linux-kernel - Re: [PATCH] tpm_tis: Disable interrupts on ThinkPad T490s

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Date:   Tue, 24 Nov 2020 10:10:21 -0800
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Jerry Snitselaar <jsnitsel@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>
Cc:     Matthew Garrett <mjg59@...gle.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Huewe <peterhuewe@....de>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Hans de Goede <hdegoede@...hat.com>
Subject: Re: [PATCH] tpm_tis: Disable interrupts on ThinkPad T490s

On Tue, 2020-11-24 at 10:52 -0700, Jerry Snitselaar wrote:
> Before diving further into that though, does anyone else have an
> opinion on ripping out the irq code, and just using polling? We've
> been only polling since 2015 anyways.

Well only a biased one, obviously: polling causes large amounts of busy
waiting, which is a waste of CPU resources and does increase the time
it takes us to do TPM operations ... not a concern if you're doing long
computation ones, like signatures, but it is a problem for short
operations like bulk updates of PCRs.  The other potential issue, as we
saw with atmel is that if you prod the chip too often (which you have
to do with polling) you risk upsetting it.  We've spent ages trying to
tune the polling parameters to balance reduction of busy wait with chip
upset and still, apparently, not quite got it right.  If the TPM has a
functioning IRQ then it gets us out of the whole polling mess entirely.
The big question is how many chips that report an IRQ actually have a
malfunctioning one?

James