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

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Message-ID: <3551e685-fed1-de9c-a30e-12025ee480fb@redhat.com>
Date:   Tue, 21 Feb 2017 11:29:20 -0800
From:   Laura Abbott <labbott@...hat.com>
To:     Chen Feng <puck.chen@...ilicon.com>,
        Will Deacon <will.deacon@....com>,
        John Stultz <john.stultz@...aro.org>,
        Zhuangluan Su <suzhuangluan@...ilicon.com>,
        Guodong Xu <guodong.xu@...aro.org>,
        Yiping Xu <xuyiping@...ilicon.com>, saberlily.xia@...ilicon.com
Cc:     devel@...verdev.osuosl.org, Jon Medhurst <tixy@...aro.org>,
        Jeremy Gebben <jgebben@...eaurora.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dan Zhao <dan.zhao@...ilicon.com>,
        Rohit kumar <rohit.kr@...sung.com>,
        linux-kernel@...r.kernel.org, Russell King <linux@...linux.org.uk>,
        linaro-mm-sig@...ts.linaro.org,
        Arve Hjønnevåg <arve@...roid.com>,
        Eun Taik Lee <eun.taik.lee@...sung.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Riley Andrews <riandrews@...roid.com>,
        Liviu Dudau <Liviu.Dudau@....com>,
        Android Kernel Team <kernel-team@...roid.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [Linaro-mm-sig] [RFCv3][PATCH 3/5] arm64: Implement
 ARCH_HAS_FORCE_CACHE

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?

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@...hat.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
>> _______________________________________________
>> Linaro-mm-sig mailing list
>> Linaro-mm-sig@...ts.linaro.org
>> https://lists.linaro.org/mailman/listinfo/linaro-mm-sig
> 
> 
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> 

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