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

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Message-ID: <87o6vsejke.fsf@bootlin.com>
Date: Fri, 16 May 2025 16:06:41 +0200
From: Miquel Raynal <miquel.raynal@...tlin.com>
To: "Mahapatra, Amit Kumar" <amit.kumar-mahapatra@....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

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.

Thanks,
Miquèl