-support/155738/44?u=daniel