[PATCH v4 5/5] dma-buf: Clarify that dma-buf sg lists are page aligned

[Christian König]

Am 15.10.20 um 18:03 schrieb Daniel Vetter:
> On Wed, Oct 14, 2020 at 09:16:01AM -0700, Jianxin Xiong wrote:
>> The dma-buf API have been used under the assumption that the sg lists
>> returned from dma_buf_map_attachment() are fully page aligned. Lots of
>> stuff can break otherwise all over the place. Clarify this in the
>> documentation and add a check when DMA API debug is enabled.
>>
>> Signed-off-by: Jianxin Xiong <jianxin.xiong at intel.com>

Reviewed-by: Christian König <christian.koenig at amd.com>

> lgtm, thanks for creating this and giving it a spin.
>
> I'll queue this up in drm-misc-next for 5.11, should show up in linux-next
> after the merge window is closed.

Thanks, I'm currently without landline internet and need to rely on my 
mobile.

Christian.

>
> Cheers, Daniel
>
>> ---
>>   drivers/dma-buf/dma-buf.c | 21 +++++++++++++++++++++
>>   include/linux/dma-buf.h   |  3 ++-
>>   2 files changed, 23 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>> index 844967f..7309c83 100644
>> --- a/drivers/dma-buf/dma-buf.c
>> +++ b/drivers/dma-buf/dma-buf.c
>> @@ -851,6 +851,9 @@ void dma_buf_unpin(struct dma_buf_attachment *attach)
>>    * Returns sg_table containing the scatterlist to be returned; returns ERR_PTR
>>    * on error. May return -EINTR if it is interrupted by a signal.
>>    *
>> + * On success, the DMA addresses and lengths in the returned scatterlist are
>> + * PAGE_SIZE aligned.
>> + *
>>    * A mapping must be unmapped by using dma_buf_unmap_attachment(). Note that
>>    * the underlying backing storage is pinned for as long as a mapping exists,
>>    * therefore users/importers should not hold onto a mapping for undue amounts of
>> @@ -904,6 +907,24 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
>>   		attach->dir = direction;
>>   	}
>>   
>> +#ifdef CONFIG_DMA_API_DEBUG
>> +	{
>> +		struct scatterlist *sg;
>> +		u64 addr;
>> +		int len;
>> +		int i;
>> +
>> +		for_each_sgtable_dma_sg(sg_table, sg, i) {
>> +			addr = sg_dma_address(sg);
>> +			len = sg_dma_len(sg);
>> +			if (!PAGE_ALIGNED(addr) || !PAGE_ALIGNED(len)) {
>> +				pr_debug("%s: addr %llx or len %x is not page aligned!\n",
>> +					 __func__, addr, len);
>> +			}
>> +		}
>> +	}
>> +#endif /* CONFIG_DMA_API_DEBUG */
>> +
>>   	return sg_table;
>>   }
>>   EXPORT_SYMBOL_GPL(dma_buf_map_attachment);
>> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
>> index a2ca294e..4a5fa70 100644
>> --- a/include/linux/dma-buf.h
>> +++ b/include/linux/dma-buf.h
>> @@ -145,7 +145,8 @@ struct dma_buf_ops {
>>   	 *
>>   	 * A &sg_table scatter list of or the backing storage of the DMA buffer,
>>   	 * already mapped into the device address space of the &device attached
>> -	 * with the provided &dma_buf_attachment.
>> +	 * with the provided &dma_buf_attachment. The addresses and lengths in
>> +	 * the scatter list are PAGE_SIZE aligned.
>>   	 *
>>   	 * On failure, returns a negative error value wrapped into a pointer.
>>   	 * May also return -EINTR when a signal was received while being
>> -- 
>> 1.8.3.1
>>
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