Re: [linux-next:master 4897/5417] drivers/i2c/busses/i2c-designware-amdpsp.c:165:25: sparse: sparse: incorrect type in argument 1 (different address spaces)

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Mon, Feb 14, 2022 at 02:59:34PM +0100, Jan Dąbroś wrote:
> pon., 14 lut 2022 o 14:28 Andy Shevchenko
> <andriy.shevchenko@linux.intel.com> napisał(a):
> >
> > On Mon, Feb 14, 2022 at 01:27:35PM +0100, Jan Dąbroś wrote:
> > > pt., 11 lut 2022 o 22:24 kernel test robot <lkp@intel.com> napisał(a):
> >
> > > >    159
> > > >    160  /* Helper to verify status returned by PSP */
> > > >    161  static int check_i2c_req_sts(struct psp_i2c_req *req)
> > > >    162  {
> > > >    163          int status;
> > > >    164
> > > >  > 165          status = readl(&req->hdr.status);
> > >
> > > Actually the above error points to something hidden but important -
> > > for reading from command-response buffer, we shouldn't use __iomem
> > > specifier (nor readl() family of functions) since this is normal
> > > memory - however updated by PSP. Thus I will refactor this to use
> > > 'volatile u32 *' and reading status by de-referencing pointer.
> >
> > Not sure volatile is a good idea. Perhaps READ_ONCE() is what you need.
> > Is this a system memory?
> 
> Yes, this is system memory.
> 
> Actually looking at asm-generic/rwonce.h:
> #define __READ_ONCE(x)  (*(const volatile __unqual_scalar_typeof(x) *)&(x))
> it is more-less based on volatile, so that compiler will not be able
> to (among others) optimize out such reads of memory which may be
> changed outside of the scope of "program".
> 
> I believe that I will get the same outcome from using READ_ONCE and
> explicit volatile, is the first way preferred in the kernel?

READ_ONCE() may be different on different arches. I believe that's why
it's preferred.

-- 
With Best Regards,
Andy Shevchenko