Re: [PATCH] fb.h: ARM uses __raw_{read/write}

On Mon, Jun 13, 2011 at 01:00:04PM +0900, Paul Mundt wrote:

On Fri, Jun 10, 2011 at 05:31:08PM -0700, H Hartley Sweeten wrote:

quoted

ARM provides __raw_{read/write}* functions for memory access. These
should be used instead of the default '(*(volatile' stuff to make sure the
memory accesses are typesafe (void __iomem *).

This also fixes a number of sparse warning like:

  warning: cast removes address space of expression

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Paul Mundt <redacted>
Cc: Russell King <redacted>

I'm not sure what semantics are desirable for ARM here, so I'll wait for
Russell to reply.

This wrapping will basically mean that the fb_read/write ops are using
__raw_xxx variants while the memset and memcpy wrappers will be using the
regular read/write[bwl] routines which contain __iormb() calls. Given
that ioread/write and friends all wrap in to the normal versions with the
barriers, I would suppose that this is the default behaviour that is
desired, as opposed to wrapping in to the __raw_xxx variants directly.

The intention for the __iormb/__iowmb calls is in relation to DMA
transfers where a buffer in normal RAM is filled in with data and the
transfer started by a writel() to a device. We need to make sure that
the normal RAM writing completes before the writel().

The change proposed by Hartley wouldn't make much difference from the
current volatile accesses (__raw_* accessors are implemented as volatile
on ARM).

I think the memcpy_(from|to)io could be optimised on ARM to only add a
barrier before or after he copying loop.

-- 
Catalin