Re: Print from DXE_DRIVER

["David A. Van Arnem" <dvanarnem@cmlab.biz>]

On 02/08/2017 05:15 PM, Laszlo Ersek wrote:
> On 02/08/17 23:10, David A. Van Arnem wrote:
>> Hello,
>>
>> I am working on a DXE_DRIVER for a custom device.  I like to use Print()
>> statements to trace code execution during development.  Thus I have put
>> a print statement in each of my Supported(), Start(), and Stop()
>> functions for the driver binding protocol.  Currently I am building the
>> driver as part of the UefiCpuPkg, with no changes to the current
>> UefiCpuPkg.dsc except for adding my driver under [Components].  I have
>> also added the PrintLib and UefiLib to the [LibraryClasses] section in
>> my driver's INF file, and included the necessary headers.
>>
>> When I load the driver from the shell (load <drivername>.efi), I get a
>> message indicating it loaded successfully, but no output from the
>> Print() messages.  The documentation for the shell says load should test
>> both the Supported() and Start() functions, so I would expect to see the
>> output, but I am not sure I am using the correct library instances to
>> accomplish this.  Is it possible to use Print() from a DXE_DRIVER, and
>> which library instance should I use in the UefiCpuPkg.dsc file?  If not,
>> would changing it to a UEFI_DRIVER help?  Any other recommendations?
>>
>> If there is an example in edk2 that does this that you could point me
>> to, that would be sufficient as well.  Thanks!
>>
> 
> I never tried to use Print() from drivers, only applications
> (UEFI_APPLICATION modules). In drivers I always use DEBUG, which, based
> on the DebugLib instance in use, can be routed to some debug port,
> serial port, etc.
> 
> But, I think Print() can be made work too. The only Print() function
> that should matter is in "MdePkg/Library/UefiLib/UefiLibPrint.c". It uses
> 
>   gST->ConOut
> 
> as output console. Since this comes from the UEFI system table, it's
> clear that you shouldn't be writing a DXE_DRIVER, since those can run
> much earlier than the UEFI architectural protocols become available.
> DXE_DRIVER modules are practically platform drivers; they are not
> governed by the UEFI spec by the PI spec, but the system table is
> defined in the UEFI spec. (The above sounds a bit hand-wavy, but it
> should work for this discussion.)
> 
> Another sign that you should be writing on a UEFI_DRIVER module is that
> the Start(), Stop(), and Supported() member functions belong to the
> driver binding protocol, which exists for UEFI drivers that conform to
> the UEFI driver model. IOW, you're not only working on a UEFI_DRIVER
> already, but one that falls into a specific class of UEFI_DRIVERs.
> 
> The third sign that your module type should be UEFI_DRIVER is the intro
> text in "MdePkg/Library/UefiLib/UefiLib.inf", to which library Print()
> belongs. Rewrapped here for readability:
> 
> #  The UEFI Library provides functions and macros that simplify the
> #  development of UEFI Drivers and UEFI Applications.  These functions
> #  and macros help manage EFI events, build simple locks utilizing EFI
> #  Task Priority Levels (TPLs), install EFI Driver Model related
> #  protocols, manage Unicode string tables for UEFI Drivers, and print
> #  messages on the console output and standard error devices.
> 
> I recommend always reading the INF file comments, they are very-very
> useful most of the time.
> 
> Finally, just loading a UEFI_DRIVER into memory and running it will do
> nothing for binding devices (as long as the driver conforms to the UEFI
> driver model, which I assume your driver does, based on the above). Such
> drivers only install their driver binding protocol instance(s) in their
> entry points, and then exit. Those Supported(), Start(), and Stop()
> functions have to be called by something. They are called by the
> gBS->ConnectController() boot service, which in turn is exposed in the
> shell with the CONNECT command. Please see the documentation / help text
> on it.
> 
> So I recommend:
> - flip your module type to UEFI_DRIVER,
> - once the driver is loaded, try a recursive CONNECT from the shell.
> 
> Hope this helps,

Definitely!  I appreciate your detailed explanation, that should be
plenty to get me in the right direction.  UEFI_DRIVER is definitely the
module type I should be using based on your info.

One follow-up question: the LOAD command in the UEFI Shell 2.2 spec says
LOAD without -nc tries to connect the driver to a proper device; is this
different from ConnectController() that the CONNECT command uses?

Regards,
David

> Laszlo
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-- 
Regards,
David Van Arnem
Development Engineer IV
Computer Measurement Laboratory, LLC