[Bug] VCHIQ functional test broken

[Stefan Wahren]

Hi Rabin,

> Rabin Vincent <rabin at rab.in> hat am 14. April 2017 um 09:41 geschrieben:
> 
> 
> On Thu, Apr 13, 2017 at 11:29:15PM +0100, Russell King - ARM Linux wrote:
> > > 00a19f3e25c0c40e0ec77f52d4841d23ad269169 is the first bad commit
> > > commit 00a19f3e25c0c40e0ec77f52d4841d23ad269169
> > > Author: Rabin Vincent <rabinv at axis.com>
> > > Date:   Tue Nov 8 09:21:19 2016 +0100
> > > 
> > >     ARM: 8627/1: avoid cache flushing in flush_dcache_page()
> > >     
> > >     When the data cache is PIPT or VIPT non-aliasing, and cache operations
> > >     are broadcast by the hardware, we can always postpone the flush in
> > >     flush_dcache_page().  A similar change was done for ARM64 in commit
> > >     b5b6c9e9149d ("arm64: Avoid cache flushing in flush_dcache_page()").
> > >     
> > >     Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
> > >     Signed-off-by: Rabin Vincent <rabinv at axis.com>
> > >     Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>
> > > 
> > > It seems that staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm
> > > relies on the behavior of flush_dcache_page before this patch get
> > > applied. Any advices to fix this issues are appreciated.
> > 
> > Any ideas why this causes a problem for this driver?  From what I can see,
> > it doesn't make use of flush_dcache_page().
> 
> The driver's create_pagelist() uses get_free_pages(), and
> get_free_pages() calls flush_dcache_page().

i think you mean get_user_pages().

> 
> The problem is that the driver fails to flush the pages which it
> acquires via get_free_pages().  It's clear that the driver needs to do
> it, since there is a flush in the is_vmalloc_addr() path in the same
> function.  The driver probably worked earlier because of the unecessary
> flush in flush_dcache_page() which existed before this patch, but the
> purpose of that flush was not DMA coherency and it was never guaranteed
> that it would flush all the way to the point that devices could see the
> data.
> 
> See radeon_ttm_tt_pin_userptr() in drivers/gpu/drm/radeon/radeon_ttm.c
> for an example of how a driver can ensure cache coherency using the DMA
> mapping API if it intends to DMA from/to pages acquired by
> get_free_pages().

Thanks for your explanation and the example, but i can't see the relevant difference. So i think i'm not the right person to fix this issue, but i will test any patches regarding to this issue.

> 
> The rest of the driver should also be converted to the DMA mapping API
> instead of abusing the API's private functions (dmac_map_area etc.)

I will add this to the TODO file of the driver.

Regards
Stefan