Re: [PATCH 2/3] udmabuf: Sync buffer mappings for attached devices

[Andrew Davis <afd@ti.com>]

On 1/24/24 5:05 PM, Kasireddy, Vivek wrote:
> Hi Andrew,
> 
>> Currently this driver creates a SGT table using the CPU as the
>> target device, then performs the dma_sync operations against
>> that SGT. This is backwards to how DMA-BUFs are supposed to behave.
>> This may have worked for the case where these buffers were given
>> only back to the same CPU that produced them as in the QEMU case.
>> And only then because the original author had the dma_sync
>> operations also backwards, syncing for the "device" on begin_cpu.
>> This was noticed and "fixed" in this patch[0].
>>
>> That then meant we were sync'ing from the CPU to the CPU using
>> a pseudo-device "miscdevice". Which then caused another issue
>> due to the miscdevice not having a proper DMA mask (and why should
>> it, the CPU is not a DMA device). The fix for that was an even
>> more egregious hack[1] that declares the CPU is coherent with
>> itself and can access its own memory space..
>>
>> Unwind all this and perform the correct action by doing the dma_sync
>> operations for each device currently attached to the backing buffer.
> Makes sense.
> 
>>
>> [0] commit 1ffe09590121 ("udmabuf: fix dma-buf cpu access")
>> [1] commit 9e9fa6a9198b ("udmabuf: Set the DMA mask for the udmabuf
>> device (v2)")
>>
>> Signed-off-by: Andrew Davis <afd@ti.com>
>> ---
>>   drivers/dma-buf/udmabuf.c | 41 +++++++++++++++------------------------
>>   1 file changed, 16 insertions(+), 25 deletions(-)
>>
>> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
>> index 3a23f0a7d112a..ab6764322523c 100644
>> --- a/drivers/dma-buf/udmabuf.c
>> +++ b/drivers/dma-buf/udmabuf.c
>> @@ -26,8 +26,6 @@ MODULE_PARM_DESC(size_limit_mb, "Max size of a
>> dmabuf, in megabytes. Default is
>>   struct udmabuf {
>>   	pgoff_t pagecount;
>>   	struct page **pages;
>> -	struct sg_table *sg;
>> -	struct miscdevice *device;
>>   	struct list_head attachments;
>>   	struct mutex lock;
>>   };
>> @@ -169,12 +167,8 @@ static void unmap_udmabuf(struct
>> dma_buf_attachment *at,
>>   static void release_udmabuf(struct dma_buf *buf)
>>   {
>>   	struct udmabuf *ubuf = buf->priv;
>> -	struct device *dev = ubuf->device->this_device;
>>   	pgoff_t pg;
>>
>> -	if (ubuf->sg)
>> -		put_sg_table(dev, ubuf->sg, DMA_BIDIRECTIONAL);
> What happens if the last importer maps the dmabuf but erroneously
> closes it immediately? Would unmap somehow get called in this case?
> 

Good question, had to scan the framework code a bit here. I thought
closing a DMABUF handle would automatically unwind any current
attachments/mappings, but it seems nothing in the framework does that.

Looks like that is up to the importing drivers[0]:

> Once a driver is done with a shared buffer it needs to call
> dma_buf_detach() (after cleaning up any mappings) and then
> release the reference acquired with dma_buf_get() by
> calling dma_buf_put().

So closing a DMABUF after mapping without first unmapping it would
be a bug in the importer, it is not the exporters problem to check
for (although some more warnings in the framework checking for that
might not be a bad idea..).

Andrew

[0] https://www.kernel.org/doc/html/v6.7/driver-api/dma-buf.html

> Thanks,
> Vivek
> 
>> -
>>   	for (pg = 0; pg < ubuf->pagecount; pg++)
>>   		put_page(ubuf->pages[pg]);
>>   	kfree(ubuf->pages);
>> @@ -185,33 +179,31 @@ static int begin_cpu_udmabuf(struct dma_buf
>> *buf,
>>   			     enum dma_data_direction direction)
>>   {
>>   	struct udmabuf *ubuf = buf->priv;
>> -	struct device *dev = ubuf->device->this_device;
>> -	int ret = 0;
>> -
>> -	if (!ubuf->sg) {
>> -		ubuf->sg = get_sg_table(dev, buf, direction);
>> -		if (IS_ERR(ubuf->sg)) {
>> -			ret = PTR_ERR(ubuf->sg);
>> -			ubuf->sg = NULL;
>> -		}
>> -	} else {
>> -		dma_sync_sg_for_cpu(dev, ubuf->sg->sgl, ubuf->sg->nents,
>> -				    direction);
>> -	}
>> +	struct udmabuf_attachment *a;
>>
>> -	return ret;
>> +	mutex_lock(&ubuf->lock);
>> +
>> +	list_for_each_entry(a, &ubuf->attachments, list)
>> +		dma_sync_sgtable_for_cpu(a->dev, a->table, direction);
>> +
>> +	mutex_unlock(&ubuf->lock);
>> +
>> +	return 0;
>>   }
>>
>>   static int end_cpu_udmabuf(struct dma_buf *buf,
>>   			   enum dma_data_direction direction)
>>   {
>>   	struct udmabuf *ubuf = buf->priv;
>> -	struct device *dev = ubuf->device->this_device;
>> +	struct udmabuf_attachment *a;
>>
>> -	if (!ubuf->sg)
>> -		return -EINVAL;
>> +	mutex_lock(&ubuf->lock);
>> +
>> +	list_for_each_entry(a, &ubuf->attachments, list)
>> +		dma_sync_sgtable_for_device(a->dev, a->table, direction);
>> +
>> +	mutex_unlock(&ubuf->lock);
>>
>> -	dma_sync_sg_for_device(dev, ubuf->sg->sgl, ubuf->sg->nents,
>> direction);
>>   	return 0;
>>   }
>>
>> @@ -307,7 +299,6 @@ static long udmabuf_create(struct miscdevice
>> *device,
>>   	exp_info.priv = ubuf;
>>   	exp_info.flags = O_RDWR;
>>
>> -	ubuf->device = device;
>>   	buf = dma_buf_export(&exp_info);
>>   	if (IS_ERR(buf)) {
>>   		ret = PTR_ERR(buf);
>> --
>> 2.39.2
>