Re: [Linaro-mm-sig] [RFCv3][PATCH 3/5] arm64: Implement ARCH_HAS_FORCE_CACHE

[Chen Feng <puck.chen@hisilicon.com>]

Hi Laura,

When we enable kernel v4.4 or newer version on our platform, we meet the issue
of flushing cache without reference device. It seems that this patch set is
a solution. I'm curious the progress of the discussion. Do you have any plan
to fix it in v4.4 and newer kernel verison?

On 2016/9/14 2:41, Laura Abbott wrote:
> On 09/13/2016 08:14 AM, Will Deacon wrote:
>> On Tue, Sep 13, 2016 at 08:02:20AM -0700, Laura Abbott wrote:
>>> On 09/13/2016 02:19 AM, Will Deacon wrote:
>>>> On Mon, Sep 12, 2016 at 02:32:56PM -0700, Laura Abbott wrote:
>>>>>
>>>>> arm64 may need to guarantee the caches are synced. Implement versions of
>>>>> the kernel_force_cache API to allow this.
>>>>>
>>>>> Signed-off-by: Laura Abbott <labbott@redhat.com>
>>>>> ---
>>>>> v3: Switch to calling cache operations directly instead of relying on
>>>>> DMA mapping.
>>>>> ---
>>>>> arch/arm64/include/asm/cacheflush.h |  8 ++++++++
>>>>> arch/arm64/mm/cache.S               | 24 ++++++++++++++++++++----
>>>>> arch/arm64/mm/flush.c               | 11 +++++++++++
>>>>> 3 files changed, 39 insertions(+), 4 deletions(-)
>>>>
>>>> I'm really hesitant to expose these cache routines as an API solely to
>>>> support a driver sitting in staging/. I appreciate that there's a chicken
>>>> and egg problem here, but we *really* don't want people using these routines
>>>> in preference to the DMA API, and I fear that we'll simply grow a bunch
>>>> more users of these things if we promote it as an API like you're proposing.
>>>>
>>>> Can the code not be contained under staging/, as part of ion?
>>>>
>>>
>>> I proposed that in V1 and it was suggested I make it a proper API
>>>
>>> http://www.mail-archive.com/driverdev-devel@linuxdriverproject.org/msg47654.html
>>> http://www.mail-archive.com/driverdev-devel@linuxdriverproject.org/msg47672.html
>>
>> :/ then I guess we're in disagreement. If ion really needs this stuff
>> (which I don't fully grok), perhaps we should be exposing something at
>> a higher level from the architecture, so it really can't be used for
>> anything other than ion.
> 
> I talked/complained about this at a past plumbers. The gist is that Ion
> ends up acting as a fake DMA layer for clients. It doesn't match nicely
> because clients can allocate both coherent and non-coherent memory.
> Trying to use dma_map doesn't work because a) a device for coherency isn't
> known at allocation time b) it kills performance. Part of the motivation
> for taking this approach is to avoid the need to rework the existing
> Android userspace and keep the existing behavior, as terrible as it
> is. Having Ion out of staging and not actually usable isn't helpful.
> 
> I'll give this all some more thought and hopefully have one or two more
> proposals before Connect/Plumbers.
> 
>>
>> Will
>>
> 
> Thanks,
> Laura
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