Re: [PATCH] mtd: bcm47xxsflash: use devm_ioremap_nocache() instead of KSEG0ADDR()

From: Maciej W. Rozycki
Date: Sun Jan 24 2016 - 18:07:34 EST


On Sun, 24 Jan 2016, Rafał Miłecki wrote:

> On 24 January 2016 at 21:26, Maciej W. Rozycki <macro@xxxxxxxxxx> wrote:
> > On Sat, 23 Jan 2016, Brian Norris wrote:
> >
> >> IIUC, this could be solved by:
> >> (a) using an uncached mapping or
> >> (b) explicitly invalidating the relevant region after doing flash writes
> >> or erasures
> >
> > Flash writes are usually much, much less frequent than reads, so
> > optimising for reads is IMO the right direction. So a cached mapping is a
> > good choice, however invalidation must then be done after a write.
>
> Can you give me some hint where to look at for cache invalidation?

There is `flush_data_cache_page' only it would seem, which is also
supported by the MIPS platform only. It makes unnecessary writebacks
before invalidation, however these aren't really supposed to happen as no
cache line involved is expected to be dirty.

Implementing `invalidate_data_cache_page', which would avoid these
unnecessary writebacks, should be straightforward as hardware provides the
necessary operations and actually the MIPS port has suitable low-level
helpers already implemented, for use by `dma_cache_inv'. So that would
merely be a semi-mechanical copy, paste, rename operation applied to our
source.

The bigger problem is the lack of portability of this interface to other
platforms, although I suspect some hardware may simply fail to provide
required operations. For example x86 only defines the sledgehammer
INVD/WBINVD instructions, which operate on the whole cache hierarchy at
once rather than on a line-by-line and cache level/part basis.

Maciej