[edk2-devel] [PATCH v3 2/6] OvmfPkg/VirtNorFlashDxe: clarify block write logic & fix shadowbuffer reads

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

Introduce 'Start' and 'End' variables to make it easier to follow the
logic and code flow.  Also add a ascii art diagram (based on a
suggestion by Laszlo).

This also fixes the 'Size' calculation for the NorFlashRead() call.
Without this patch the code will read only one instead of two
P30_MAX_BUFFER_SIZE_IN_BYTES blocks in case '*NumBytes' is smaller than
P30_MAX_BUFFER_SIZE_IN_BYTES but 'Offset + *NumBytes' is not, i.e. the
update range crosses a P30_MAX_BUFFER_SIZE_IN_BYTES boundary.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
---
 OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c | 36 ++++++++++++++++++++------
 1 file changed, 28 insertions(+), 8 deletions(-)

diff --git a/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c b/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c
index 7f4743b00399..88a4d2c23fc0 100644
--- a/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c
+++ b/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c
@@ -520,6 +520,7 @@ NorFlashWriteSingleBlock (
   UINTN       BlockSize;
   UINTN       BlockAddress;
   UINT8       *OrigData;
+  UINTN       Start, End;
 
   DEBUG ((DEBUG_BLKIO, "NorFlashWriteSingleBlock(Parameters: Lba=%ld, Offset=0x%x, *NumBytes=0x%x, Buffer @ 0x%08x)\n", Lba, Offset, *NumBytes, Buffer));
 
@@ -555,7 +556,28 @@ NorFlashWriteSingleBlock (
   // 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.
-  if ((*NumBytes + (Offset & BOUNDARY_OF_32_WORDS)) <= (2 * P30_MAX_BUFFER_SIZE_IN_BYTES)) {
+  //
+  //    0               128              256
+  //    [----------------|----------------]
+  //    ^         ^             ^         ^
+  //    |         |             |         |
+  //    |         |             |        End, the next "word" boundary beyond
+  //    |         |             |        the (logical) update
+  //    |         |             |
+  //    |         |     (Offset & BOUNDARY_OF_32_WORDS) + NumBytes;
+  //    |         |     i.e., the relative offset inside (or just past)
+  //    |         |     the *double-word* such that it is the
+  //    |         |     *exclusive* end of the (logical) update.
+  //    |         |
+  //    |         Offset & BOUNDARY_OF_32_WORDS; i.e., Offset within the "word";
+  //    |         this is where the (logical) update is supposed to start
+  //    |
+  //    Start = Offset & ~BOUNDARY_OF_32_WORDS; i.e., Offset truncated to "word" boundary
+
+  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)) {
     // 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.
@@ -565,8 +587,8 @@ NorFlashWriteSingleBlock (
     Status = NorFlashRead (
                Instance,
                Lba,
-               Offset & ~BOUNDARY_OF_32_WORDS,
-               (*NumBytes | BOUNDARY_OF_32_WORDS) + 1,
+               Start,
+               End - Start,
                Instance->ShadowBuffer
                );
     if (EFI_ERROR (Status)) {
@@ -601,7 +623,7 @@ NorFlashWriteSingleBlock (
 
     Status = NorFlashWriteBuffer (
                Instance,
-               BlockAddress + (Offset & ~BOUNDARY_OF_32_WORDS),
+               BlockAddress + Start,
                P30_MAX_BUFFER_SIZE_IN_BYTES,
                Instance->ShadowBuffer
                );
@@ -609,12 +631,10 @@ NorFlashWriteSingleBlock (
       goto Exit;
     }
 
-    if ((*NumBytes + (Offset & BOUNDARY_OF_32_WORDS)) > P30_MAX_BUFFER_SIZE_IN_BYTES) {
-      BlockAddress += P30_MAX_BUFFER_SIZE_IN_BYTES;
-
+    if ((End - Start) > P30_MAX_BUFFER_SIZE_IN_BYTES) {
       Status = NorFlashWriteBuffer (
                  Instance,
-                 BlockAddress + (Offset & ~BOUNDARY_OF_32_WORDS),
+                 BlockAddress + Start + P30_MAX_BUFFER_SIZE_IN_BYTES,
                  P30_MAX_BUFFER_SIZE_IN_BYTES,
                  Instance->ShadowBuffer + P30_MAX_BUFFER_SIZE_IN_BYTES
                  );
-- 
2.43.0



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