[edk2-devel] [PATCH v1 1/1] ArmVirtPkg: Enable Early Serial For DxeCore

[Ard Biesheuvel]

On Mon, 11 Sept 2023 at 07:30, Laszlo Ersek <lersek at redhat.com> wrote:
>
> On 9/8/23 01:58, Sean Brogan wrote:
> >
> > On 9/7/2023 1:54 PM, Laszlo Ersek wrote:
> >> On 9/7/23 19:50, Sean Brogan wrote:
> >>> I would argue that by declaring that your library class supports type
> >>> DXE_CORE (or any core type) that you have declared you understand the
> >>> uniqueness of the environment and have accounted for it.
> >>>
> >>> For instances that support DXE_CORE or MM_CORE module types we often use
> >>> a global variable (to the library) to determine if the init routine has
> >>> been completed.  This does require a single byte check on each serial
> >>> message write (hot path) but given all the code on that path this is not
> >>> noticeable in performance measurements.   In the case below this pattern
> >>> could be used by the FdtPL011SerialPortLib to detect if it's init
> >>> routine has been called.
> >> Good point, but then I still claim that the "init check in each API"
> >> should be done in a dedicated "DxeCoreDebugLibSerialPort" instance, and
> >> not in a SerialPortLib instance. Here's why:
> >>
> >> (1) The SerialPortLib *class* requires SerialPortInitialize() to be
> >> called before the other APIs.
> >
> > Where do you see this? Looking at the file here:
> > edk2/MdePkg/Include/Library/SerialPortLib.h at master · tianocore/edk2
> > (github.com)
> > <https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Library/SerialPortLib.h> I don't see that.  I don't necessarily disagree with you but I am just trying to find out where this is documented.
>
> I consider this an implicit requirement, from the first two sentences
> (and from the fact that the function is declared at the top of the
> header) -- "Initialize the serial device hardware. If no initialization
> is required, then return RETURN_SUCCESS."
>

Agreed. Even if the header file does not spell it out literally, it is
implied: SerialPortInitialize() needs to be implemented even if no
initialization is required, in which case the routine does nothing
before returning SUCCESS. So the caller cannot distinguish a library
class implementation that requires initialization from one that does
not, and it can only assume that calling SerialPortInitialize() is
needed before using any other part of the API.


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