linux-kernel - Re: [PATCH] initrd: Remove erroneous comment

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Message-ID: <20200622204034.GL27801@bill-the-cat>
Date:   Mon, 22 Jun 2020 16:40:34 -0400
From:   Tom Rini <trini@...sulko.com>
To:     ron minnich <rminnich@...il.com>
Cc:     lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Borislav Petkov <bp@...e.de>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        "H . Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] initrd: Remove erroneous comment

On Mon, Jun 22, 2020 at 01:02:16PM -0700, ron minnich wrote:

> The other thing you ought to consider fixing:
> initrd is documented as follows:
> 
>         initrd=         [BOOT] Specify the location of the initial ramdisk
> 
> for bootloaders only.
> 
> UEFI consumes initrd from the command line as well. As ARM servers
> increasingly use UEFI, there may be situations in which the initrd
> option doesn't make its way to the kernel? I don't know, UEFI is such
> a black box to me. But I've seen this "initrd consumption" happen.
> 
> Based on docs, and the growing use of bootloaders that are happy to
> consume initrd= and not pass it to the kernel, you might be better off
> trying to move to the new command line option anyway.
> 
> IOW, this comment may not be what people want to see, but ... it might
> also be right. Or possibly changed to:
> 
> /*
>  * The initrd keyword is in use today on ARM, PowerPC, and MIPS.
>  * It is also reserved for use by bootloaders such as UEFI and may
>  * be consumed by them and not passed on to the kernel.
>  * The documentation also shows it as reserved for bootloaders.
>  * It is advised to move to the initrdmem= option whereever possible.
>  */

Fair warning, one of the other hats I wear is the chief custodian of the
U-Boot project.

Note that on most architectures in modern times the device tree is used
to pass in initrd type information and "initrd=" on the command line is
quite legacy.

But what do you mean UEFI "consumes" initrd= ?  It's quite expected that
when you configure grub/syslinux/systemd-boot/whatever via extlinux.conf
or similar with "initrd /some/file" something reasonable happens to
read that in to memory and pass along the location to Linux (which can
vary from arch to arch, when not using device tree).  I guess looking at 
Documentation/x86/boot.rst is where treating initrd= as a file that
should be handled and ramdisk_image / ramdisk_size set came from.  I do
wonder what happens in the case of ARM/ARM64 + UEFI without device tree.

That said, no the comment is wrong.  It's not "since 11/2018" but "since
the 1990s".  And it doesn't provide any sort of link / context to the
boot loader specification project or similar that explains the cases
when a non-filename "initrd=" would reasonably (or unreasonably but
happens in reality) be removed.

I would go so far as to suggest that adding special handling for some
x86 setups is the wrong to place to start / further deprecate how other
architectures and firmwares handle a given situation.  I'm only chiming
in here as I saw this commit go by on LWN and wanted to see how this was
different from the traditional usage of initrd= in the rest of the
kernel (it's not) and then saw the otherwise unrelated new comment being
added.

-- 
Tom

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