Re: Storing Non volatile variables on SD/NAND

[Vladimir Olovyannikov <vladimir.olovyannikov@broadcom.com>]

> -----Original Message-----
> From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf Of
> Ard Biesheuvel
> Sent: Monday, September 18, 2017 8:43 AM
> To: Udit Kumar
> Cc: grant.likely@linaro.org.; edk2-devel@lists.01.org;
> Olivier.Martin@arm.com
> Subject: Re: [edk2] Storing Non volatile variables on SD/NAND
>
> On 18 September 2017 at 06:52, Udit Kumar <udit.kumar@nxp.com> wrote:
> > Hi EDK-2 Experts,
> > I am looking to store NV variables on SD/NAND device.
> >
> > While browsing, I came across some old post at link,
> > http://feishare.com/efimail/messages/20130319-1700-
> Re__edk2__Regarding
> > _storing_Boot_Device_Config_in_persistent_memory-Olivier_Martin.html
> >
> > Looks like, this is possible easily.
>
> That's a bold statement dude :-)
>
> >>> What you need to support Non-Volatile UEFI variables is a
Non-Volatile
> Memory. And also a driver that implements the EFI Firmware Volume Block
> protocol for this NVM device.
> >
> > But MdeModulePkg does Copymem from NV variable start memory to
> some
> > allocated buffers.  With SD/NAND Copymem is not possible, Is this
> > something changes since 2013 or there are some other way to use
> > SD/NAND
> >
>
> No, SD/MMC cannot currently be used as the backing store for the EFI
> variable store. The problem is that the variable protocols are
architectural
> protocols in PI that need to be present before any driver model drivers
are
> dispatched, and so putting the variable store on block devices is not
> something that the PI software architecture currently supports (unless
you
> reimplement the whole driver stack as DXE drivers).
>
> On top of that, it is almost impossible to share a block device that
sits behind
> a controller between the firmware and the OS at runtime (i.e., for
> SetVariable() calls made by efibootmgr under Linux), because only a
single
> agent can take ownership of the controller at any given time. (You
/could/
> dedicate the SD/MMC to the firmware entirely, and boot from SATA or USB,
> but this is out of the question on most platforms that need to use
SD/MMC
> for that variable backing store, i.e., mobile platforms)
>
> The best thing would be for you to convince the hardware architects in
your
> company to design and implement dual-ported SD/MMC controllers that
> allow a single SD/MMC to have two logical views that are independent
> (although I'm unsure if that is even possible in the context of the
SD/MMC
> specifications)
>
> Thanks,
> Ard.
Udit,
Ard is absolutely right.

Following design I had to implement variable store on the EMMC boot
partition 1 (not exactly SD card, but close; the same is applied to NAND I
guess).
To do that I forked VariableRuntime driver and changed it to do cache
writes into the store (just modifying store memory) at runtime.
This is because you cannot share eMMC store between OS (Linux)  and the
UEFI service at the same time, and you cannot predict when OS writes/reads
to/from the EMMC device.
So I do cache writes at runtime (just modifying store in memory), and
flush the store on reboot/shutdown.
For this purpose my ResetSystemLib calls into the Variable service and
forces flush. Variable service reinitiates eMMC controller
and really writes into the store on the eMMC. Real flush is also performed
on ExitBootServices().

As a big drawback an OS should either reset or shutdown (get into UEFI
ResetSystemLib) in order for variable store to be updated physically;
Physical flush is also forced on ExitBootServices - be it a reset command,
or booting an OS.
Just pressing a reset button (example: Linux Kernel panic with no reset)
would cause variable changes for this session to get lost.

Thank you,
Vladimir
> _______________________________________________
> edk2-devel mailing list
> edk2-devel@lists.01.org
> https://lists.01.org/mailman/listinfo/edk2-devel