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

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Message-ID: <20201129032145.GC39488@kernel.org>
Date:   Sun, 29 Nov 2020 05:21:45 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     James Bottomley <James.Bottomley@...senpartnership.com>
Cc:     Jerry Snitselaar <jsnitsel@...hat.com>,
        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, Nov 24, 2020 at 10:10:21AM -0800, James Bottomley wrote:
> 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

Do we have a way to know is Windows TPM code using IRQ's?

/Jarkko