Re: [PATCH v2] PCI: Add DMA alias for Intel Corporation 8 Series HECI

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Nov 29, 2022 at 12:27:56PM -0600, Bjorn Helgaas wrote:
> Does the TPM work under Windows?  If so, it would suggest that there's
> a different way to use it that doesn't require the quirk or the DMAR
> override.  Or maybe it only works on Windows without the IOMMU being
> enabled?

Haswell was the first generation having firmware TPM, and had fTPM
implementation that was flakky in many ways (e.g. Intel TXT still required
a discrete TPM so servers never used it). We have some workarounds for it
already in place.

I'm almost 100% confident nobody seriously tried to use it on server
side and/or together with MMIO.

> Naive question: apparently the TPM is doing DMA reads/writes.  I see
> tpm_crb.c doing MMIO mappings (ioremap()), but I don't see any DMA
> mappings.  Is that implicit or done elsewhere?

Firmware does this. Kernel does not and should not care how it does
it because it is not part of the specification [1]. Kernel cares only
of getting a buffer pointed out by the firmware.

[1] Section 6.5.3 in https://trustedcomputinggroup.org/resource/pc-client-platform-tpm-profile-ptp-specification/

BR, Jarkko