On Mar 10, 2014, at 7:04 PM, Zeng, Star <[email protected]> wrote:
> Hi Olivier,
>
> As we know, the BlockSize is the unit of FVB, FVB needs to depend on lower
> protocol(SPI protocol) to do real write operation,
SPI Protocol does not seem to be part of the Open source or industry standard.
FirmwareVolumeBlock is the last standardized/open source hop.
Thanks,
Andrew Fish
> but the unit of write operation may be one byte<image001.png>, or other
> bytes. Anyway, we should not assume the operation unit of real flash chip. To
> keep the atomicity, we need to be very careful.
>
> Thanks,
> Star
> From: Tim Lewis [mailto:[email protected]]
> Sent: Tuesday, March 11, 2014 5:42 AM
> To: [email protected]
> Subject: Re: [edk2] MdeModulePkg: (at least) 4 writes to Flash to update a NV
> variable in UpdateVariable()
>
> Olivier –
>
> I don’t have the whole code section memorized, but if I remember right there
> is the problem when there is a power failure as the variable header is being
> written. If VAR_HEADER_VALID_ONLY was set in this case, but the entire
> variable header was not written, wouldn’t this be another inconsistent state?
>
> Tim
>
> From: Olivier Martin [mailto:[email protected]]
> Sent: Monday, March 10, 2014 12:52 PM
> To: [email protected]
> Subject: [edk2] MdeModulePkg: (at least) 4 writes to Flash to update a NV
> variable in UpdateVariable()
>
> Dear MdeModulePkg maintainers,
>
> We have seen on some platforms that flash writing/erasing counts for most of
> the boot time.
> We have also noticed UpdateVariable() (in
> MdeModulePkg/Universal/RuntimeDxe/Variable.c) might make 4 accesses to the
> same region of Flash to update a non-volatile variable:
>
> // 1. Write variable header
> UpdateVariableStore (...,
> mVariableModuleGlobal->NonVolatileLastVariableOffset,
> sizeof (VARIABLE_HEADER),
> (UINT8 *) NextVariable
> );
> // 2. Set variable state to header valid
> NextVariable->State = VAR_HEADER_VALID_ONLY;
> UpdateVariableStore (...,
> mVariableModuleGlobal->NonVolatileLastVariableOffset + OFFSET_OF
> (VARIABLE_HEADER, State),
> sizeof (UINT8),
> &NextVariable->State
> );
> // 3. Write variable data
> UpdateVariableStore (...,
> mVariableModuleGlobal->NonVolatileLastVariableOffset + sizeof
> (VARIABLE_HEADER),
> (UINT32) VarSize - sizeof (VARIABLE_HEADER),
> (UINT8 *) NextVariable + sizeof (VARIABLE_HEADER)
> );
> // 4. Set variable state to valid
> NextVariable->State = VAR_ADDED;
> UpdateVariableStore (...,
> mVariableModuleGlobal->NonVolatileLastVariableOffset + OFFSET_OF
> (VARIABLE_HEADER, State),
> sizeof (UINT8),
> &NextVariable->State
> );
>
> I understand the 4 steps are to prevent the flash to be inconsistent in case
> of accidental reset.
> Actually the steps 1 and 2 could easily be merged (ie: do
> 'NextVariable->State = VAR_HEADER_VALID_ONLY' in step 1).
>
> For most variables, it is likely these 4 accesses would be into the same
> block of flash. It means this block of flash would be written 4 times.
> Could we potentially check if we do a write to the same block of flash and do
> a single write?
>
> For instance (untested code):
> BlockSize = Fvb->GetBlockSize ();
> BlockStart = mVariableModuleGlobal->NonVolatileLastVariableOffset &
> ~(BlockSize – 1);
> BlockEnd = (mVariableModuleGlobal->NonVolatileLastVariableOffset + VarSize) &
> ~(BlockSize – 1);
> if (BlockStart == BlockEnd) {
> NextVariable->State = VAR_ADDED;
> UpdateVariableStore (...,
> mVariableModuleGlobal->NonVolatileLastVariableOffset,
> VarSize,
> (UINT8 *) NextVariable
> );
> } else {
> // 1. Write variable header
> NextVariable->State = VAR_HEADER_VALID_ONLY;
> UpdateVariableStore (...,
> mVariableModuleGlobal->NonVolatileLastVariableOffset,
> sizeof (VARIABLE_HEADER),
> (UINT8 *) NextVariable
> );
> // 2. Write variable data
> UpdateVariableStore (...,
> mVariableModuleGlobal->NonVolatileLastVariableOffset + sizeof
> (VARIABLE_HEADER),
> (UINT32) VarSize - sizeof (VARIABLE_HEADER),
> (UINT8 *) NextVariable + sizeof (VARIABLE_HEADER)
> );
> // 3. Set variable state to valid
> NextVariable->State = VAR_ADDED;
> UpdateVariableStore (...,
> mVariableModuleGlobal->NonVolatileLastVariableOffset + OFFSET_OF
> (VARIABLE_HEADER, State),
> sizeof (UINT8),
> &NextVariable->State
> );
> }
>
> Regards,
> Olivier
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