Re: [PATCH 2/4] usb: typec: tcpci_rt1711h: Convert enum->pointer for data in the match tables

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Wed, Aug 23, 2023 at 04:49:05PM +0200, Andi Shyti wrote:
> Hi,
> 
> On Tue, Aug 22, 2023 at 03:08:38PM +0300, Andy Shevchenko wrote:
> > On Tue, Aug 22, 2023 at 02:00:05PM +0200, Geert Uytterhoeven wrote:
> > > On Tue, Aug 22, 2023 at 1:44 PM Andy Shevchenko
> > > <andriy.shevchenko@linux.intel.com> wrote:
> > > > On Tue, Aug 22, 2023 at 09:21:19AM +0200, Geert Uytterhoeven wrote:
> > > > > On Mon, Aug 21, 2023 at 7:09 PM Andy Shevchenko
> > > > > <andriy.shevchenko@linux.intel.com> wrote:
> > > > > > On Mon, Aug 21, 2023 at 05:40:05PM +0200, Geert Uytterhoeven wrote:
> > > > > > > On Mon, Aug 21, 2023 at 5:25 PM Andy Shevchenko
> > > > > > > <andriy.shevchenko@linux.intel.com> wrote:
> > > > > > > > On Mon, Aug 21, 2023 at 03:27:43PM +0200, Geert Uytterhoeven wrote:
> > > > > > > > > On Mon, Aug 21, 2023 at 3:04 PM Andy Shevchenko
> > > > > > > > > <andriy.shevchenko@linux.intel.com> wrote:
> > > > > > > > > > On Sun, Aug 20, 2023 at 07:44:00PM +0100, Biju Das wrote:
> > 
> > ...
> > 
> > > > > > > > > > For all your work likes this as I noted in the reply to Guenter that
> > > > > > > > > > the couple of the selling points here are:
> > > > > > > > > > 1) avoidance of the pointer abuse in OF table
> > > > > > > > > >    (we need that to be a valid pointer);
> > > > > > > > >
> > > > > > > > > There is no pointer abuse: both const void * (in e.g. of_device_id)
> > > > > > > > > and kernel_ulong_t (in e.g. i2c_device_id) can be used by drivers
> > > > > > > > > to store a magic cookie, being either a pointer, or an integer value.
> > > > > > > > > The same is true for the various unsigned long and void * "driver_data"
> > > > > > > > > fields in subsystem-specific driver structures.
> > > > > > > >
> > > > > > > > (void *)5 is the abuse of the pointer.
> > > > > > > > We carry something which is not a valid pointer from kernel perspective.
> > > > > > >
> > > > > > > But the data field is not required to be a valid pointer.
> > > > > > > What kind and type of information it represents is specific to the driver.
> > > > > >
> > > > > > Where to find necessary information which is not always an integer constant.
> > > > > > For example, for the driver data that has callbacks it can't be invalid pointer.
> > > > >
> > > > > If the driver uses it to store callbacks, of course it needs to be a
> > > > > valid pointer. But that is internal to the driver.  It is not that
> > > > > we're passing random integer values to a function that expects a
> > > > > pointer that can actually be dereferenced.
> > > > >
> > > > > > Since OF ID table structure is universal, it uses pointers. Maybe you need to
> > > > > > update it to use plain integer instead?
> > > > >
> > > > > It is fairly common in the kernel to use void * to indicate a
> > > > > driver-specific cookie, being either a real pointer or an integral
> > > > > value, that is passed verbatim.  See also e.g. the "dev" parameter
> > > > > of request_irq().
> > > >
> > > > Yes, that parameter is void * due to calling kfree(free_irq(...)).
> > > > So, that's argument for my concerns.
> > > 
> > > Sorry, I don't understand this comment.
> > > (kfree(free_irq(...)) is only called in pci_free_irq()?)
> > 
> > Passing void * for a "driver cookie" makes sense due to possibility of the
> > passing it to other functions that want to have void * as your example shows.
> > And that supports my idea of having void * over the unsigned long.
> 
> I actually agree with Andy here... not much to add to his
> arguments but if a void * is used as an integer then just change
> the type.
> 
> I also was quite puzzled when I started seeing this flow of
> patches.
> 
> I would rather prefer to store pointers in u64 variables rather
> than integers in a pointer.

I think pointers should be stored in pointers, and preserve constness.
The reason that many legacy device id structures use kernel_ulong_t to
store driver "cookie" is shortsightedness on our part long time ago.
Back then we just needed "a simple piece of data" to be attached, a
true/false flag or maybe a combination of flags. Nowadays we often
want to attach a structure describing a chip's parameters there with
several flags, maybe some methods, etc. So OF/DT folks did the right
thing, and used const void *, and we should try and convert the rest to
use const void * as well.

I posted RFC for this for I2C here:

https://lore.kernel.org/all/20230814-i2c-id-rework-v1-0-3e5bc71c49ee@gmail.com/

And then we can unify OF and legacy handling if driver tables.

Thanks.

-- 
Dmitry