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SubjectRe: [PATCH v2 2/4] litex: add common LiteX header
On Tue, Nov 26, 2019 at 10:02:18AM +0100, Mateusz Holenko wrote:
> śr., 20 lis 2019 o 20:26 Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> napisał(a):
> >
> > On Wed, Oct 23, 2019 at 11:47:04AM +0200, Mateusz Holenko wrote:
> > > +#ifdef __LITTLE_ENDIAN
> > > +# define LITEX_READ_REG(addr) ioread32(addr)
> > > +# define LITEX_READ_REG_OFF(addr, off) ioread32(addr + off)
> > > +# define LITEX_WRITE_REG(val, addr) iowrite32(val, addr)
> > > +# define LITEX_WRITE_REG_OFF(val, addr, off) iowrite32(val, addr + off)
> > > +#else
> > > +# define LITEX_READ_REG(addr) ioread32be(addr)
> > > +# define LITEX_READ_REG_OFF(addr, off) ioread32be(addr + off)
> > > +# define LITEX_WRITE_REG(val, addr) iowrite32be(val, addr)
> > > +# define LITEX_WRITE_REG_OFF(val, addr, off) iowrite32be(val, addr + off)
> > > +#endif
> >
> > I just noticed this.
> >
> > Ick, this is not good. You will run into problems in the future with
> > this, I can guarantee it. What about systems where the CPU is one
> > endian and the hardware in the other? It will happen trust us.
>
> As mentioned in the previous comment, LiteX CSRs are guaranteed to be
> always little-endian - this includes configurations with both
> big-endian and little-endian CPUs.

What enforces that guarantee?

> The aim of including the ifdef section was exactly to target situation
> where endianness is different for CPU and devices. As such this
> approach *should* work.

"should" :)

We have seen it happen all the time that some hardware team hooks this
up backwards, no matter what the "spec" required. So be careful here.

good luck!

greg k-h

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