linux-kernel - RE: [PATCH v12 3/3] mtd: Add driver for concatenating devices

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Message-ID:
 <IA0PR12MB76996538D556ABC8E9C29624DC65A@IA0PR12MB7699.namprd12.prod.outlook.com>
Date: Mon, 26 May 2025 14:27:37 +0000
From: "Mahapatra, Amit Kumar" <amit.kumar-mahapatra@....com>
To: Miquel Raynal <miquel.raynal@...tlin.com>
CC: "richard@....at" <richard@....at>, "vigneshr@...com" <vigneshr@...com>,
	"robh@...nel.org" <robh@...nel.org>, "krzk+dt@...nel.org"
	<krzk+dt@...nel.org>, "conor+dt@...nel.org" <conor+dt@...nel.org>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "git
 (AMD-Xilinx)" <git@....com>, "amitrkcian2002@...il.com"
	<amitrkcian2002@...il.com>, Bernhard Frauendienst <kernel@...pam.obeliks.de>
Subject: RE: [PATCH v12 3/3] mtd: Add driver for concatenating devices

[AMD Official Use Only - AMD Internal Distribution Only]

> > [AMD Official Use Only - AMD Internal Distribution Only]
> >
> >> On 13/05/2025 at 14:45:39 GMT, "Mahapatra, Amit Kumar" <amit.kumar-
> >> mahapatra@....com> wrote:
> >>
> >> > [AMD Official Use Only - AMD Internal Distribution Only]
> >> >
> >> > Hello Miquel,
> >> >
> >> >> >> > +           mtd->dev.parent = concat->subdev[0]->dev.parent;
> >> >> >> > +           mtd->dev = concat->subdev[0]->dev;
> >> >> >> > +
> >> >> >> > +           /* Register the platform device */
> >> >> >> > +           ret = mtd_device_register(mtd, NULL, 0);
> >> >> >> > +           if (ret)
> >> >> >> > +                   goto destroy_concat;
> >> >> >> > +   }
> >> >> >> > +
> >> >> >> > +   return 0;
> >> >> >> > +
> >> >> >> > +destroy_concat:
> >> >> >> > +   mtd_concat_destroy(mtd);
> >> >> >> > +
> >> >> >> > +   return ret;
> >> >> >> > +}
> >> >> >> > +
> >> >> >> > +late_initcall(mtd_virt_concat_create_join);
> >> >> >>
> >> >> >> The current implementation does not support probe deferrals, I
> >> >> >> believe it should be handled.
> >> >> >
> >> >> > I see that the parse_mtd_partitions() API can return
> >> >> > -EPROBE_DEFER during MTD device registration, but this behavior
> >> >> > is specific to the parse_qcomsmem_part parser. None of the other
> >> >> > parsers appear to support probe deferral. As discussed in RFC
> >> >> > [1], the virtual concat feature is purely a fixed-partition
> >> >> > capability, and based on my understanding, the fixed-partition parser does
> not support probe deferral.
> >> >> > Please let me know if you can think of any other probe deferral
> >> >> > scenarios that might impact the virtual concat driver.
> >> >>
> >> >> That's true, but I kind of dislike the late_initcall, I fear it
> >> >> might break in creative
> >> ways.
> >> >
> >> > I understand, but since we require the partition information to be
> >> > available, late_initcall seems to be the most suitable choice among
> >> > the initcall levels—if we decide to proceed with using an initcall.
> >> > Regarding potential failures, as far as I can tell, the operation
> >> > would fail if, at the time of concatenation, one or more of the MTD
> >> > devices involved in the concat are not yet available. In such a
> >> > scenario, we can issue a kernel warning and exit gracefully. But,
> >> > However, if you prefer to move away from using initcalls and have
> >> > an alternative implementation approach in mind, please let us know.
> >>
> >> I am sorry but this does not work with modules, and we cannot ignore
> >> this case I believe. More specifically, if a controller probe is
> >> deferred (with EPROBE_DEFER or just prevented because some
> >> dependencies are not yet satisfied), you'll get incorrectly defined mtd devices.
> >
> > Ok, an alternative solution could be to remove the initcall
> > registration and instead invoke mtd_virt_concat_create_join()—which
> > was previously registered as a late_initcall—directly from
> mtd_device_parse_register().
> > I believe this approach would address both of your concerns regarding
> > module support and probe deferral. Additionally, we could consider
> > moving the entire code from mtd_virt_concat.c into mtdconcat.c.
> > Please let us know your take on this.
>
> What would this bring?
>
> Maybe we should trigger some kind of notifier after registering an mtd device and in
> there attempt to gather all mtd devices required for the concatenation. Can you
> please propose something like that?

In the current patch, during MTD registration, if a device is
part of a concatenated (concat) device, it is not registered individually.
Instead, its information is stored in a concat-specific data structure, as
it is not meant to be exposed as a standalone MTD device. As per my
earlier proposal, once all individual MTD devices are registered,
mtd_virt_concat_create_join() is called from
mtd_device_parse_register() to scan this data structure and create the
corresponding concat devices. Just to confirm, are you suggesting that
mtd_virt_concat_create_join() should be triggered through a notifier
instead? At the point when all individual MTD devices are registered,
we already have the complete information required for concatenation.
So, rather than relying on a listener notification, we cac directly call the
API. Please let me know if I am missing anything here.

Regards,
Amit
>
> Thanks,
> Miquèl