Re: [PATCH 1/1] PCI: Use FIELD_PREP() and remove *_SHIFT defines

[Ilpo Järvinen]

On Fri, 3 Nov 2023, Ilpo Järvinen wrote:

> On Tue, 31 Oct 2023, Bjorn Helgaas wrote:
> > On Fri, Oct 27, 2023 at 11:38:11AM +0300, Ilpo Järvinen wrote:
> > > Instead of open-coded masking and shifting with PCI_CONF1_* bitfields,
> > > use GENMASK() and FIELD_PREP(), and then remove the *_SHIFT defines
> > > that are no longer needed.
> 
> > > @@ -797,19 +799,15 @@ static inline pci_power_t mid_pci_get_power_state(struct pci_dev *pdev)
> > >   * Section 3.2.2.3.2, Figure 3-2, p. 50.
> > >   */
> > >  
> > > -#define PCI_CONF1_BUS_SHIFT	16 /* Bus number */
> > > -#define PCI_CONF1_DEV_SHIFT	11 /* Device number */
> > > -#define PCI_CONF1_FUNC_SHIFT	8  /* Function number */
> > > -
> > > -#define PCI_CONF1_BUS_MASK	0xff
> > > -#define PCI_CONF1_DEV_MASK	0x1f
> > > -#define PCI_CONF1_FUNC_MASK	0x7
> > > +#define PCI_CONF1_BUS_MASK	GENMASK(23, 16)
> > > +#define PCI_CONF1_DEV_MASK	GENMASK(15, 11)
> > > +#define PCI_CONF1_FUNC_MASK	GENMASK(10, 8)
> > >  #define PCI_CONF1_REG_MASK	0xfc /* Limit aligned offset to a maximum of 256B */
> > >  
> > >  #define PCI_CONF1_ENABLE	BIT(31)
> > > -#define PCI_CONF1_BUS(x)	(((x) & PCI_CONF1_BUS_MASK) << PCI_CONF1_BUS_SHIFT)
> > > -#define PCI_CONF1_DEV(x)	(((x) & PCI_CONF1_DEV_MASK) << PCI_CONF1_DEV_SHIFT)
> > > -#define PCI_CONF1_FUNC(x)	(((x) & PCI_CONF1_FUNC_MASK) << PCI_CONF1_FUNC_SHIFT)
> > > +#define PCI_CONF1_BUS(x)	FIELD_PREP(PCI_CONF1_BUS_MASK, (x))
> > > +#define PCI_CONF1_DEV(x)	FIELD_PREP(PCI_CONF1_DEV_MASK, (x))
> > > +#define PCI_CONF1_FUNC(x)	FIELD_PREP(PCI_CONF1_FUNC_MASK, (x))
> > >  #define PCI_CONF1_REG(x)	((x) & PCI_CONF1_REG_MASK)
> > 
> > I love getting rid of the _SHIFT #defines.
> >
> > I'm a dinosaur and haven't been completely converted to the wonders of
> > GENMASK yet.
> 
> Okay but would it convince even "a dinosaur" like you :-) if you imagine
> a Bit Location column in some spec which says:
> 	23:16
> 
> GENMASK just happens to mystically repeat those two numbers:
> 	GENMASK(23, 16)
> 
> Pretty magic, isn't it?
> 
> > PCI_CONF1_ADDRESS is the only user of PCI_CONF1_BUS etc,
> > so I think this would be simpler overall:
> > 
> >   #define PCI_CONF1_BUS  0x00ff0000
> >   #define PCI_CONF1_DEV  0x0000f800
> >   #define PCI_CONF1_FUNC 0x00000700
> >   #define PCI_CONF1_REG  0x000000ff
> > 
> >   #define PCI_CONF1_ADDRESS(bus, dev, func, reg) \
> >     (FIELD_PREP(PCI_CONF1_BUS, (bus)) | \
> >      FIELD_PREP(PCI_CONF1_DEV, (dev)) | \
> >      FIELD_PREP(PCI_CONF1_FUNC, (func)) | \
> >      FIELD_PREP(PCI_CONF1_REG, (reg & ~0x3)))

This ended up not working, because FIELD_PREP() detects ext regs not 
fitting into PCI_CONF1_REG:
	FIELD_PREP(PCI_CONF1_REG, (reg) & ~0x3)

There are two partially overlapping things here when it comes to reg 
(addressing side and parameter input side):

#define PCI_CONF1_REG_ADDR	0x000000ff
// for PCI_CONF1_EXT_ADDRESS:
#define PCI_CONF1_EXT_REG_ADDR	0x0f000000

/* PCI Config register (parameter input side) */
#define PCI_CONF1_REG		0x0fc
#define PCI_CONF1_EXT_REG	0xf00

Would those 4 defines be acceptable? Or should I mark the input side with
_IN or use different prefix for the defines?

> Yes, it makes sense to remove the extra layer.
> 
> One additional thing, I just noticed lots of arch/ code is duplicating 
> this calculation so perhaps this should also be moved outside of 
> drivers/pci/ into include/linux/pci.h ? (Not in the same patch.)

I also noticed you took PCI_CONF1_ENABLE away from PCI_CONF1_ADDRESS(),
did you intend for it to be moved at the caller site?

Moving it outside of PCI_CONF1_ADDRESS() would certainly help reusing this 
code as notall arch code wants PCI_CONF1_ENABLE I think.

> > The v6.7 merge window just opened, and I won't start merging v6.8
> > material until v6.7-rc1 (probably Nov 12), so no hurry on this.
> 
> Yes I knew I was sending it quite late because I tried to meet your 
> request in getting it all done in the same merge window (which I 
> obviously failed but it isn't the end of the world).
> 
> 

-- 
 i.