Re: [PATCH v2 1/2] PCI: designware-ep: Fix find_first_zero_bit() usage

[Lorenzo Pieralisi]

[+Kishon - please CC him next time]

On Fri, Nov 17, 2017 at 04:00:40PM +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 size in bytes rather than bits,
> 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.
> 
> 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.
> 
> Fixes: f8aed6ec624f ("PCI: dwc: designware: Add EP mode support")
> Signed-off-by: Niklas Cassel <niklas.cassel@xxxxxxxx>
> ---
>  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);

You fix the bug at hand but code suffers from the same issue since we
should be checking the value against ~0UL first ie if size in bits <
num_ib_windows (that I think it is unlikely anyway but while at it let's
think if we can improve this).

@Kishon: maybe we should add a static upper size and declare a
static bitmap accordingly (and still use find_first_zero_bit()) ?

I think the check against ~0UL should be added anyway, let's get
Kishon's opinion before a v3 (if any).

Thanks,
Lorenzo

>  	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
>