Re: [PATCH v2 1/1] OvmfPkg/QemuFlashFvbServicesRuntimeDxe: refine flash detection

["Oliver Steffen" <osteffen@redhat.com>]

Quoting Gerd Hoffmann (2023-06-01 08:08:03)
> Flash can be write-protected in qemu (which is usually the case for
> code).  In case the variable store flash block is configured read-only
> ovmf wouldn't be able to store EFI variables there, so not setting up
> fvb in that case (and fallhack to emulation) is the better option.

I like the term "fallhack", I'll keep that in mind for future use >,<

> It'll avoid problems later due to flash writes failing.
>
> The patch tries to write back the original value read earlier, so flash
> content doesn't change in case the write succeeds.  But the status we
> read back after the attempt to write will tell us whenever flash is
> writable or not.
>
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c
> index 54f859de9ff9..a577aea55614 100644
> --- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c
> +++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c
> @@ -114,9 +114,17 @@ QemuFlashDetected (
>        DEBUG ((DEBUG_INFO, "QemuFlashDetected => FD behaves as RAM\n"));
>        *Ptr = OriginalUint8;
>      } else if (ProbeUint8 == CLEARED_ARRAY_STATUS) {
> -      DEBUG ((DEBUG_INFO, "QemuFlashDetected => FD behaves as FLASH\n"));
> -      FlashDetected = TRUE;
> -      *Ptr          = READ_ARRAY_CMD;
> +      *Ptr       = WRITE_BYTE_CMD;
> +      *Ptr       = OriginalUint8;
> +      *Ptr       = READ_STATUS_CMD;
> +      ProbeUint8 = *Ptr;
> +      *Ptr       = READ_ARRAY_CMD;
> +      if (ProbeUint8 & 0x10 /* programming error */) {
> +        DEBUG ((DEBUG_INFO, "QemuFlashDetected => FD behaves as FLASH, write-protected\n"));
> +      } else {
> +        DEBUG ((DEBUG_INFO, "QemuFlashDetected => FD behaves as FLASH, writable\n"));
> +        FlashDetected = TRUE;
> +      }
>      }
>    }
>
> --
> 2.40.1
>

- Oliver