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From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date: Wed, 20 Sep 2017 10:39:57 -0700
Message-ID: <CAKv+Gu8Vjf6gyKQ4jRTibLNgk78rESujr-c4BTs_w2-_iG9Eog@mail.gmail.com>
To: Udit Kumar <udit.kumar@nxp.com>
Cc: Pankaj Bansal <pankaj.bansal@nxp.com>, Andrew Fish <afish@apple.com>, 
 "Olivier.Martin@arm.com" <Olivier.Martin@arm.com>, 
 Vladimir Olovyannikov <vladimir.olovyannikov@broadcom.com>, 
 "edk2-devel@lists.01.org" <edk2-devel@lists.01.org>
Subject: Re: Storing Non volatile variables on SD/NAND
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On 20 September 2017 at 10:39, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> On 20 September 2017 at 10:34, Udit Kumar <udit.kumar@nxp.com> wrote:
>>
>> When we want to have UEFI and OS accessing same media ,
>> Possibilities I see
>>
>> 1- Patch OS For status check of media (diversion from generic OS), Good case will be modify low level driver.
>> But we may end up some surprises on synchronization.
>>
>> 2- no runtime service for OS . I guess this will not be possible
>>
>> 3- Way the  Vladimir implemented for eMMC, This has risk of losing data in case of AC power off.
>>
>> 4- update hardware with dual view (Ard suggestion)
>>
>
> 5 - abstract direct block device access into a firmware service that
> is exposed via a DXE_RUNTIME_DRIVER.
>
> The UEFI spec allows you to expose entry points into a
> DXE_RUNTIME_DRIVER module via a UEFI configuration table, and the OS
> can use a driver that uses the abstracted device rather than the real
> device. Performance is going to be terrible, probably, and lots of
> things that are specific to SD/MMC will no longer work, but it is a
> possibility nonetheless.

BTW this would go beyond the UEFI spec, and would effectively be a
PI/UEFI dependent feature.