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Message-ID: <bba1e67f-3ce4-4b90-b402-543faf239b3c@xs4all.nl>
Date: Mon, 15 Jan 2024 16:14:34 +0100
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Subject: Re: [PATCH v16 4/8] media: core: Add bitmap manage bufs array entries
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To: Benjamin Gaignard <benjamin.gaignard@collabora.com>, mchehab@kernel.org
Cc: linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
 kernel@collabora.com
References: <20231215090813.15610-1-benjamin.gaignard@collabora.com>
 <20231215090813.15610-5-benjamin.gaignard@collabora.com>
 <94e9f612-5daf-414a-a8b9-26330e697884@xs4all.nl>
 <8f683397-f0e2-4701-9a4b-5b5c32d25915@collabora.com>
From: Hans Verkuil <hverkuil@xs4all.nl>
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In-Reply-To: <8f683397-f0e2-4701-9a4b-5b5c32d25915@collabora.com>
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On 15/01/2024 15:51, Benjamin Gaignard wrote:
> 
> Le 15/01/2024 à 13:21, Hans Verkuil a écrit :
>> On 15/12/2023 10:08, Benjamin Gaignard wrote:
>>> Add a bitmap field to know which of bufs array entries are
>>> used or not.
>>> Remove no more used num_buffers field from queue structure.
>>> Use bitmap_find_next_zero_area() to find the first possible
>>> range when creating new buffers to fill the gaps.
>>>
>>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
>>> ---
>>>   .../media/common/videobuf2/videobuf2-core.c   | 37 ++++++++++++++++---
>>>   include/media/videobuf2-core.h                | 17 +++++----
>>>   2 files changed, 41 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
>>> index cd2b9e51b9b0..9509535a980c 100644
>>> --- a/drivers/media/common/videobuf2/videobuf2-core.c
>>> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
>>> @@ -421,11 +421,12 @@ static void init_buffer_cache_hints(struct vb2_queue *q, struct vb2_buffer *vb)
>>>    */
>>>   static void vb2_queue_add_buffer(struct vb2_queue *q, struct vb2_buffer *vb, unsigned int index)
>>>   {
>>> -    WARN_ON(index >= q->max_num_buffers || q->bufs[index] || vb->vb2_queue);
>>> +    WARN_ON(index >= q->max_num_buffers || test_bit(index, q->bufs_bitmap) || vb->vb2_queue);
>>>         q->bufs[index] = vb;
>>>       vb->index = index;
>>>       vb->vb2_queue = q;
>>> +    set_bit(index, q->bufs_bitmap);
>>>   }
>>>     /**
>>> @@ -434,6 +435,7 @@ static void vb2_queue_add_buffer(struct vb2_queue *q, struct vb2_buffer *vb, uns
>>>    */
>>>   static void vb2_queue_remove_buffer(struct vb2_buffer *vb)
>>>   {
>>> +    clear_bit(vb->index, vb->vb2_queue->bufs_bitmap);
>>>       vb->vb2_queue->bufs[vb->index] = NULL;
>>>       vb->vb2_queue = NULL;
>>>   }
>>> @@ -462,7 +464,8 @@ static int __vb2_queue_alloc(struct vb2_queue *q, enum vb2_memory memory,
>>>       num_buffers = min_t(unsigned int, num_buffers,
>>>                   q->max_num_buffers - vb2_get_num_buffers(q));
>>>   -    index = vb2_get_num_buffers(q);
>>> +    index = bitmap_find_next_zero_area(q->bufs_bitmap, q->max_num_buffers,
>>> +                       0, num_buffers, 0);
>> Shouldn't this check if this call fails to find an area of 'num_buffers' 0-bits?
>> Or, alternatively, keep reducing num_buffers until it finds a free range. I'm
>> not sure what is best.
> 
> I will add a check on the return value. If it can't allocate the requested number of buffers
> it will fail. Userspace can decide if it wants to try allocated less buffers or not.

I'm not sure if that's the right solution. Currently create_bufs (and reqbufs for that matter)
will reduce the number of requested buffers if not all can be allocated. E.g. if you
want 10 buffers, but there is memory for only 5, then it will still allocate but it
returns 'count' with value 5.

Shouldn't that happen with this as well? The documentation is quite explicit that
you might get fewer buffers than requested.

