Re: [PATCH v6 08/10] misc: rp1: RaspberryPi RP1 misc driver

[Herve Codina <herve.codina@bootlin.com>]

Hi Andrea,

On Tue, 21 Jan 2025 15:49:04 +0100
Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:

> On Tue, Jan 21, 2025 at 03:38:42PM +0100, Andrea della Porta wrote:
> > Hi Greg,
> > 
> > On 15:18 Tue 21 Jan     , Greg Kroah-Hartman wrote:  
> > > On Tue, Jan 21, 2025 at 02:59:21PM +0100, Andrea della Porta wrote:  
> > > > Hi Greg,
> > > > 
> > > > On 09:48 Tue 21 Jan     , Greg Kroah-Hartman wrote:  
> > > > > On Tue, Jan 21, 2025 at 09:43:37AM +0100, Andrea della Porta wrote:  
> > > > > > Hi Greg,
> > > > > > 
> > > > > > On 12:47 Fri 17 Jan     , Greg Kroah-Hartman wrote:  
> > > > > > > On Mon, Jan 13, 2025 at 03:58:07PM +0100, Andrea della Porta wrote:  
> > > > > > > > The RaspberryPi RP1 is a PCI multi function device containing
> > > > > > > > peripherals ranging from Ethernet to USB controller, I2C, SPI
> > > > > > > > and others.
> > > > > > > > 
> > > > > > > > Implement a bare minimum driver to operate the RP1, leveraging
> > > > > > > > actual OF based driver implementations for the on-board peripherals
> > > > > > > > by loading a devicetree overlay during driver probe.
> > > > > > > > 
> > > > > > > > The peripherals are accessed by mapping MMIO registers starting
> > > > > > > > from PCI BAR1 region.
> > > > > > > > 
> > > > > > > > With the overlay approach we can achieve more generic and agnostic
> > > > > > > > approach to managing this chipset, being that it is a PCI endpoint
> > > > > > > > and could possibly be reused in other hw implementations. The
> > > > > > > > presented approach is also used by Bootlin's Microchip LAN966x
> > > > > > > > patchset (see link) as well, for a similar chipset.
> > > > > > > > 
> > > > > > > > For reasons why this driver is contained in drivers/misc, please
> > > > > > > > check the links.  
> > > > > > > 
> > > > > > > Links aren't always around all the time, please document it here why
> > > > > > > this is needed, and then links can "add to" that summary.  
> > > > > > 
> > > > > > Ack.
> > > > > >   
> > > > > > >   
> > > > > > > > This driver is heavily based on downstream code from RaspberryPi
> > > > > > > > Foundation, and the original author is Phil Elwell.
> > > > > > > > 
> > > > > > > > Link: https://datasheets.raspberrypi.com/rp1/rp1-peripherals.pdf  
> > > > > > 
> > > > > > ...
> > > > > >   
> > > > > > > > diff --git a/drivers/misc/rp1/rp1_pci.c b/drivers/misc/rp1/rp1_pci.c
> > > > > > > > new file mode 100644
> > > > > > > > index 000000000000..3e8ba3fa7fd5
> > > > > > > > --- /dev/null
> > > > > > > > +++ b/drivers/misc/rp1/rp1_pci.c
> > > > > > > > @@ -0,0 +1,305 @@
> > > > > > > > +// SPDX-License-Identifier: GPL-2.0
> > > > > > > > +/*
> > > > > > > > + * Copyright (c) 2018-24 Raspberry Pi Ltd.
> > > > > > > > + * All rights reserved.
> > > > > > > > + */
> > > > > > > > +
> > > > > > > > +#include <linux/err.h>
> > > > > > > > +#include <linux/interrupt.h>
> > > > > > > > +#include <linux/irq.h>
> > > > > > > > +#include <linux/irqchip/chained_irq.h>
> > > > > > > > +#include <linux/irqdomain.h>
> > > > > > > > +#include <linux/module.h>
> > > > > > > > +#include <linux/msi.h>
> > > > > > > > +#include <linux/of_platform.h>
> > > > > > > > +#include <linux/pci.h>
> > > > > > > > +#include <linux/platform_device.h>
> > > > > > > > +
> > > > > > > > +#include "rp1_pci.h"  
> > > > > > > 
> > > > > > > Why does a self-contained .c file need a .h file?  Please put it all in
> > > > > > > here.  
> > > > > > 
> > > > > > I agree with you. Indeed, the very first version of this patch had the header
> > > > > > file placed inside the .c, but I received concerns about it and some advice to
> > > > > > do it differently, as you can see here:
> > > > > > https://lore.kernel.org/all/ZtWDpaqUG9d9yPPf@apocalypse/
> > > > > > so I've changed it accordingly in V2. So right now I'm not sure what the
> > > > > > acceptable behaviour should be ...  
> > > > > 
> > > > > It's a pretty simple rule:
> > > > > 	Only use a .h file if multiple .c files need to see the symbol.
> > > > > 
> > > > > So no .h file is needed here.  
> > > > 
> > > > Perfect, I'll revert back that two lines to V1 then. Please be aware
> > > > though that this will trigger the following checkpatch warning:
> > > > 
> > > > WARNING: externs should be avoided in .c files  
> > > 
> > > Well where are those externs defined at?  Shouldn't there be a .h file
> > > for them somewhere in the tree if they really are global?  
> > 
> > Those symbols are deined in drivers/misc/rp1/rp1-pci.dtbo.S (added by
> > this patchset) and created by cmd_wrap_S_dtb in scripts/Makefile.lib.
> > They are just placeholders that contains rp1-pci.dtbo as
> > a binary blob, in order for the driver (rp1_pci.c) to be able to use
> > the binary buffer representing the overlay and address it from the
> > driver probe function.
> > So there's no other reference from outside rp1_pci.c to those two symbols.
> > In comparison, this is the very same approach used by a recently accepted
> > patch involving drivers/misc/lan966x_pci.c, which also has the two externs
> > in it and triggers the same checkpatch warning.  
> 
> Ok, that's fine, checkpatch is just a hint, not a hard-and-fast-rule.
> 

Maybe just to avoid confusion for future readers, you can add a comment as I
did for the lan966x:
  https://elixir.bootlin.com/linux/v6.13-rc3/source/drivers/misc/lan966x_pci.c#L21

This will not avoid the warning but will give an explanation to people
looking closer at this checkpatch warning.

Best regards,
Hervé