Re: [edk2-devel] [PATCH v2 2/6] OvmfPkg/VirtNorFlashDxe: clarify block write logic & fix shadowbuffer reads

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

On 1/15/24 16:59, Gerd Hoffmann wrote:
> 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>
> ---
>  OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c | 35 ++++++++++++++++++++------
>  1 file changed, 27 insertions(+), 8 deletions(-)
>
> diff --git a/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c b/OvmfPkg/VirtNorFlashDxe/VirtNorFlash.c
> index 7f4743b00399..54251633d0ee 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 +556,27 @@ 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 & 0x7F) + NumBytes; i.e., the Offset inside
> +  //    |         |     (or just past) the *double-word* such that Offset is
> +  //    |         |     the *exclusive* end of the (logical) update.

Obviously, when I proposed this diagram, I messed up this text.

Clearly, there's no better time for making a mistake in a comment than
when complaining about comments... :)

Two warts:

- 0x7F has not been replaced with BOUNDARY_OF_32_WORDS

- the uppercase "Offset" identifier (= proper noun), from my original
proposal, is misleading here. The common noun "offset" is what's need.

So I suggest refreshing it as:

  //    |         |     (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.

With that comment update:

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

Thanks!
Laszlo

> +  //    |         |
> +  //    |         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 +586,8 @@ 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 +622,7 @@ NorFlashWriteSingleBlock (
>
>      Status = NorFlashWriteBuffer (
>                 Instance,
> -               BlockAddress + (Offset & ~BOUNDARY_OF_32_WORDS),
> +               BlockAddress + Start,
>                 P30_MAX_BUFFER_SIZE_IN_BYTES,
>                 Instance->ShadowBuffer
>                 );
> @@ -609,12 +630,10 @@ 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
>                   );



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