Re: [PATCH 06/13] PCI: aardvark: Do not clear status bits of masked interrupts

Lorenzo Pieralisi
2021-10-05T16:04:37Z

On Tue, Oct 05, 2021 at 03:15:45PM +0200, Pali Rohár wrote:
> On Tuesday 05 October 2021 13:42:02 Marc Zyngier wrote:
> > On Tue, 05 Oct 2021 13:13:40 +0100,
> > Marek Behún <kabel@kernel.org> wrote:
> > >
> > > On Mon, 04 Oct 2021 16:31:54 +0100
> > > Marc Zyngier <maz@kernel.org> wrote:
> > >
> > > > On Mon, 04 Oct 2021 15:06:53 +0100,
> > > > Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> wrote:
> > > > >
> > > > > [+Marc - always better to have his eyes on IRQ handling code]
> > > > >
> > > > > On Fri, Oct 01, 2021 at 09:58:49PM +0200, Marek Behún wrote:
> > > > > > From: Pali Rohár <pali@kernel.org>
> > > > > >
> > > > > > It is incorrect to clear status bits of masked interrupts.
> > > > > >
> > > > > > The aardvark driver clears all status interrupt bits if no
> > > > > > unmasked status bit is set. Masked bits should never be cleared.
> > > > > >
> > > > > > Fixes: 8c39d710363c ("PCI: aardvark: Add Aardvark PCI host
> > > > > > controller driver") Signed-off-by: Pali Rohár <pali@kernel.org>
> > > > > > Reviewed-by: Marek Behún <kabel@kernel.org>
> > > > > > Signed-off-by: Marek Behún <kabel@kernel.org>
> > > > > > Cc: stable@vger.kernel.org
> > > > > > ---
> > > > > > drivers/pci/controller/pci-aardvark.c | 5 +----
> > > > > > 1 file changed, 1 insertion(+), 4 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/pci/controller/pci-aardvark.c
> > > > > > b/drivers/pci/controller/pci-aardvark.c index
> > > > > > d5d6f92e5143..e4986806a189 100644 ---
> > > > > > a/drivers/pci/controller/pci-aardvark.c +++
> > > > > > b/drivers/pci/controller/pci-aardvark.c @@ -1295,11 +1295,8 @@
> > > > > > static void advk_pcie_handle_int(struct advk_pcie *pcie)
> > > > > > isr1_mask = advk_readl(pcie, PCIE_ISR1_MASK_REG); isr1_status =
> > > > > > isr1_val & ((~isr1_mask) & PCIE_ISR1_ALL_MASK);
> > > > > > - if (!isr0_status && !isr1_status) {
> > > > > > - advk_writel(pcie, isr0_val, PCIE_ISR0_REG);
> > > > > > - advk_writel(pcie, isr1_val, PCIE_ISR1_REG);
> > > > >
> > > > > This looks fine - on the other hand if no interrupt is set in the
> > > > > status registers (that are filtered with the masks) we are dealing
> > > > > with a spurious IRQ right ? Just gauging how severe this is.
> > > > >
> > > > > Lorenzo
> > > > >
> > > > > > + if (!isr0_status && !isr1_status)
> > > > > > return;
> > > >
> > > > The whole thing is a bit odd. What the commit message doesn't say is
> > > > whether the status register shows the status of the line before
> > > > masking, or after masking.
> > >
> > > I don't quite understand what you are asking about.
> > > If you are asking about the register itself:
> > > the PCIE_ISR1_REG says which interrupts are currently set / active,
> > > including those which are masked.
> >
> > Then please say so in the commit message.
>
> Very well, we shall do so.
>
> > > If you are asking about the isr1_status variable, it is the
> > > status of the line after masking. I.e. masked interrupts are not set in
> > > this variable, only active interrupts which are also unmasked. That is
> > > obvious from the code.
> >
> > Which is what I have said... two lines below. If you are going to
> > reply, please do so in context.
> >
> > >
> > > > The code seems to imply the former, but then the behaviour is
> > > > awkward. How did we end-up here the first place?
> > >
> > > I answered this in reply to Lorenzo's comment on this patch, see
> > > https://lore.kernel.org/linux-pci/20211004171823.0288684e@thinkpad/
> >
> > It did grace my inbox, thanks.
> >
> > > > if that's only a
> > > > spurious interrupt, then I'd probably simplify the code altogether,
> > > > and drop all the early return code. Something like below, as usual
> > > > completely untested.
> > > >
> > > > M.
> > > >
> > > > diff --git a/drivers/pci/controller/pci-aardvark.c
> > > > b/drivers/pci/controller/pci-aardvark.c index
> > > > 596ebcfcc82d..1d8f257ecb63 100644 ---
