On Thursday 22 September 2016, Ard Biesheuvel wrote:

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NOTE: this is a work in progress, and not fully functional yet. In particular,
the actual MMC host protocol methods are stubbed out at the moment, and need to
be wired up to the Linux device drivers.
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On mobile and embedded systems, there is usually only a single MMC device for
non-volatile storage, which sits behind a controller that is owned by the OS at
runtime. This makes it difficult to host the UEFI variable store on MMC as well,
since the UEFI runtime services routines expect ownership of the underlying
device as well.

This series proposes an approach to work around this. It implements the UEFI
MMC host protocol in the kernel, in a way that makes it possible to expose it
to the firmware. At the same time, the firmware needs be set up for this, i.e.,
it needs to expose its MMC host protocol pointer via a UEFI configuration table,
so that the kernel can override it if it decides to expose this functionality
to the firmware.

Note that these patches are based on patches in the EFI tree that are queued
for v4.9, which replace the runtime wrappers spinlock with a semaphore. This
allows us to sleep in the firmware callbacks.

Would it be possible to implement the UEFI varstore more generally on top
of the Linux pstore interface and then have a pstore backend on top of
MMC? That could give us very similar behavior, but also a bit more flexibility.
It would be a bit confusing to have the 'dmesg' pstore target on top of
UEFI varstore which in turn is on top of pstore on MMC, but I think that's
ok as long as we prevent recursion.

	Arnd