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:olivier.mar...@arm.com]
Sent: Monday, March 10, 2014 12:52 PM
To: edk2-devel@lists.sourceforge.net
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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