UEFI News and Commentary

Thursday, August 29, 2013

UEFI 2.4 Review, Part 6: HII Forms op-code for displaying a warning message

This is the sixth article in a series reviewing the changes in the UEFI 2.4 specification. This time, we look at a topic near and dear to my heart: a new IFR opcode, EFI_IFR_WARNING_IF_OP. For those of you who have followed this blog, you know I've spent a lot of time assembling and disassembling the opcodes of the Internal Forms Representation (IFR) with my tools. Part of that is because I spent a lot of time as a part of the UEFI Configuration Sub-Team trying to work out how to make a specification that was powerful enough to do what was needed, small enough to actually fit in a flash device, and flexible enough to allow alternative implementations from a 1-line DVD player to a full-blown GUI interface.

UEFI encodes the configuration settings information in a byte-code called IFR. IFR consists of a stream of binary-encoded opcodes. Opcodes can be nested inside each other. So drivers can produce form packages which contain zero or more form sets, each of which contains zero or more forms (pages), each of which contains zero or more statements (static items) or questions (items with an associated value). Questions contain a lot of information about their value's data type as well as expressions that perform range checking. Starting in the UEFI 2.1 specification, there were two IFR opcodes that allowed for question value error checking. The EFI_IFR_INCONSISTENT_OP checked the value immediately after it was changed by the user. The EFI_IFR_NO_SUBMIT_OP checked the value before the form containing the question was closed. Both of these also provide for an error message that will be presented to the user.

Now, with the UEFI 2.4 specification, there is a third: EFI_IFR_WARNING_IF_OP. Like the other two, this opcode has an expression that is evaluated and a message to present to the user. The expression is evaluated when the user attempts to leave the question or leave the form. But rather than an error condition, where the user cannot proceed without correcting something, this merely presents a warning to the user which the user must either acknowledge before leaving the form or which can optionally timeout.

This opcode presents additional concerns about a proposed change on the part of the user. For example, if the user disabled security settings or was about to do something which would cause loss of data. Without this opcode, driver writers often resorted to popping up messages in the Callback() member of their EFI_CONFIG_ACCESS_PROTOCOL. but without actually knowing the type of display being used (as well as the style and color palette of the setup utility) this was risky or garish.

On older browser implementations, this opcode will be ignored.

This update to UEFI 2.4 gives the driver better control over the user experience when they want to change certain configuration settings.

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