Re: [PATCH 05/15] musb: Do not use musb_read[b|w] / _write[b|w] wrappers in generic fifo functions

[Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>]

On Tuesday 10 March 2015 08:43:22 Hans de Goede wrote:
> On 09-03-15 22:50, Arnd Bergmann wrote:
> > On Monday 09 March 2015 21:40:18 Hans de Goede wrote:
> >> The generic fifo functions already use non wrapped accesses in various
> >> cases through the iowrite#_rep functions, and all platforms which override
> >> the default musb_read[b|w] / _write[b|w] functions also provide their own
> >> fifo access functions, so we can safely drop the unnecessary indirection
> >> from the fifo access functions.
> >>
> >> Signed-off-by: Hans de Goede <hdegoede-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> >>
> >
> > The patch looks reasonably, but the description seem misleading.
> > I believe the real reason why it's ok to use __raw_writew for the
> > FIFO is that a FIFO by definition is using CPU endian access for
> > copying byte streams from memory, which is unlike any other MMIO
> > register that requires fixed-endian accessors.
> 
> I'm not sure that that is the case here, this fifo allows reading
> 4 bytes at a time using 32 bit word access, so endianness may come
> into play. This patch is safe however since all existing users of
> the generic fifo_read / write helpers which this patch touches, are
> also using the generic musb_read[b|w] / _write[b|w] functions which
> are just __raw_foo wrappers, so there is no functional change, which
> is what the commit message tries to say.

This probably means that the generic musb helpers are not safe to use
on big-endian ARM systems (but may work on MIPS). The only one currently
not using the generic helpers is blackfin, which is fixed to little
endian.
 
> Note that sunxi needs this function because of the register address
> translation the sunxi_musb_readb / writeb wrappers are doing which
> does not know how to deal with fifo data, and besides that it should
> make the generic read / write fifo helpers somewhat faster by removing
> an indirect function call.

Your sunxi_musb_readw/writew functions however also are endian-safe
and seem to get this part right, unlike all other platforms that use
the generic __raw_*() accessors. With this patch applied, it should
actually work fine, and it would work on other platforms as well
if we change all __raw_*() calls outside of musb_default_write_fifo()
and musb_default_read_fifo() to use *_relaxed() instead.

	Arnd