[if gte mso 9]>

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

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

From: Tim Lewis <tim.lewis@insyde.com>

To: "edk2-devel@lists.sourceforge.net" <edk2-devel@lists.sourceforge.net>

Date: 2014-03-12 00:26:56

Olivier –

 

Actually, the current NOR technology assumes that we can do a byte level write only when changing bits from 1 to 0. In order to change from 0 to 1, you need to do an erase. I understood (correct me if I’m wrong) that NAND works the opposite way (can change from 0 to 1, but 1 to 0 requires and erase)

 

That is why there is a polarity bit in the FV header, to tell the FV code which way to do it.

 

Tim

 

From: Olivier Martin [mailto:olivier.martin@arm.com]
Sent: Tuesday, March 11, 2014 7:47 AM
To: edk2-devel@lists.sourceforge.net
Subject: Re: [edk2] MdeModulePkg: (at least) 4 writes to Flash to update a NV variable in UpdateVariable()

 

As Andrew says you have assumed a hardware specific technology (you assume you have SPI Flash which allows to write a single byte in NV memory) to manage your atomicity.

 

For most block devices, it is not possible to update a single byte without reading/writing the full block.

It means the current logic of UpdateVariable() is not correct.

 

If we keep the assumption that we can only do block access then it would be more efficient/faster to use a checksum to ensure the consistency of our variables (actually there is a spare field into VARIABLE_HEADER).

 

Most Flash memory needs their block to be erased before to be written. It means with the current flow, we have (example of 256 Byte page):

 

Step 1: Write the header: Erase 256B. Write 256B

Step 2: Update the state: Erase 256B. Write 256B

Step 3: Write the Data:   Erase 256B. Write 256B

Step 4: Update the state: Erase 256B. Write 256B

 

 

 

From: Andrew Fish [mailto:afish@apple.com]
Sent: 11 March 2014 02:29
To: edk2-devel@lists.sourceforge.net
Subject: Re: [edk2] MdeModulePkg: (at least) 4 writes to Flash to update a NV variable in UpdateVariable()

 

 

On Mar 10, 2014, at 7:04 PM, Zeng, Star <star.zeng@intel.com> 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:tim.lewis@insyde.com] 
Sent: Tuesday, March 11, 2014 5:42 AM
To: edk2-devel@lists.sourceforge.net
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:olivier.martin@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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