linux-kernel - Re: [PATCH] PCI: designware-ep: use ffz() instead of find_first_zero

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Date:   Tue, 21 Nov 2017 17:05:03 +0000
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Joe Perches <joe@...ches.com>
Cc:     Niklas Cassel <niklas.cassel@...s.com>,
        Jingoo Han <jingoohan1@...il.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Niklas Cassel <niklass@...s.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: designware-ep: use ffz() instead of
 find_first_zero_bit()

On Tue, Nov 21, 2017 at 08:12:10AM -0800, Joe Perches wrote:
> On Fri, 2017-11-17 at 11:38 +0000, Lorenzo Pieralisi wrote:
> > Please follow Bjorn's request for patch formatting:
> > 
> > https://marc.info/?l=linux-pci&m=150905742808166&w=2
> > 
> > I would mention in the commit title that this is a fix.
> > 
> > eg "PCI: designware-ep: Fix find_first_zero_bit() usage"
> > 
> > On Thu, Nov 16, 2017 at 07:34:38PM +0100, Niklas Cassel wrote:
> > > find_first_zero_bit()'s parameter 'size' is defined in bits,
> > > not in bytes.
> > > 
> > > find_first_zero_bit() was called with bytes rather than bits,
> > 
> > "was called with size in bytes"...
> 
> This code also uses set_bit/clear_bit.
> 
> I suggest not intermixing set_bit/clear_bit calls and ffz
> on the same object.
> 
> Use either (&/~/ffz) or (set_bit/clear_bit/find_first_zero_bit)
> 
> BITS_PER_LONG would work fine for find_first_zero_bit

That would do as a fix and I agree with you that we should not mix the
APIs usage (a check for return value < BITS_PER_LONG should be added too
though).

Actually we can just allocate a bitmap given that we know the bitmap
required size (num_{ib/ob}_windows) and update the find_first_zero_bit()
call with the appropriate size in bits parameter but that can be
a subsequent refactoring patch.

Thanks,
Lorenzo

> > > which thus defined a too low upper limit, causing
> > > dw_pcie_ep_inbound_atu() to assign iatu index #4 to both bar 4
> > > and bar 5, which made bar 5 overwrite the settings set by bar 4.
> > > 
> > > ./pcitest.sh
> > > BAR tests
> > > BAR0:		OKAY
> > > BAR1:		OKAY
> > > BAR2:		OKAY
> > > BAR3:		OKAY
> > > BAR4:		NOT OKAY
> > > BAR5:		OKAY
> > 
> > Sorry but this example is pretty much useless - either you add
> > a self-explanatory kernel log or you just remove it.
> > 
> > > Fix this by using replacing find_first_zero_bit() with ffz(),
> > > since ffz() only works on a single 'unsigned long' and therefore
> > > does not need a size argument.
> > > 
> > > Signed-off-by: Niklas Cassel <niklas.cassel@...s.com>
> > 
> > We need a Fixes: tag at least, see Bjorn's post above.
> > 
> > Thanks,
> > Lorenzo
> > 
> > > ---
> > >  drivers/pci/dwc/pcie-designware-ep.c | 6 ++----
> > >  1 file changed, 2 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/pci/dwc/pcie-designware-ep.c b/drivers/pci/dwc/pcie-designware-ep.c
> > > index d53d5f168363..ab9a9e160daf 100644
> > > --- a/drivers/pci/dwc/pcie-designware-ep.c
> > > +++ b/drivers/pci/dwc/pcie-designware-ep.c
> > > @@ -70,8 +70,7 @@ static int dw_pcie_ep_inbound_atu(struct dw_pcie_ep *ep, enum pci_barno bar,
> > >  	u32 free_win;
> > >  	struct dw_pcie *pci = to_dw_pcie_from_ep(ep);
> > >  
> > > -	free_win = find_first_zero_bit(&ep->ib_window_map,
> > > -				       sizeof(ep->ib_window_map));
> > > +	free_win = ffz(ep->ib_window_map);
> > >  	if (free_win >= ep->num_ib_windows) {
> > >  		dev_err(pci->dev, "no free inbound window\n");
> > >  		return -EINVAL;
> > > @@ -96,8 +95,7 @@ static int dw_pcie_ep_outbound_atu(struct dw_pcie_ep *ep, phys_addr_t phys_addr,
> > >  	u32 free_win;
> > >  	struct dw_pcie *pci = to_dw_pcie_from_ep(ep);
> > >  
> > > -	free_win = find_first_zero_bit(&ep->ob_window_map,
> > > -				       sizeof(ep->ob_window_map));
> > > +	free_win = ffz(ep->ob_window_map);
> > >  	if (free_win >= ep->num_ob_windows) {
> > >  		dev_err(pci->dev, "no free outbound window\n");
> > >  		return -EINVAL;
> > > -- 
> > > 2.14.2
> > >