linux-kernel - Re: [PATCH v4 1/6] tpm, tpm_tis

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Date:   Tue, 17 May 2022 21:23:18 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
Cc:     peterhuewe@....de, jgg@...pe.ca, stefanb@...ux.vnet.ibm.com,
        linux@...ewoehner.de, linux-integrity@...r.kernel.org,
        linux-kernel@...r.kernel.org, lukas@...ner.de,
        p.rosenberger@...bus.com, Lino Sanfilippo <l.sanfilippo@...bus.com>
Subject: Re: [PATCH v4 1/6] tpm, tpm_tis_spi: Request threaded irq

On Mon, 2022-05-16 at 21:52 +0200, Lino Sanfilippo wrote:
> Hi,
> 
> On 13.05.22 at 20:08, Jarkko Sakkinen wrote:
> > On Wed, May 11, 2022 at 09:18:39PM +0200, Lino Sanfilippo wrote:
> > > Hi,
> > > 
> > > On 11.05.22 at 13:22, Jarkko Sakkinen wrote:
> > > > On Mon, May 09, 2022 at 10:05:54AM +0200, Lino Sanfilippo wrote:
> > > > > From: Lino Sanfilippo <l.sanfilippo@...bus.com>
> > > > > 
> > > > > Interrupt handling at least includes reading and writing the interrupt
> > > > > status register within the interrupt routine. Since accesses over the SPI
> > > > > bus are synchronized by a mutex, request a threaded interrupt handler to
> > > > > ensure a sleepable context during interrupt processing.
> > > > > 
> > > > > Fixes: 1a339b658d9d ("tpm_tis_spi: Pass the SPI IRQ down to the driver")
> > > > > Signed-off-by: Lino Sanfilippo <l.sanfilippo@...bus.com>
> > > > 
> > > > When you state that it needs a sleepable context, you should bring a
> > > > context why it needs it. This not to disregard the code change overally but
> > > > you cannot make even the most obvious claim without backing data.
> > > > 
> > > 
> > > so what kind of backing data do you have in mind? Would it help to emphasize more
> > > that the irq handler is running in hard irq context in the current code and thus
> > > must not access registers over SPI since SPI uses a mutex (I consider it as basic
> > > knowledge that a mutex must not be taken in hard irq context)?
> > 
> > There's zero mention about specific lock you are talking about. Providing
> > the basic knowledge what you are trying to do is the whole point of the
> > commit message in the first place. I'd presume this patch is related to the
> > use bus_lock_mutex but it is fully ingored here.
> > 
> 
> Ok, understood. I will amend the commit message to make more clear that
> reading and writing registers from the interrupt handler results in
> a call to tpm_tis_spi_transfer() which uses the bus_lock_mutex of the
> spi device and thus requires a sleepable context.

Yeah, please be always as explicit as possible, so that it is impossible
to get it wrong. Then a reader of your patch saves time from seeking e.g.
the current name of the data structure. Just dumb things down as far as
you can.

Commit messages have a dual function:

1. They *lower* the barrier to look into a code change, which helps
the patches get any attention.
2. Proper commit messages save tons of time from the maintainer, when
you have revisit years old commits, e.g. when bisecting a bug.

Comparing to e.g. Github the key difference is this: in Github you have
commits and issues. In kernel commit is both issue and the commit bundled
together. Therefore, every commit also needs to have a "bug report"
included.

BR, Jarkko