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

[Laura Abbott]

On 02/22/2017 05:01 PM, Chen Feng wrote:
> 
> 
> On 2017/2/22 3:29, Laura Abbott wrote:
>> On 02/20/2017 10:05 PM, Chen Feng wrote:
>>> 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?
>>>
>>
>> No, I've abandoned this approach based on feedback. The APIs had too much
>> potential for incorrect usage. I'm ripping out the implicit caching in Ion
>> and switching it to a model where there should always be a device available.
>>
>> What's your use case where you don't have a device structure?
>>
> Userspace use ioctl to flush cache for device.
> 
> ion_sync_for_device
> 	dma_sync_sg_for_device(NULL, buffer->sg_table->sgl,
> 		 buffer->sg_table->nents, DMA_BIDIRECTIONAL);
> 
> And sys-heap when allocate a zero buffer flush zero data to ddr.
> alloc_buffer_page
> 	ion_pages_sync_for_device(NULL, page, PAGE_SIZE << order,
> 		DMA_BIDIRECTIONAL);
> 
> 

Yes, those calls are being removed. This is what I proposed back in
December https://marc.info/?l=linux-kernel&m=148176054902921&w=2
I never heard any feedback on it so I assume everyone was okay with
the general direction. I plan on pushing a revised and expanded
version of that series once the merge window closes.

Thanks,
Laura

> 
>> Thanks,
>> Laura
>>
>>> 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@xxxxxxxxxx>
>>>>>>>> ---
>>>>>>>> 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@xxxxxxxxxxxxxxxxxxxxxx/msg47654.html
>>>>>> http://www.mail-archive.com/driverdev-devel@xxxxxxxxxxxxxxxxxxxxxx/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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>>>
>>>
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>>
>>
>> .
>>
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