Re: [PATCH] i2c: tegra: Add i2c support

[Ben Dooks <ben-i2c-elnMNo+KYs3YtjvyW6yDsg@public.gmane.org>]

On Sun, Feb 20, 2011 at 04:28:46PM -0800, Colin Cross wrote:
> On Sun, Feb 20, 2011 at 3:57 PM, Colin Cross <ccross-z5hGa2qSFaRBDgjK7y7TUQ@public.gmane.org> wrote:
> > On Sun, Feb 20, 2011 at 3:38 PM, Ben Dooks <ben-i2c-elnMNo+KYs3YtjvyW6yDsg@public.gmane.org> wrote:
> > <snip>
> >
> >>> > you know, there's a readsl() function that does this.
> >>> I think readsl can't handle the possibly unaligned buf pointer.
> >>
> >> it does. see arch/arm/lib/io-writesl.S for proof.
> > You're right, I traced the wrong definition of readsl/writesl
> >
> >>> > this whole function gives me the creeps, is there any reason why
> >>> > we can't use the readsl or similar functions for this?
> >>> Same here - readsl can't handle the alignment requirements, readsb
> >>> can't  handle the required 32 bit register read, and the bytes in the
> >>> same word but after the end buf may not be part of buf, so byte writes
> >>> to buf are required.
> >>
> >> You'll find it can, too. arch/arm/lib/io-readsl.S.
> > I still think readsl doesn't work here.  It may need to read 7 bytes,
> > requiring 2 calls to readl, but 1 word write and 3 byte writes to buf.
> >  Using readsl for the whole buffer would overwrite the 8th byte.  I
> > can use readsl for the main loop, but I will still need to write the
> > last 1-3 bytes separately.  I could use readl and memcpy to do the
> > last writes.
> 
> Actually, the same problem applies to writesl.  If the buffer is not
> aligned, reading an entire word to get the partial word at the end of
> the buffer may cross a page boundary and fault.  I'll have to use
> writesl for the whole words and then handle the last partial word with
> memcpy and writel.

Hmm, that's an interesting one, however I suspect this is something
that will never happen as the kernel over-allocates (rounds up) buffers
when allocating memory blocks.

-- 
Ben Dooks, ben-elnMNo+KYs3YtjvyW6yDsg@public.gmane.org, http://www.fluff.org/ben/

Large Hadron Colada: A large Pina Colada that makes the universe disappear.