Re: endianness swapped

[Greg Ungerer]

Hi Arnd,

On 29/4/19 4:44 am, Arnd Bergmann wrote:
> On Sun, Apr 28, 2019 at 3:59 PM Greg Ungerer <gerg@xxxxxxxxxxxxxx> wrote:
> > On 28/4/19 7:21 pm, Arnd Bergmann wrote:
> > > On Sun, Apr 28, 2019 at 10:46 AM Geert Uytterhoeven
> > > <geert@xxxxxxxxxxxxxx> wrote:
> > > > On Sat, Apr 27, 2019 at 10:22 PM Angelo Dureghello <angelo@xxxxxxxx> wrote:
> > > > > On Sat, Apr 27, 2019 at 05:32:22PM +0200, Angelo Dureghello wrote:
> > >
> > > Coldfire makes the behavior of readw()/readl() depend on the
> > > MMIO address, presumably since that was the easiest way to
> > > get drivers working originally, but it breaks the assumption
> > > in the asm-generic code.
> >
> > Yes, that is right.
> >
> > There is a number of common hardware modules that Freescale have
> > used in the ColdFire SoC parts and in their ARM based parts (iMX
> > families). The ARM parts are pretty much always little endian, and
> > the ColdFire is always big endian. The hardware registers in those
> > hardware blocks are always accessed in native endian of the processor.
>
> In later Freescale/NXP ARM SoCs (i.MX and Layerscape), we
> also get a lot of devices pulled over from PowerPC, with random
> endianess. In some cases, the same device that had big-endian
> registers originally ends up in two different ARM products and one of
> them uses big-endian while the other one uses little-endian registers.
>
> > So the address range checks are to deal with those internal
> > hardware blocks (i2c, spi, dma, etc), since we know those are
> > at fixed addresses. That leaves the usual endian swapping in place for
> > other general (ie external) devices (PCI devices, network chips, etc).
>
> Is there a complete list of coldfire on-chip device drivers?
>
> Looking at some of the drivers:
>
> - drivers/i2c/busses/i2c-imx.c uses only 8-bit accesses and works either way,
>    same for drivers/tty/serial/mcf.c
> - drivers/spi/spi-coldfire-qspi.c is apparently coldfire-only and could use
>    ioread32be for a portable to do big-endian register access.
> - edma-common has a wrapper to support both big-endian and little-endian
>    configurations in the same kernel image, but the mcf interrupt handler
>    is hardcoded to the (normally) little-endian ioread32 function.
> - drivers/net/ethernet/freescale/fec_main.c is shared between coldfire
>    and i.MX (but not mpc52xx), and is hardcoded to readl/writel, and
>    would need the same trick as edma to make it portable.

That matches up with what we list out in arch/m68k/coldfire/devices.c.
I can't think of any other drivers.

There is a lot of use readl/writel and friends in the architecture
specific code too, in arch/m68k/coldfire. At first I used __raw_readl/
__raw_writel to always get native endianess. But quote a few uses of
readl/writel have crept in over the years.

Regards
Greg