Re: [PATCH v2 4/6] PCI: brcmstb: add Broadcom STB PCIe host controller driver

[Robin Murphy]

On 21/11/2019 9:07 pm, Jim Quinlan wrote:
[...]
> > As for [...]_NUM_MASK_BITS I'm looking for a smart/generic way to calculate it
> > from the actual mask. No luck so far. If not, I think I'll simply leave it as
> > is for now.

HWEIGHT()?

> > > > FYI, What's happening here is that we have to save the CPU address range
> > > > (which
> > > > is already shifted right 20 positions) in two parts, the lower 12 bits go
> > > > into
> > > > PCIE_MISC_CPU_2_PCIE_MEM_WIN0_BASE_LIMIT while the higher 8 bits go into
> > > > PCIE_MISC_CPU_2_PCIE_MEM_WIN0_BASE_HI or
> > > > PCIE_MISC_CPU_2_PCIE_MEM_WIN0_LIMIT_HI.
> > >
> > > The hardware spec require bits 31:20 of the address, and the high registers
> > > require 39:32 right?
> >
> > Yes, that's it.
> >
> > > (Apologies, the indirection by the WR_FLD_** macros easily confuses me. These
> > > type of macros are helpful, or rather would be if the whole kernel used them.
> > > I think they can add confusion when each driver has its own set of similar
> > > macros. This is why its *really* helpful to use any existing macros in the
> > > kernel - and only invent new ones if needed).
> >
> > I agree it's pretty confusing, I think v3, using bitfield.h as much as
> > possible, looks substantially more welcoming.
>
> The reason we use custom macros is because we'd like to keep the
> register names the same as the HW declares and our internal tools
> support.  As you may have noticed, our register names are unusually
> long and it is hard to fit a simple read or write field assignment
> within 80 columns w/o using custom macros tailored to our register
> names' format.
>
> Perhaps Nicolas can pull a rabbit out of a hat and use Linux macros
> while keeping our long register names, but if he has to use his own
> shorter register names it will become harder for Broadcom developers
> to debug this driver.

Regardless of the length of the names, the standard bitfield helpers can 
still make things easier to reason about - in this particular case I 
think you could lose some boilerplate and indirection with essentially 
no change to the readability you're concerned for - compare:

#define REG_NAME ...
#define REG_NAME_FIELD_NAME_MASK ...
#define REG_NAME_FIELD_NAME_SHIFT ...

	val = RD_FIELD(base, REG_NAME,
		       FIELD_NAME);

vs.

#define REG_NAME ...
#define   FIELD_NAME ...

	reg = bcm_readl(base + REG_NAME);
	val = FIELD_GET(FIELD_NAME, reg);

Robin.