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 09:56:52 Hans de Goede wrote:
> On 10-03-15 09:50, Arnd Bergmann wrote:
> > 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.
> 
> I think that that change falls outside of the scope of this patchset.

Yes, sorry for not being clear about that.

> I agree that it would be probably a good idea to get rid of the
> __raw_foo usage in musb, which is why I've used the non __raw
> versions in the sunxi glue, but as said I believe this falls outside
> of the scope of this patchset. All my preparation patches for adding
> sunxi support carefully do not make any functional changes, as I
> do not want to cause regressions on hardware which I cannot test.

Right.

	Arnd
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