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From:   Seunghun Han <kkamagui@gmail.com>
Date:   Mon, 16 Sep 2019 17:18:33 +0900
Message-ID: <CAHjaAcQ07nDbb1NAg4m7bK8szSKVi7b55m7epzxPJjxxttaT0A@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] tpm: tpm_crb: enhance resource mapping mechanism
 for supporting AMD's fTPM
To:     Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc:     Peter Huewe <peterhuewe@gmx.de>,
        Matthew Garrett <mjg59@srcf.ucam.org>,
        "open list:TPM DEVICE DRIVER" <linux-integrity@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Vanya Lazeev <ivan.lazeev@gmail.com>
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> > > Matthew pointed out that having a hook in NVS driver is better solution
> > > because it is nil functionality if the TPM driver is loaded. We need
> > > functions to:
> > >
> > > 1. Request a region from the NVS driver (when tpm_crb loads)
> > > 2. Release a region back to the NVS Driver (when tpm_crb unloads).
> > >
> > > My proposal would unnecessarily duplicate code and also leave a
> > > side-effect when TPM is not used in the first place.
> > >
> > > I see this as the overally best solution. If you can come up with a
> > > patch for the NVS side and changes to CRB drivers to utilize the new
> > > hooks, then combined with Vanya's changes we have a sustainable solution
> > > for AMD fTPM.
> >
> > It's a great solution. I will update this patch on your advice and
> > send it to you soon.
> >
> > By the way, I have a question about your advice.
> > If we handle the NVS region with NVS driver, calling devm_ioremap()
> > function is fine like crb_ioremap_resource() function in this patch?
>
> No, you should reclaim the resource that conflicts and return it back
> when tpm_crb is unregistered (e.g. rmmod tpm_crb).
>
> I would try something like enumerating iomem resources with
> walk_iomem_res_desc(). I would advice to peek at arch/x86/kernel/crash.c
> for an example how to use this for NVS regions
>
> Then you could __release_region() to unallocate the source. When tpm_crb
> is removed you can then allocate and insert a resource with data
> matching it had.

Thank you for your sincere advice, and I have some questions about it.
As you know, the core reason of this ACPI NVS problem is that a busy
bit is set to the ACPI NVS area. So, devm_ioremap_resource() function
fails because of it.

If we want to call devm_ioremap_resource() for this case, we maybe
need to rearrange the existing memory layout from the child
relationship to the sibling relationship below. We also need to get
back when tpm_crb unloads.
[ ACPI NVS (parent) [ TPM CMD buffer (child of NVS) ] [ TPM RSP buffer
(child of NVS) ] ]   <--->   [ ACPI NVS head ] [ CMD buffer ] [ NVS
middle ] [ RSP buffer ] [ NVS tails ]

Our concern is a race condition between NVS driver and TPM CRB driver.
In my view, we could solve this problem if we only make and call the
functions you said, requesting and releasing a region from NVS driver.
NVS driver doesn't rely on iomem layout, and it relies on internal
nvs_list data.

Therefore, I added some details to your guide. How about this sequence?
1) When tpm_crb driver loads, the driver checks if command/response
buffers are in ACPI NVS area. If so, it requests (or removes) the
buffer regions from NVS driver's nvs_list (with
suspend_nvs_unregister() function I will add to the nvs.c driver).

2) If command/response buffers are in ACPI NVS area, tpm_crb driver
calls devm_ioremap() instead of devm_ioremap_resource() like this
patch.

3) When tpm_crb driver unloads, the driver releases (or adds) the
buffer regions to NVS driver's nvs_list (with existing
suspend_nvs_register() function in the nvs.c driver).

I think the sequence could solve the problem we concerned.
What do you think about the sequence?

Seunghun