linux-kernel - Re: [PATCH v3] staging: android: ion: Add implementation of dma_buf_vmap and dma_buf

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Date:   Thu, 1 Feb 2018 08:10:07 +0200
From:   Alexey Skidanov <alexey.skidanov@...el.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     devel@...verdev.osuosl.org, labbott@...hat.com, tkjos@...roid.com,
        rve@...roid.com, linux-kernel@...r.kernel.org, maco@...roid.com,
        sumit.semwal@...aro.org
Subject: Re: [PATCH v3] staging: android: ion: Add implementation of
 dma_buf_vmap and dma_buf_vunmap


On 01/31/2018 03:00 PM, Greg KH wrote:
> On Wed, Jan 31, 2018 at 02:03:42PM +0200, Alexey Skidanov wrote:
>> Any driver may access shared buffers, created by ion, using dma_buf_vmap and
>> dma_buf_vunmap dma-buf API that maps/unmaps previosuly allocated buffers into
>> the kernel virtual address space. The implementation of these API is missing in
>> the current ion implementation.
>>
>> Signed-off-by: Alexey Skidanov <alexey.skidanov@...el.com>
>> ---
> 
> No review from any other Intel developers? :(
Will add.
> 
> Anyway, what in-tree driver needs access to these functions?
I'm not sure that there are the in-tree drivers using these functions 
and ion as buffer exporter because they are not implemented in ion :) 
But there are some in-tree drivers using these APIs (gpu drivers) with 
other buffer exporters.
> 
> And are you sure that you don't need to do any "real" logic in the
> vmap/vunmap calls?  That feels like there would be some reference
> counting problems here.
dma_buf_start_cpu_access is called before the call to dma_buf_vmap. It 
actually increments the reference count. dma_buf_end_cpu_access is 
called after the dma_buf_vunmap. It actually decrements the reference 
count.>>
>> diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c
>> index f480885..4f1dc7f 100644
>> --- a/drivers/staging/android/ion/ion.c
>> +++ b/drivers/staging/android/ion/ion.c
>> @@ -327,6 +327,17 @@ static void ion_dma_buf_kunmap(struct dma_buf *dmabuf, unsigned long offset,
>>   {
>>   }
>>   
>> +static void *ion_dma_buf_vmap(struct dma_buf *dmabuf)
>> +{
>> +	struct ion_buffer *buffer = dmabuf->priv;
>> +
>> +	return buffer->vaddr;
> 
> Just call ion_dma_buf_kmap(dmabuf, 0)?
Sure.
> 
> Again, please get this reviewed by someone else in Intel first.  Don't
> ignore the resources you have, to do so would be foolish :)
Sure. Will do.
> 
> thanks,
> 
> greg k-h
> 

Thanks,
Alexey