Rabin Vincent <rabin@xxxxxx> 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@xxxxxxxx>
>> > 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@xxxxxxx>
>> > Signed-off-by: Rabin Vincent <rabinv@xxxxxxxx>
>> > Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>
>> >
>> > 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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