Re: [PATCH v5 2/5] efi: Add embedded peripheral firmware support

Luis R. Rodriguez
2018-05-08T17:12:06Z

On Fri, May 04, 2018 at 07:54:28AM +0200, Ard Biesheuvel wrote:
> On 4 May 2018 at 01:29, Luis R. Rodriguez <mcgrof@kernel.org> wrote:
> > On Sun, Apr 29, 2018 at 11:35:55AM +0200, Hans de Goede wrote:
> [...]
> >> diff --git a/Documentation/driver-api/firmware/request_firmware.rst b/Documentation/driver-api/firmware/request_firmware.rst
> >> index c8bddbdcfd10..560dfed76e38 100644
> >> --- a/Documentation/driver-api/firmware/request_firmware.rst
> >> +++ b/Documentation/driver-api/firmware/request_firmware.rst
> >> @@ -73,3 +73,69 @@ If something went wrong firmware_request() returns non-zero and fw_entry
> >> is set to NULL. Once your driver is done with processing the firmware it
> >> can call call firmware_release(fw_entry) to release the firmware image
> >> and any related resource.
> >> +
> >> +EFI embedded firmware support
> >> +=============================
> >
> > This is a new fallback mechanism, please see:
> >
> > Documentation/driver-api/firmware/fallback-mechanisms.rst
> >
> > Refer to the section "Types of fallback mechanisms", augument the list there
> > and then move the section "Firmware sysfs loading facility" to a new file, and
> > then add a new file for your own.
> >
> >> +
> >> +On some devices the system's EFI code / ROM may contain an embedded copy
> >> +of firmware for some of the system's integrated peripheral devices and
> >> +the peripheral's Linux device-driver needs to access this firmware.
> >
> > You in no way indicate this is a just an invented scheme, a custom solution and
> > nothing standard. I realize Ard criticized that the EFI Firmware Volume Protocol
> > is not part of the UEFI spec -- however it is a bit more widely used right?
> > Why can't Linux support it instead?
> >
>
> Most implementations of UEFI are based on PI,

That seems to be the UEFI Platform Initialization specification:

http://www.uefi.org/sites/default/files/resources/PI_Spec_1_6.pdf

> and so it is likely that
> the protocols are available. However, the PI spec does not cover
> firmware blobs,

Indeed, I cannot find anything about it on the PI Spec, but I *can* easily
find a few documents referring to the Firmware Volume Protocol:

http://wiki.phoenix.com/wiki/index.php/EFI_FIRMWARE_VOLUME_PROTOCOL

But this has no references at all...

I see stupid patents over some of this and authentication mechanisms for it:

https://patents.google.com/patent/US20170098084

> and so it is undefined whether such blobs are self
> contained (i.e., in separate files in the firmware volume), statically
> linked into the driver or maybe even encrypted or otherwise
> encapsulated, and the actual loadable image only lives in memory.

Got it, thanks this helps! There are two things then:

1) The "EFI Firmware Volume Protocol" ("FV" for short in your descriptions
below), and whether to support it or not in the future and recommend it
for future use cases.

b) Han's EFI scraper to help support 2 drivers, and whether or not to
recommend it for future use cases.

> Hans's case is the second one, i.e., the firmware is at an arbitrary
> offset in the driver image. Using the FV protocol in this case would
> result in a mix of both approaches: look up the driver file by GUID
> [which could change btw between different versions of the system
> firmware, although this is unlikely] and then still use the prefix/crc
> based approach to sift through the image itself.

Got it. And to be clear its a reversed engineered solution to what
two vendors decided to do.

> But my main objection is simply that from the UEFI forum point of
> view, there is a clear distinction between the OS visible interfaces
> in the UEFI spec and the internal interfaces in the PI spec (which for
> instance are not subject to the same rules when it comes to backward
> compatibility), and so I think we should not depend on PI at all.

Ah I see.

> This
> is all the more important considering that we are trying to encourage
> the creation of other implementations of UEFI that are not based on PI
> (e.g., uboot for arm64 implements the required UEFI interfaces for
> booting the kernel via GRUB), and adding dependencies on PI protocols
> makes that a moving target.

Got it!

> So in my view, we either take a ad-hoc approach which works for the
> few platforms we expect to support, in which case Hans's approach is
> sufficient,

Modulo it needs some work for ARM as it only works for x86 right now ;)

> or we architect it properly, in which case we shouldn't
> depend on PI because it does not belong in a properly architected
> OS<->firmware exchange.

OK, it sounds to me like we have room to then implement our own de-facto
standard for letting vendors stuff firmware into EFI as we in the Linux
community see fit.

We can start out by supporting existing drivers, but also consider customizing
this in the future for our own needs, so long as we document it and set
expectations well.

So we need to support what Hans is implementing for two reasons then:

a) The FV Protocol cannot be used to support the two drivers he's
trying to provide support for -- I believe Hans tried and it didn't work,
Hans, correct me if I'm wrong?

b) The FV Protocol relies on *internal* interfaces of PI spec, and since:
1) The PI spec does not define firmware at all
2) The internal interfaces of PI Spec does not guarantee any backward
compatibility
Any implementation details in FV may be subject to change, and may vary
system to system. Supporting the FV Protocol would be difficult as it
purposely ambiguous.

If accurate, Hans, can you capture this in your documentation somehow?

And.. we should review Hans' changes in light of not only supporting 2
drivers but also helping advance the framework to properly expand it
in the future as we see fit. And ensuring backward compatibility, should
we change or enhance the interface.

In that case I believe that, other than enhanced documentation justifying
these new changes over supporting the FV protocol, the remaining sticking
points to this patch series was the stronger hashing algorithms used for
which Andy already provided feedback and recommendations for.

Luis