Re: [edk2-devel] [PATCH v2 4/6] OvmfPkg/VirtNorFlashDxe: allow larger writes without block erase

["Laszlo Ersek" <lersek@redhat.com>]

On 1/15/24 16:59, Gerd Hoffmann wrote:
> Raise the limit for writes without block erase from two to four
> P30_MAX_BUFFER_SIZE_IN_BYTES blocks.  With this in place almost all efi
> variable updates are handled without block erase.  With the old limit
> some variable updates (with device paths) took the block erase code
> path.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c b/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c
> index 67610d6920f7..d80e9f0a2f3a 100644
> --- a/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c
> +++ b/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c
> @@ -550,13 +550,15 @@ NorFlashWriteSingleBlock (
>      return EFI_BAD_BUFFER_SIZE;
>    }
>  
> -  // Pick P30_MAX_BUFFER_SIZE_IN_BYTES (== 128 bytes) as a good start for word
> -  // operations as opposed to erasing the block and writing the data regardless
> -  // if an erase is really needed.  It looks like most individual NV variable
> -  // writes are smaller than 128 bytes.
> -  // To avoid pathological cases were a 2 byte write is disregarded because it
> -  // occurs right at a 128 byte buffered write alignment boundary, permit up to
> -  // twice the max buffer size, and perform two writes if needed.
> +  // Pick 4 * P30_MAX_BUFFER_SIZE_IN_BYTES (== 512 bytes) as a good
> +  // start for word operations as opposed to erasing the block and
> +  // writing the data regardless if an erase is really needed.
> +  //
> +  // Many NV variable updates are small enough for a a single
> +  // P30_MAX_BUFFER_SIZE_IN_BYTES block write.  In case the update is
> +  // larger than a single block, or the update crosses a
> +  // P30_MAX_BUFFER_SIZE_IN_BYTES boundary (as shown in the diagram
> +  // below), or both, we might have to write two or more blocks.
>    //
>    //    0               128              256
>    //    [----------------|----------------]
> @@ -577,7 +579,7 @@ NorFlashWriteSingleBlock (
>    Start = Offset & ~BOUNDARY_OF_32_WORDS;
>    End   = ALIGN_VALUE (Offset + *NumBytes, P30_MAX_BUFFER_SIZE_IN_BYTES);
>  
> -  if ((End - Start) <= (2 * P30_MAX_BUFFER_SIZE_IN_BYTES)) {
> +  if ((End - Start) <= (4 * P30_MAX_BUFFER_SIZE_IN_BYTES)) {
>      // Check to see if we need to erase before programming the data into NOR.
>      // If the destination bits are only changing from 1s to 0s we can just write.
>      // After a block is erased all bits in the block is set to 1.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>



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