Re: [PATCH net-next v7 3/7] arch: x86: add IPC mailbox accessor function and add SoC register access

[Choong Yong Liang]

On 20/2/2025 1:01 am, David E. Box wrote:
> On Thu, 2025-02-06 at 08:46 -0800, Dave Hansen wrote:
> > On 2/6/25 05:18, Choong Yong Liang wrote:
> > > - Exports intel_pmc_ipc() for host access to the PMC IPC mailbox
> > > - Add support to use IPC command allows host to access SoC registers
> > > through PMC firmware that are otherwise inaccessible to the host due
> > > to security policies.
> > I'm not quite parsing that second bullet as a complete sentence.
> >
> > But that sounds scary! Why is the fact that they are "otherwise
> > inaccessible" relevant here?
>
> The PMC IPC mailbox is a host interface to the PMC. Its purpose is to allow the
> host to access certain areas of the PMC that are restricted from direct MMIO
> access due to security policies. Other parts of the PMC are accessible via MMIO
> (most of what the intel_pmc_core driver touches with is MMIO), so I think the
> original statement was trying to explain why the mailbox is needed instead of
> MMIO in this case. However, I agree that the mention of security policies is not
> relevant to the change itself.
>
> > ...
> > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > > index 87198d957e2f..631c1f10776c 100644
> > > --- a/arch/x86/Kconfig
> > > +++ b/arch/x86/Kconfig
> > > @@ -688,6 +688,15 @@ config X86_AMD_PLATFORM_DEVICE
> > >   	  I2C and UART depend on COMMON_CLK to set clock. GPIO driver is
> > >   	  implemented under PINCTRL subsystem.
> > >   
> > > +config INTEL_PMC_IPC
> > > +	tristate "Intel Core SoC Power Management Controller IPC mailbox"
> > > +	depends on ACPI
> > > +	help
> > > +	  This option enables sideband register access support for Intel
> > > SoC
> > > +	  power management controller IPC mailbox.
> > > +
> > > +	  If you don't require the option or are in doubt, say N.
> >
> > Could we perhaps beef this up a bit to help users figure out if they
> > want to turn this on? Really the only word in the entire help text
> > that's useful is "Intel".
> >
> > I'm not even sure we *want* to expose this to users. Can we just leave
> > it as:
> >
> > 	config INTEL_PMC_IPC
> > 		def_tristate n
> > 		depends on ACPI
> >
> > so that it only gets enabled by the "select" in the other patches?
>
> I agree with this change Choong. This can be selected by the driver that's using
> it. There's no need to expose it to users.
>
> > > + * Authors: Choong Yong Liang <yong.liang.choong@xxxxxxxxxxxxxxx>
> > > + *          David E. Box <david.e.box@xxxxxxxxxxxxxxx>
> >
> > I'd probably just leave the authors bit out. It might have been useful
> > in the 90's, but that's what git is for today.
> >
> > > +	obj = buffer.pointer;
> > > +	/* Check if the number of elements in package is 5 */
> > > +	if (obj && obj->type == ACPI_TYPE_PACKAGE && obj->package.count ==
> > > 5) {
> > > +		const union acpi_object *objs = obj->package.elements;
> > > +
> >
> > The comment there is just not super useful. It might be useful to say
> > *why* the number of elements needs to be 5.
> >
> > > +EXPORT_SYMBOL(intel_pmc_ipc);
> > > +
> > > +MODULE_LICENSE("GPL");
> > > +MODULE_DESCRIPTION("Intel PMC IPC Mailbox accessor");
> >
> > Honestly, is this even worth being a module? How much code are we
> > talking about here?
>
> Yeah, this doesn't need to be a module either.
>
> David

Hi David,

Thank you for the confirmation.
Let's work together to address the comments.