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SubjectRe: [PATCH] staging: android: ion: add buffer flag update ioctl
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On 12/23/18 6:47 PM, Zengtao (B) wrote:
> Hi laura:
>
>> -----Original Message-----
>> From: Laura Abbott [mailto:labbott@redhat.com]
>> Sent: Friday, December 21, 2018 4:50 AM
>> To: Zengtao (B) <prime.zeng@hisilicon.com>; sumit.semwal@linaro.org
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>; Arve Hjønnevåg
>> <arve@android.com>; Todd Kjos <tkjos@android.com>; Martijn Coenen
>> <maco@android.com>; Joel Fernandes <joel@joelfernandes.org>;
>> devel@driverdev.osuosl.org; dri-devel@lists.freedesktop.org;
>> linaro-mm-sig@lists.linaro.org; linux-kernel@vger.kernel.org
>> Subject: Re: [PATCH] staging: android: ion: add buffer flag update ioctl
>>
>> On 12/19/18 5:39 PM, Zengtao (B) wrote:
>>> Hi laura:
>>>
>>>> -----Original Message-----
>>>> From: Laura Abbott [mailto:labbott@redhat.com]
>>>> Sent: Thursday, December 20, 2018 2:10 AM
>>>> To: Zengtao (B) <prime.zeng@hisilicon.com>;
>> sumit.semwal@linaro.org
>>>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>; Arve
>> Hjønnevåg
>>>> <arve@android.com>; Todd Kjos <tkjos@android.com>; Martijn
>> Coenen
>>>> <maco@android.com>; Joel Fernandes <joel@joelfernandes.org>;
>>>> devel@driverdev.osuosl.org; dri-devel@lists.freedesktop.org;
>>>> linaro-mm-sig@lists.linaro.org; linux-kernel@vger.kernel.org
>>>> Subject: Re: [PATCH] staging: android: ion: add buffer flag update
>>>> ioctl
>>>>
>>>> On 12/19/18 9:19 AM, Zeng Tao wrote:
>>>>> In some usecases, the buffer cached attribute is not determined at
>>>>> allocation time, it's determined just before the real cpu mapping.
>>>>> And from the memory view of point, a buffer should not have the
>>>> cached
>>>>> attribute util is really mapped by the cpu. So in this patch, we
>>>>> introduced the new ioctl command to target the requirement.
>>>>>
>>>>
>>>> This is racy and error prone. Can you explain more what problem you
>>>> are trying to solve?
>>>
>>> My use case is like this:
>>> 1. There are two process A and B, A takes case of ion buffer allocation,
>> and
>>> pass the buffer fd to B, then B maps and uses it.
>>> 2. Process B need to map the buffer with different cached attribute
>>> for different use case, for example, if the buffer is used for pure
>>> software algorithm, then we need to map it as cached, otherwise
>>> non-cached, and B needs to deal with both cases.
>>> And unfortunately the mmap syscall takes no cached flags and we can't
>>> decide the cache attribute when we are doing the mmap, so I introduce
>>> new the ioctl even though I think the solution is not as good.
>>>
>>>
>>
>> Thanks for the explanation, this was about the use case I expected.
>> I'm pretty sure I had this exact problem once upon a time and we didn't
>> come up with a solution. I'd still like to get rid of uncached buffers in
>> general and just use cached buffers (see
>> http://driverdev.linuxdriverproject.org/pipermail/driverdev-devel/2018-N
>> ovember/128842.html)
>> What's your usecase for uncached buffers?
>
> Some buffers are only used by HW, in this case, we tend to use uncached buffers.
> But sometimes the SW need to read few buffer contents for debug purpose and
> no synchronization is needed.
>

If this is primarily for debug, that's not really a compelling reason to
support uncached buffers. It's incredibly difficult to do uncached
buffers correctly I'd rather spend effort on making the cached
use cases work better.

