Re: [PATCH] drivers: block: save return value of pci_find_capability() in u8

[Bjorn Helgaas <helgaas@kernel.org>]

On Sun, Dec 06, 2020 at 11:08:14PM +0000, Chaitanya Kulkarni wrote:
> On 12/6/20 11:45, Puranjay Mohan wrote:
> > Callers of pci_find_capability should save the return value in u8.
> > change type of variables from int to u8 to match the specification.
> 
> I did not understand this, pci_find_capability() does not return u8. 
> 
> what is it that we are achieving by changing the variable type ?
> 
> This patch will probably also generate type mismatch warning with
> 
> certain static analyzers.

There's a patch pending via the PCI tree to change the return type to
u8.  We can do one of:

  - Ignore this.  It only changes something on the stack, so no real
    space saving and there's no problem assigning the u8 return value
    to the "int".

  - The maintainer could ack it and I could merge it via the PCI tree
    so it happens in the correct order (after the interface change).

  - The PCI core interface change will be merged for v5.11, so we
    could hold this until v5.12.

I don't really have a preference.  The only place there would really
be a benefit would be if we store the return value in a struct, where
we could potentially save three bytes.

Bjorn

> > Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
> > ---
> >  drivers/block/mtip32xx/mtip32xx.c | 2 +-
> >  drivers/block/skd_main.c          | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/block/mtip32xx/mtip32xx.c b/drivers/block/mtip32xx/mtip32xx.c
> > index 153e2cdecb4d..da57d37c6d20 100644
> > --- a/drivers/block/mtip32xx/mtip32xx.c
> > +++ b/drivers/block/mtip32xx/mtip32xx.c
> > @@ -3936,7 +3936,7 @@ static DEFINE_HANDLER(7);
> >  
> >  static void mtip_disable_link_opts(struct driver_data *dd, struct pci_dev *pdev)
> >  {
> > -	int pos;
> > +	u8 pos;
> >  	unsigned short pcie_dev_ctrl;
> >  
> >  	pos = pci_find_capability(pdev, PCI_CAP_ID_EXP);
> > diff --git a/drivers/block/skd_main.c b/drivers/block/skd_main.c
> > index a962b4551bed..16d59569129b 100644
> > --- a/drivers/block/skd_main.c
> > +++ b/drivers/block/skd_main.c
> > @@ -3136,7 +3136,7 @@ MODULE_DEVICE_TABLE(pci, skd_pci_tbl);
> >  
> >  static char *skd_pci_info(struct skd_device *skdev, char *str)
> >  {
> > -	int pcie_reg;
> > +	u8 pcie_reg;
> >  
> >  	strcpy(str, "PCIe (");
> >  	pcie_reg = pci_find_capability(skdev->pdev, PCI_CAP_ID_EXP);
> 
>