HI Michael,
On Thu, Dec 8, 2016 at 8:46 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> On Wed, Dec 07, 2016 at 10:59:12PM -0800, Wendy Liang wrote:
>> If sg is already dma mapped, do not call dma_map_page() in
>> vring_map_one_sg().
>>
>> In case of rpmsg, rpmsg uses dma_alloc_coherent() to allocate
>> memory to share with the remote. There is no pages setup
>> in dma_alloc_coherent().
>>
>> In this case, we cannot convert the virtual address back to the
>> physical address. In this case, we can setup the sg_dma_addr to
>> store the DMA address, and also mark the sg is already mapped.
>>
>> In the vring, we can detect if the address is already mapped
>> by checking the sg_dma_addr. If yes, do not call dma_map_page().
>>
>> Signed-off-by: Wendy Liang <jliang@xxxxxxxxxx>
>> ---
>> drivers/virtio/virtio_ring.c | 6 ++++++
>> 1 file changed, 6 insertions(+)
>>
>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>> index 489bfc6..9793e1f 100644
>> --- a/drivers/virtio/virtio_ring.c
>> +++ b/drivers/virtio/virtio_ring.c
>> @@ -180,6 +180,12 @@ static dma_addr_t vring_map_one_sg(const struct vring_virtqueue *vq,
>> if (!vring_use_dma_api(vq->vq.vdev))
>> return (dma_addr_t)sg_phys(sg);
>>
>> + /* If the sg is already mapped, return the DMA address */
>
> How come we even reach this code for rpmsg?
> Does vring_use_dma_api return true for rpmsg?
I used vdev feature bit VIRTIO_F_IOMMU_PLATFORM.
>
>> + if (sg_dma_address(sg)) {
>> + sg->length = sg_dma_len(sg);
>> + return sg_dma_address(sg);
>> + }
>> +
>
> Is there a rule that says 0 is not a valid address?
OK, i see. Is it better to check the sg_dma_len() instead?
However, I just noticed yesterday, there is another patch to
virtio_rpmsg_bus posted to linux-remoteproc mailing list trying to
solve the same issue:
[PATCH v1 2/6] rpmsg: virtio_rpmsg_bus: fix sg_set_buf() when addr is
not a valid kernel address. he didn't use sg_dma_address. And doesn't
require to update the virtio_ring implementation.
>
>> /*
>> * We can't use dma_map_sg, because we don't use scatterlists in
>> * the way it expects (we don't guarantee that the scatterlist
>> --
>> 1.9.1
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