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

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

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);



> 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@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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>>
>>
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> 
> 
> .
>