Thanks,
Laura

>>
>>>>
>>>>> Signed-off-by: Zeng Tao <prime.zeng@hisilicon.com>
>>>>> ---
>>>>> drivers/staging/android/ion/ion-ioctl.c | 4 ++++
>>>>> drivers/staging/android/ion/ion.c | 17
>> +++++++++++++++++
>>>>> drivers/staging/android/ion/ion.h | 1 +
>>>>> drivers/staging/android/uapi/ion.h | 22
>>>> ++++++++++++++++++++++
>>>>> 4 files changed, 44 insertions(+)
>>>>>
>>>>> diff --git a/drivers/staging/android/ion/ion-ioctl.c
>>>>> b/drivers/staging/android/ion/ion-ioctl.c
>>>>> index a8d3cc4..60bb702 100644
>>>>> --- a/drivers/staging/android/ion/ion-ioctl.c
>>>>> +++ b/drivers/staging/android/ion/ion-ioctl.c
>>>>> @@ -12,6 +12,7 @@
>>>>>
>>>>> union ion_ioctl_arg {
>>>>> struct ion_allocation_data allocation;
>>>>> + struct ion_buffer_flag_data update;
>>>>> struct ion_heap_query query;
>>>>> };
>>>>>
>>>>> @@ -83,6 +84,9 @@ long ion_ioctl(struct file *filp, unsigned int
>>>>> cmd, unsigned long arg)
>>>>>
>>>>> break;
>>>>> }
>>>>> + case ION_IOC_BUFFER_UPDATE:
>>>>> + ret = ion_buffer_update(data.update.fd, data.update.flags);
>>>>> + break;
>>>>> case ION_IOC_HEAP_QUERY:
>>>>> ret = ion_query_heaps(&data.query);
>>>>> break;
>>>>> diff --git a/drivers/staging/android/ion/ion.c
>>>>> b/drivers/staging/android/ion/ion.c
>>>>> index 9907332..f1404dc 100644
>>>>> --- a/drivers/staging/android/ion/ion.c
>>>>> +++ b/drivers/staging/android/ion/ion.c
>>>>> @@ -436,6 +436,23 @@ int ion_alloc(size_t len, unsigned int
>>>> heap_id_mask, unsigned int flags)
>>>>> return fd;
>>>>> }
>>>>>
>>>>> +int ion_buffer_update(unsigned int fd, unsigned int flags) {
>>>>> + struct dma_buf *dmabuf;
>>>>> + struct ion_buffer *buffer;
>>>>> +
>>>>> + dmabuf = dma_buf_get(fd);
>>>>> +
>>>>> + if (!dmabuf)
>>>>> + return -EINVAL;
>>>>> +
>>>>> + buffer = dmabuf->priv;
>>>>> + buffer->flags = flags;
>>>>> + dma_buf_put(dmabuf);
>>>>> +
>>>>> + return 0;
>>>>> +}
>>>>> +
>>>>> int ion_query_heaps(struct ion_heap_query *query)
>>>>> {
>>>>> struct ion_device *dev = internal_dev; diff --git
>>>>> a/drivers/staging/android/ion/ion.h
>>>>> b/drivers/staging/android/ion/ion.h
>>>>> index c006fc1..99bf9ab 100644
>>>>> --- a/drivers/staging/android/ion/ion.h
>>>>> +++ b/drivers/staging/android/ion/ion.h
>>>>> @@ -199,6 +199,7 @@ int ion_heap_pages_zero(struct page *page,
>>>> size_t size, pgprot_t pgprot);
>>>>> int ion_alloc(size_t len,
>>>>> unsigned int heap_id_mask,
>>>>> unsigned int flags);
>>>>> +int ion_buffer_update(unsigned int fd, unsigned int flags);
>>>>>
>>>>> /**
>>>>> * ion_heap_init_shrinker
>>>>> diff --git a/drivers/staging/android/uapi/ion.h
>>>>> b/drivers/staging/android/uapi/ion.h
>>>>> index 5d70098..99753fc 100644
>>>>> --- a/drivers/staging/android/uapi/ion.h
>>>>> +++ b/drivers/staging/android/uapi/ion.h
>>>>> @@ -74,6 +74,20 @@ struct ion_allocation_data {
>>>>> __u32 unused;
>>>>> };
>>>>>
>>>>> +/**
>>>>> + * struct ion_buffer_flag_data - metadata passed from userspace for
>>>>> +update
>>>>> + * buffer flags
>>>>> + * @fd: file descriptor of the buffer
>>>>> + * @flags: flags passed to the buffer
>>>>> + *
>>>>> + * Provided by userspace as an argument to the ioctl */
>>>>> +
>>>>> +struct ion_buffer_flag_data {
>>>>> + __u32 fd;
>>>>> + __u32 flags;
>>>>> +}
>>>>> +
>>>>> #define MAX_HEAP_NAME 32
>>>>>
>>>>> /**
>>>>> @@ -116,6 +130,14 @@ struct ion_heap_query {
>>>>> struct ion_allocation_data)
>>>>>
>>>>> /**
>>>>> + * DOC: ION_IOC_BUFFER_UPDATE - update the specified ion buffer
>>>> flags
>>>>> + *
>>>>> + * Takes an ion_buffer_flag_data structure and returns the result
>>>>> +of the
>>>>> + * buffer flag update operation.
>>>>> + */
>>>>> +#define ION_IOC_BUFFER_UPDATE _IOWR(ION_IOC_MAGIC, 1, \
>>>>> + struct ion_buffer_flag_data)
>>>>> +/**
>>>>> * DOC: ION_IOC_HEAP_QUERY - information about available
>> heaps
>>>>> *
>>>>> * Takes an ion_heap_query structure and populates information
>>>> about
>>>>>
>>>
>

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