> 
>>>         *first_index = index;
>>>   @@ -664,7 +667,6 @@ static void __vb2_queue_free(struct vb2_queue *q, unsigned int buffers)
>>>           kfree(vb);
>>>       }
>>>   -    q->num_buffers -= buffers;
>>>       if (!vb2_get_num_buffers(q)) {
>>>           q->memory = VB2_MEMORY_UNKNOWN;
>>>           INIT_LIST_HEAD(&q->queued_list);
>>> @@ -882,6 +884,14 @@ int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
>>>           q->bufs = kcalloc(q->max_num_buffers, sizeof(*q->bufs), GFP_KERNEL);
>>>       if (!q->bufs)
>>>           ret = -ENOMEM;
>>> +
>>> +    if (!q->bufs_bitmap)
>>> +        q->bufs_bitmap = bitmap_zalloc(q->max_num_buffers, GFP_KERNEL);
>>> +    if (!q->bufs_bitmap) {
>>> +        ret = -ENOMEM;
>>> +        kfree(q->bufs);
>>> +        q->bufs = NULL;
>>> +    }
>>>       q->memory = memory;
>>>       mutex_unlock(&q->mmap_lock);
>>>       if (ret)
>>> @@ -951,7 +961,6 @@ int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
>>>       }
>>>         mutex_lock(&q->mmap_lock);
>>> -    q->num_buffers = allocated_buffers;
>>>         if (ret < 0) {
>>>           /*
>>> @@ -978,6 +987,10 @@ int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
>>>       mutex_lock(&q->mmap_lock);
>>>       q->memory = VB2_MEMORY_UNKNOWN;
>>>       mutex_unlock(&q->mmap_lock);
>>> +    kfree(q->bufs);
>>> +    q->bufs = NULL;
>>> +    bitmap_free(q->bufs_bitmap);
>>> +    q->bufs_bitmap = NULL;
>>>       return ret;
>>>   }
>>>   EXPORT_SYMBOL_GPL(vb2_core_reqbufs);
>>> @@ -1014,9 +1027,19 @@ int vb2_core_create_bufs(struct vb2_queue *q, enum vb2_memory memory,
>>>           q->memory = memory;
>>>           if (!q->bufs)
>>>               q->bufs = kcalloc(q->max_num_buffers, sizeof(*q->bufs), GFP_KERNEL);
>>> -        if (!q->bufs)
>>> +        if (!q->bufs) {
>>>               ret = -ENOMEM;
>>> +            goto unlock;
>>> +        }
>>> +        if (!q->bufs_bitmap)
>>> +            q->bufs_bitmap = bitmap_zalloc(q->max_num_buffers, GFP_KERNEL);
>>> +        if (!q->bufs_bitmap) {
>>> +            ret = -ENOMEM;
>>> +            kfree(q->bufs);
>>> +            q->bufs = NULL;
>>> +        }
>> The same code is used in reqbufs and create_bufs, so perhaps creating a helper
>> function is better.
> 
> I will add vb2_core_allocated_queue_buffers_storage() and vb2_core_free_queue_buffers_storage().
> 
>>
>>>           mutex_unlock(&q->mmap_lock);
>>> +unlock:
>>>           if (ret)
>>>               return ret;
>>>           q->waiting_for_buffers = !q->is_output;
>>> @@ -1078,7 +1101,6 @@ int vb2_core_create_bufs(struct vb2_queue *q, enum vb2_memory memory,
>>>       }
>>>         mutex_lock(&q->mmap_lock);
>>> -    q->num_buffers += allocated_buffers;
>>>         if (ret < 0) {
>>>           /*
>>> @@ -2567,6 +2589,9 @@ void vb2_core_queue_release(struct vb2_queue *q)
>>>       __vb2_queue_free(q, vb2_get_num_buffers(q));
>>>       kfree(q->bufs);
>>>       q->bufs = NULL;
>>> +    bitmap_free(q->bufs_bitmap);
>>> +    q->bufs_bitmap = NULL;
>>> +
>> And perhaps also a helper function to free the memory.
>>
>>>       mutex_unlock(&q->mmap_lock);
>>>   }
>>>   EXPORT_SYMBOL_GPL(vb2_core_queue_release);
>>> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
>>> index 607f2ba7a905..e4c1fc7ae82f 100644
>>> --- a/include/media/videobuf2-core.h
>>> +++ b/include/media/videobuf2-core.h
>>> @@ -346,8 +346,8 @@ struct vb2_buffer {
>>>    *            describes the requested number of planes and sizes\[\]
>>>    *            contains the requested plane sizes. In this case
>>>    *            \*num_buffers are being allocated additionally to
>>> - *            q->num_buffers. If either \*num_planes or the requested
