Re: S3 wake restore operations, regarding S3IoLib and S3PciLib

[Leif Lindholm <leif.lindholm@linaro.org>]

Not my area of expertise.

Liming, Mike?

On Tue, Jan 16, 2018 at 12:01:51AM +0000, Marvin H?user wrote:
> Dear developers,
> 
> First off, I'm writing this mail for educational purposes, so I
> would be very thankful if I got a reply, though you will not miss
> anything if you skip it.
> 
> I was browsing several initialization codes of Intel platforms (the
> ones included in the main edk2 repository, as well as MinPlatformPkg
> from edk2-platforms), as well as recent imports of library code into
> MdePkg (S3IoLib and S3PciLib).
> While checking out latter, I noticed that all functions, including
> 'And' and 'Or' operations, do not store the actual operations, but
> just the value that results from applying them to the register at
> the current time in execution.
> While I checked out former, I noticed that actions are mixed (some
> operations using SaveMemWrite, which saves the entire register's
> value, some operations using SaveMemReadWrite, which saves the
> operations (and/or) involved).
> 
> Now, I do not have a lot of knowledge and/or experience with
> low-level x86 initialization. I was checking out the PantherPoint
> PCH specification, trying to find a clue on when to use which
> operation precisely. Every register (be it a MMIO or PCI register) I
> was looking at (with a few exceptions, as I have just noticed), had
> a clear, defined value upon power-up. I'm also certain that there
> are no conditional executions possible by the BootScript table,
> which makes me think that storing the and/or operations and storing
> just the register values is basically achieving the same state in
> the end.
> 
> I hope for your experience regarding the following questions:
> 
>   1.  Is my quite poorly researched assumption that there is no
>       actual difference between these two types of operation (for
>       registers with a clearly defined default value) usually/always
>       correct (for x86 platforms)?
>      *   If they are, why are there different opcodes in the
>          BootScript table? To support platforms for which that is
>          not the case?
>      *   If they are not, how could one be sure on what to use?
>          Which specification or datasheet contains the necessary
>          information? Is S3*Lib safe to use without explicate
>          knowledge?
>      *   If they are not, can there be a drawback to always storing
>          the and/or operations to apply, such as applying the
>          operations on a random register value (undefined default
>          value)?
> 
>                                                                i.      If there is no drawback, why are the S3*Libs using the method of storing the final value rather than the operation?
> 
>   1.  Just after composing the questions, I noticed two registers
>       that indeed do not have a defined power-up value. In that
>       case, I wonder whether it is unsafe to run a complete write,
>       as it overwrites whatever is there (it's unpredictable, I
>       suppose), or whether it is unsafe to use the and/or
>       operations, as there may be unwanted bits set due to the
>       undefined value.
> 
> Thank you in advance for your time, if you have read till here!
> 
> Best regards,
> Marvin.
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