Re: [PATCH V2] PCI: set BAR size bits correctly in Resize BAR control register

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Aug 08, 2019 at 07:01:03AM +0000, Koenig, Christian wrote:
> Am 08.08.19 um 01:01 schrieb Bjorn Helgaas:
> > On Fri, Jul 26, 2019 at 12:55:52AM +0530, Sumit Saxena wrote:
> >> In Resize BAR control register, bits[8:12] represents size of BAR.
> >> As per PCIe specification, below is encoded values in register bits
> >> to actual BAR size table:
> >>
> >> Bits  BAR size
> >> 0     1 MB
> >> 1     2 MB
> >> 2     4 MB
> >> 3     8 MB
> >> --
> >>
> >> For 1 MB BAR size, BAR size bits should be set to 0 but incorrectly
> >> these bits are set to "1f". Latest megaraid_sas and mpt3sas adapters
> >> which support Resizable BAR with 1 MB BAR size fails to initialize
> >> during system resume from S3 sleep.
> >>
> >> Fix: Correctly calculate BAR size bits for Resize BAR control register.
> >>
> >> V2:
> >> -Simplified calculation of BAR size bits as suggested by Christian Koenig.
> >>
> >> CC: stable@vger.kernel.org # v4.16+
> >> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=203939
> >> Fixes: d3252ace0bc652a1a244455556b6a549f969bf99 ("PCI: Restore resized BAR state on resume")
> >> Signed-off-by: Sumit Saxena <sumit.saxena@broadcom.com>
> >> ---
> >>   drivers/pci/pci.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> >> index 29ed5ec1ac27..e59921296125 100644
> >> --- a/drivers/pci/pci.c
> >> +++ b/drivers/pci/pci.c
> >> @@ -1438,7 +1438,7 @@ static void pci_restore_rebar_state(struct pci_dev *pdev)
> >>   		pci_read_config_dword(pdev, pos + PCI_REBAR_CTRL, &ctrl);
> >>   		bar_idx = ctrl & PCI_REBAR_CTRL_BAR_IDX;
> >>   		res = pdev->resource + bar_idx;
> >> -		size = order_base_2((resource_size(res) >> 20) | 1) - 1;
> >> +		size = order_base_2(resource_size(res) >> 20);
> > Since BAR sizes are always powers of 2, wouldn't this be simpler as:
> >
> > 		size = ilog2(resource_size(res)) - 20;
> >
> > which nicely matches the table in PCIe r5.0, sec 7.8.6.3?
> 
> Yeah, that should obviously work as well.
> 
> We would have a serious problem in the resource management if the 
> resource size is smaller than 1MB or not a power of two.

Yes, definitely.  Resizable BARs are required by spec to be 1MB or
larger, but this does niggle at me a little bit, too.  It probably
saves a few bits in pci_dev to recompute this at restore-time, but
honestly, I think it would be more obviously correct to just do the
simple-minded thing of saving and restoring the entire register.

> Feel free to add my r-b.

Done, thanks!

> Regards,
> Christian.
> 
> >
> >>   		ctrl &= ~PCI_REBAR_CTRL_BAR_SIZE;
> >>   		ctrl |= size << PCI_REBAR_CTRL_BAR_SHIFT;
> >>   		pci_write_config_dword(pdev, pos + PCI_REBAR_CTRL, ctrl);
> >> -- 
> >> 2.18.1
> >>
>