[Bug] VCHIQ functional test broken

Eric Anholt eric at anholt.net
Thu Apr 20 14:27:38 EDT 2017

Rabin Vincent <rabin at rab.in> writes:

> 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().
> 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().
> 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'm confused by what you're saying here.  The driver has already been
converted to not use dmac_map_area (commit
cf9caf1929882b66922aee698e99e6c8f357bee5), and uses dma_map_sg instead,
matching the radeon driver you give as a model as far as I can see.
That commit is in v4.11-rc6 from Stefan's regression report.
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