OvmfPkg/VirtNorFlashDxe: clarify block write logic & fix shadowbuffer reads

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>
Message-Id: <20240116171105.37831-3-kraxel@redhat.com>

1 files changed, 28 insertions, 8 deletions

diff --git a/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c b/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c
index 7f4743b003..88a4d2c23f 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

                  );