Re: [edk2] MdeModulePkg: (at least) 4 writes to Flash to update a NV variable in UpdateVariable()

Mon, 10 Mar 2014 19:07:24 -0700 

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, but the unit of write 
operation may be one byte[cid:[email protected]], 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]<mailto:[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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