Re: [v4,2/3] PCI: mediatek: Add new generation controller support

Bjorn Helgaas
2020-11-30T17:33:57Z

On Mon, Nov 30, 2020 at 09:05:48AM -0700, Rob Herring wrote:
> On Sun, Nov 22, 2020 at 11:45 PM Jianjun Wang <jianjun.wang@mediatek.com> wrote:
> > On Thu, 2020-11-19 at 14:28 -0600, Bjorn Helgaas wrote:
> > > "Add new generation" really contains no information. And "mediatek"
> > > is already used for the pcie-mediatek.c driver, so we should have a
> > > new tag for this new driver. Include useful information in the
> > > subject, e.g.,
> > >
> > > PCI: mediatek-gen3: Add MediaTek Gen3 driver for MT8192

> > > > + writel(PCIE_CFG_HEADER_FORCE_BE(devfn, bus->number, bytes),
> > > > + port->base + PCIE_CFGNUM_REG);
> > > > +
> > > > + *val = readl(port->base + PCIE_CFG_OFFSET_ADDR + (where & ~0x3));
> > >
> > > These look like they need to be atomic, since you need a writel()
> > > followed by a readl().
> > >
> > > pci_lock_config() (used in pci_bus_read_config_*(), etc) uses the
> > > global pci_lock for this unless CONFIG_PCI_LOCKLESS_CONFIG is set.
> > >
> > > But I would like to eventually move away from this implicit dependency
> > > on pci_lock. If you need to make this atomic, can you add the
> > > explicit locking here, so there's a clear connection between the lock
> > > and the things it protects?
> >
> > Sure, I will split it to a map_bus() function and use the standard
> > pci_generic_config_read32/write32 functions as Rob's suggestion. I think
> > the potential risks of atomic read/write can be avoided.
>
> The generic functions have no effect on atomicity, but using them does
> make it easier to find the non-atomic cases.
>
> I'm not sure that having host drivers do their own locking is the best
> approach. That's a recipe for more cleanups. It's a common enough
> issue that I think it's better if we have locking done in 1 place.
> Then host drivers can simply say if they need locking or not via some
> bus flag.

Yeah, you may be right. I guess we don't have to make it an issue for
this patch; we do have pci_lock that protects this, whether the
write/read occurs in the driver or in
pci_generic_config_read32/write32.

Bjorn