linux-kernel - Re: [PATCH v7 2/8] efi: Add embedded peripheral firmware support

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Date:   Fri, 15 Nov 2019 13:09:59 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Jonathan Corbet <corbet@....net>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Peter Jones <pjones@...hat.com>,
        Dave Olsthoorn <dave@...aar.me>, x86@...nel.org,
        platform-driver-x86@...r.kernel.org, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-input@...r.kernel.org
Subject: Re: [PATCH v7 2/8] efi: Add embedded peripheral firmware support

Hi,

On 14-11-2019 22:50, Luis Chamberlain wrote:
> On Thu, Nov 14, 2019 at 09:48:38PM +0100, Hans de Goede wrote:

<snip>

>> But I guess what you really want is some error to be thrown if someone
>> calls firmware_request_platform() before we are ready.
> 
> Yes.
> 
>> I guess I could make efi_check_for_embedded_firmwares() which scans
>> for known firmwares and saved a copy set a flag that it has run.
>>
>> And then combine that with making efi_get_embedded_fw() (which underpins
>> firmware_request_platform()) print a warning when called if that flag
>> is not set yet.

<snip>

> That'd be great.

So I've been working on this, my first though was to use WARN_ON as
calling this too early would be a bug, but there is a bunch of
normal circumstances where efi_check_for_embedded_firmwares() never
runs. One of the being classic BIOS boot, but e.g. also when running
paravirtualized in a paravirt env. using UEFI.

Normally we should not end up calling efi_get_embedded_fw() in those
cases, for one it is unlikely for any drivers using firmware_request_platform()
to be used in such an environment, and if we somehow do end up with
a case where firmware_request_platform() is called, since the EFI
emebedded fw fallback then will not work I would expect a copy of
the necessary fw to be under /lib/firmware so we never hit the fallback.

This all makes efi_get_embedded_fw() getting called in cases where
efi_check_for_embedded_firmwares() will never run unlikely, but not
impossible. Making a WARN_ON the wrong thing to do so for v8 of this
patch-set I will add a pr_warn for this.

Note I've looked into detecting all the circumstances where it is normal
for efi_check_for_embedded_firmwares() to never run, but after tracing
the call path leading up to it getting called I've found that a check
for that is complicated and more importantly error-prone and likely
to get out of sync with reality if any of the functions higher up
the call path ever change the conditions.

So a pr_warn it is, and since as explained one would normally not
expect to ever hit the fallback on systems where
efi_check_for_embedded_firmwares() does not get called, I see no
harm in simply always printing the warning if
efi_check_for_embedded_firmwares() was not called.

Regards,

Hans