> > > > a/drivers/pci/controller/pci-aardvark.c +++
> > > > b/drivers/pci/controller/pci-aardvark.c @@ -1275,7 +1275,8 @@ static
> > > > void advk_pcie_handle_msi(struct advk_pcie *pcie) static void
> > > > advk_pcie_handle_int(struct advk_pcie *pcie) {
> > > > u32 isr0_val, isr0_mask, isr0_status;
> > > > - u32 isr1_val, isr1_mask, isr1_status;
> > > > + u32 isr1_val, isr1_mask;
> > > > + unsigned long isr1_status;
> > > > int i;
> > > >
> > > > isr0_val = advk_readl(pcie, PCIE_ISR0_REG);
> > > > @@ -1285,22 +1286,14 @@ static void advk_pcie_handle_int(struct
> > > > advk_pcie *pcie) isr1_val = advk_readl(pcie, PCIE_ISR1_REG);
> > > > isr1_mask = advk_readl(pcie, PCIE_ISR1_MASK_REG);
> > > > isr1_status = isr1_val & ((~isr1_mask) & PCIE_ISR1_ALL_MASK);
> > > > -
> > > > - if (!isr0_status && !isr1_status) {
> > > > - advk_writel(pcie, isr0_val, PCIE_ISR0_REG);
> > > > - advk_writel(pcie, isr1_val, PCIE_ISR1_REG);
> > > > - return;
> > > > - }
> > > > + isr1_status >> 8;
>
> Hello!
>
> I dislike this approach. It adds another magic number which is just
> causing issues. Please read commit message for patch 11/13 where we
> describe why such magic constants are bad and already caused lot of
> issues in this driver.
>
> > > > /* Process MSI interrupts */
> > > > if (isr0_status & PCIE_ISR0_MSI_INT_PENDING)
> > > > advk_pcie_handle_msi(pcie);
> > > >
> > > > /* Process legacy interrupts */
> > > > - for (i = 0; i < PCI_NUM_INTX; i++) {
> > > > - if (!(isr1_status & PCIE_ISR1_INTX_ASSERT(i)))
> > > > - continue;
> > > > -
> > > > + for_each_set_bit(i, &isr1_status, PCI_NUM_INTX) {
> > > > advk_writel(pcie, PCIE_ISR1_INTX_ASSERT(i),
> > > > PCIE_ISR1_REG);
> > >
> > > 1. what you are doing here is code cleanup. We are currently in the
> > > state where we have lots of fixes for this driver, which we are
> > > hoping will go also to stable.
> >
> > Yes, it is code cleanup. Because I don't find this patch to be very
> > good, TBH. As for going into stable, that's not relevant for this
> > discussion.
> >
> > > Some of them depend on these changes.
> > > Can we please first apply those fixes (we want to send them in
> > > batches to avoid sending 60 patchs in one series, since last time
> > > nobody wanted to review all of that) and do this afterwards?
> >
> > It would be better to start with patches that are in a better
> > shape. After all, this is what the code review process is about. This
> > isn't "just take my patches".
> >
> > > 2. you are throwing away lower 8 bits of isr1_status. We have follow-up
> > > patches (not in this series, but in another batch which we want to
> > > send after this) that will be using those lower 8 bits, so we do not
> > > want to throw away them now.
> >
> > I'm discarding these bits because *in isolation*, that's the correct
> > thing to do. Feel free to propose a better patch that doesn't discard
> > these bits and still makes the code more palatable.
>
> The code pattern in this function is: compose irs*_status variable and
> then compare it with register macros defined at the top of driver. Each
> bit in this register represent some event and for each event there is
> simple macro to match.
>
> So with your proposed change it would break all macros (as they are
> going to be shifted by magic constant) and then this code disallow
> access to events represented by low bits. And also it makes code pattern
> different for isr0_status and isr1_status variables which is very
> confusing and probably source for introduction of new bugs.
>
> Also the whole early-return optimization can be removed as it does not
> change functionality. So we will do so.
>
> But we do not agree with the lower 8 bit discard of the isr1_status
> variable as explained above.
>
> So if we add the explanation to commit message and drop the early
> return, would it be ok?

Please repost a v2 of this series with the review comments you
received (and tags removed if they are not applicable as things
stand in mainline) and we will take it from there.

Thanks,
Lorenzo