>>> - *            sizes are invalid callback must return %-EINVAL.
>>> + *            the buffers already in the queue. If either \*num_planes
>> already in the queue -> already allocated
>>
>>> + *            or the requested sizes are invalid callback must return %-EINVAL.
>>>    * @wait_prepare:    release any locks taken while calling vb2 functions;
>>>    *            it is called before an ioctl needs to wait for a new
>>>    *            buffer to arrive; required to avoid a deadlock in
>>> @@ -572,7 +572,7 @@ struct vb2_buf_ops {
>>>    * @memory:    current memory type used
>>>    * @dma_dir:    DMA mapping direction.
>>>    * @bufs:    videobuf2 buffer structures
>>> - * @num_buffers: number of allocated/used buffers
>>> + * @bufs_bitmap: bitmap tracking whether each bufs[] entry is used
>>>    * @max_num_buffers: upper limit of number of allocated/used buffers.
>>>    *             If set to 0 v4l2 core will change it VB2_MAX_FRAME
>>>    *             for backward compatibility.
>>> @@ -639,7 +639,7 @@ struct vb2_queue {
>>>       unsigned int            memory;
>>>       enum dma_data_direction        dma_dir;
>>>       struct vb2_buffer        **bufs;
>>> -    unsigned int            num_buffers;
>>> +    unsigned long            *bufs_bitmap;
>>>       unsigned int            max_num_buffers;
>>>         struct list_head        queued_list;
>>> @@ -1168,7 +1168,10 @@ static inline bool vb2_fileio_is_active(struct vb2_queue *q)
>>>    */
>>>   static inline unsigned int vb2_get_num_buffers(struct vb2_queue *q)
>>>   {
>>> -    return q->num_buffers;
>>> +    if (!q->bufs_bitmap)
>>> +        return 0;
>>> +
>>> +    return bitmap_weight(q->bufs_bitmap, q->max_num_buffers);
>> I'd invert the test:
>>
>>     if (q->bufs_bitmap)
>>         return bitmap_weight(q->bufs_bitmap, q->max_num_buffers);
>>     return 0;
>>
>> It's a little bit easier to read.
>>
>>>   }
>>>     /**
>>> @@ -1271,13 +1274,13 @@ static inline void vb2_clear_last_buffer_dequeued(struct vb2_queue *q)
>>>   static inline struct vb2_buffer *vb2_get_buffer(struct vb2_queue *q,
>>>                           unsigned int index)
>>>   {
>>> -    if (!q->bufs)
>>> +    if (!q->bufs_bitmap)
>> Can you ever have q->bufs set, but not q->bufs_bitmap?
>>
>> I think the original check is just fine.
>>
>> It is probably a good idea to perhaps clarify this in the @bufs documentation:
>> if it is non-NULL, then bufs_bitmap is also non-NULL.
>>
>> And ensure that where you allocate and assign these fields that bufs_bitmap
>> is always non-NULL when assigning q->bufs. Then it is enough to just test
>> q->bufs to be certain both bufs and bufs_bitmap are non-NULL.
> 
> I will add that in the documentation.
> 
>>
>>>           return NULL;
>>>         if (index >= q->max_num_buffers)
>>>           return NULL;
>>>   -    if (index < q->num_buffers)
>>> +    if (test_bit(index, q->bufs_bitmap))
>>>           return q->bufs[index];
>>>       return NULL;
>>>   }
>> Adding support for deleting buffers also causes a odd change in behavior
>> of CREATE_BUFS w.r.t. the index field of struct v4l2_create_buffers:
>> when adding new buffers, the index field is indeed the starting buffer index,
>> as per the documentation. But if count == 0, then the index field returns
>> the total number of allocated buffers, which is really something different.
>>
>> I think the documentation of VIDIOC_CREATE_BUFS should be updated to clearly
>> state that if count == 0, then 'index' is set to the total number of
>> allocated buffers.
>>
>> I really hate VIDIOC_CREATE_BUFS, and I do plan an RFC with a proposal for
>> an alternative API.
>>
>> Regards,
>>
>>     Hans
>>

Regards,

	Hans