[PATCH 2/5] drm: add ARM flush implementation

[Russell King - ARM Linux]

On Tue, Jan 30, 2018 at 11:14:36AM +0100, Daniel Vetter wrote:
> On Tue, Jan 23, 2018 at 06:56:03PM -0800, Gurchetan Singh wrote:
> > The dma_cache_maint_page function is important for cache maintenance on
> > ARM32 (this was determined via testing).
> > 
> > Since we desire direct control of the caches in drm_cache.c, let's make
> > a copy of the function, rename it and use it.
> > 
> > v2: Don't use DMA API, call functions directly (Daniel)
> > 
> > Signed-off-by: Gurchetan Singh <gurchetansingh at chromium.org>
> 
> fwiw, in principle, this approach has my Ack from the drm side.
> 
> But if we can't get any agreement from the arch side then I guess we'll
> just have to suck it up and mandate that any dma-buf on ARM32 must be wc
> mapped, always. Not sure that's a good idea either, but should at least
> get things moving.

Let me expand on my objection, as I find your tone to be problematical
here.

The patch 2 (which is the earliest patch in this series) makes use of
facilities such as dmac_map_area(), and those are defined as macros in
arch/arm/mm/mm.h.  I see no way that drm_cache.c can pick up on that
_unless_ there's another patch (maybe that's patch 1) which moves the
definition.

dmac_map_area() is non-trivial to export (it's not a function, it's
macro which either points to a function or a function pointer structure
member) so it's likely that this patch also breaks building DRM as a
module.

We've been here before with drivers abusing the architecture private APIs,
which is _exactly_ why dmac_map_area() is defined in arch/arm/mm.h and not
in some kernel-wide asm header file - it's an implementation detail of the
architectures DMA API that drivers have no business mucking about with.

I've always said if the interfaces don't do what you want, talk to
architecture people, don't go poking about in architecture private parts
of the kernel and start abusing stuff.  I say this because years ago, we
had people doing _exactly_ that for the older virtually cached ARMs.  Then
ARMv6 came along, which needed an entire revamp of the architecture cache
interfaces, and lo and behold, drivers got broken because of this kind of
abuse.  IOW, abusing interfaces makes kernel maintenance harder.

For example, interfaces designed for flushing the cache when page tables
get torn down were abused in drivers to flush data for DMA or coherency
purposes, which meant that on ARMv6, where we no longer needed to flush
for page table maintenance, suddenly the interfaces that drivers were
using became no-ops.

In this case, dmac_map_area() exists to perform any cache maintenance
for the kernel view of that memory required for a non-coherent DMA
mapping.  What it does depends on the processsor and the requested
DMA_xxx type - it _may_ invalidate (discard) or clean (writeback but
leave in the cache) cache lines, or do nothing.

dmac_unmap_area() has the same issues - what it does depends on what
operation is being requested and what the processor requires to
achieve coherency.

The two functions are designed to work _together_, dmac_map_area()
before the DMA operation and dmac_unmap_area() after the DMA operation.
Only when they are both used together do you get the correct behaviour.

These functions are only guaranteed to operate on the kernel mapping
passed in as virtual addresses to the dmac_* functions.  They make no
guarantees about other mappings of the same memory elsewhere in the
system, which, depending on the architecture of the caches, may also
contain dirty cache lines (the same comment applies to the DMA API too.)
On certain cache architectures (VIPT) where colouring effects apply,
flushing the kernel mapping may not even be appropriate if the desired
effect is to flush data from a user mapping.

This is exactly why abusing APIs (like what is done in this patch) is
completely unacceptable from the architecture point of view - while
it may _appear_ to work, it may only work for one class of CPU or one
implementation.

Hence why the dmac_{un,}map_area() interfaces are not exported to
drivers.  You can't just abuse one of them.  They are a pair that
must be used together, and the DMA API knows that, and the DMA API
requirements ensure that happens.  It's not really surprising, these
functions were written to support the DMA API, and the DMA API is
the kernel-wide interface to these functions.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up
According to speedtest.net: 8.21Mbps down 510kbps up