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

["Gerd Hoffmann" <kraxel@redhat.com>]

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.
-- 
2.43.0



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