linux-kernel - Re: [PATCH v3 2/4] tpm: Simplify locality handling

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Message-ID: <YJNKs8bUMGOzFre+@kernel.org>
Date:   Thu, 6 May 2021 04:47:31 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
Cc:     peterhuewe@....de, jgg@...pe.ca, stefanb@...ux.vnet.ibm.com,
        James.Bottomley@...senpartnership.com, keescook@...omium.org,
        jsnitsel@...hat.com, ml.linux@...oe.vision,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/4] tpm: Simplify locality handling

On Wed, May 05, 2021 at 01:15:29AM +0200, Lino Sanfilippo wrote:
> Hi,
> 
> On 03.05.21 at 17:50, Jarkko Sakkinen wrote:
> > What the heck is "simplification" and what that has to do with fixing
> > anything? I don't understand your terminology.
> 
> 
> The intention for this patch is not to fix anything. Please read the cover
> letter and the commit message.
> This patch is about making the locality handling easier by not claiming/releasing
> it multiple times over the driver life time, but claiming it once at driver
> startup and only releasing it at driver shutdown.
> 
> Right now we have locality request/release combos in
> 
> - probe_itpm()
> - tpm_tis_gen_interrupt()
> - tpm_tis_core_init()
> - tpm_chip_start()
> 
> and there is still one combo missing for
> 
> - tpm2_get_timeouts()
> 
> which is the reason why we get the "TPM returned invalid status" bug in case
> of TPM2 (and this is the bug which is _incidentally_ fixed by this patch, see
> below).
> 
> And if we are going to enable interrupts, we have to introduce yet another combo,
> for accessing the status register in the interrupt handler, since TPM 2.0
> requires holding the locality for writing to the status register. That makes
> 6 different code places in which we take and release the locality.
> 
> With this patch applied we only take the locality at one place. Furthermore
> with interrupts enabled we dont have to claim the locality for each handler
> execution, saving us countless claim/release combinations at runtime.
> 
> Hence the term "simplification" which is perfectly justified IMO.
> 
> So again, this patch is "only" in preparation for the next patch when interrupts
> are actually enabled and we would have to take the locality in the interrupt
> handler without this patch.

So: what problem this patch does solve?

/Jarkko