On Fri, 2023-10-06 at 16:00 +0530, Vinod Koul wrote: > EXTERNAL EMAIL: Do not click links or open attachments unless you > know the content is safe > > On 05-10-23, 18:35, Kelvin.Cao@xxxxxxxxxxxxx wrote: > > > > > > +static struct dma_async_tx_descriptor * > > > > > +switchtec_dma_prep_wimm_data(struct dma_chan *c, dma_addr_t > > > > > dma_dst, u64 data, > > > > > + unsigned long flags) > > > > > > > > can you please explain what this wimm data refers to... > > > > > > > > I think adding imm callback was a mistake, we need a better > > > > justification for another user for this, who programs this, > > > > what > > > > gets > > > > programmed here > > > > > > Sure. I think it's an alternative method to prep_mem and would be > > > more > > > convenient to use when the write is 8-byte and the data to be > > > moved > > > is > > > not in a DMA mapped memory location. For example, we write to a > > > doorbell register with the value from a local variable which is > > > not > > > associated with a DMA address to notify the receiver to consume > > > the > > > data, after confirming that the previously initiated DMA > > > transactions > > > of the data have completed. I agree that the use scenario would > > > be > > > very > > > limited. > > Can you please explain more about this 'value' where is it derived > from? > Who programs it and how... Sure. Think of a producer/consumer use case where the producer is a host DMA client driver and the consumer is a PCIe EP. On the EP, there is a memory-mapped data buffer for data receiving and a memory-mapped doorbell buffer for triggering data consuming. Each time for a bulk data transfer, the DMA client driver first DMA the data of size X to the memory-mapped data buffer, then it DMA the value X to the doorbell buffer to trigger data consumption on device. On receiving a doorbell writing, the device starts to consume the data of size X from the data buffer. For the first DMA operation, the DMA client would use dma_prep_memory() like what most DMA clients do. However, for the second transfer, value X is held in a local variable like below. u64 size_to_transfer; In this case, the client would use dma_prep_wimm_data() to DMA value X to the doorbell buffer, like below. dma_prep_wimm_data(chan, dst_for_db_buffer, size_to_transfer, flag); Hope this example explains the thing. People would argue that the client could use the same dma_prep_memory() for the doorbell ringing. I would agree, this API just adds an alternative way to do so when the data is as little as 64 bit and it also saves a call site to dma_alloc_coherent() to allocate a source DMA buffer just for holding the 8-byte value, which is required by dma_prep_memcpy(). > > > > > + /* set sq/cq */ > > > > > + writel(lower_32_bits(swdma_chan->dma_addr_sq), > > > > > &chan_fw- > > > > > > sq_base_lo); > > > > > + writel(upper_32_bits(swdma_chan->dma_addr_sq), > > > > > &chan_fw- > > > > > > sq_base_hi); > > > > > + writel(lower_32_bits(swdma_chan->dma_addr_cq), > > > > > &chan_fw- > > > > > > cq_base_lo); > > > > > + writel(upper_32_bits(swdma_chan->dma_addr_cq), > > > > > &chan_fw- > > > > > > cq_base_hi); > > > > > + > > > > > + writew(SWITCHTEC_DMA_SQ_SIZE, &swdma_chan- > > > > > >mmio_chan_fw- > > > > > > sq_size); > > > > > + writew(SWITCHTEC_DMA_CQ_SIZE, &swdma_chan- > > > > > >mmio_chan_fw- > > > > > > cq_size); > > > > > > > > what is write happening in the descriptor alloc callback, that > > > > does > > > > not > > > > sound correct to me > > > > > > All the queue descriptors of a channel are pre-allocated, so I > > > think > > > it's proper to convey the queue address/size to hardware at this > > > point. > > > After this initialization, we only need to assign cookie in > > > submit > > > and > > > update queue head to hardware in issue_pending. > > Sorry that is not right, you can prepare multiple descriptors and > then > submit. Only at submit is the cookie assigned which is in order, so > this > should be moved to when we start the txn and not in this call > The hardware assumes the SQ/CQ is a contiguous circular buffer of fix sized element. And the above code passes the address and size of SQ/CQ to the hardware. At this point hardware will do nothing but take note of the SQ/CQ location/size. When do dma_issue_pending(), the actual SQ head write will occur to update hardware with the current SQ head, as below: writew(swdma_chan->head, &swdma_chan->mmio_chan_hw->sq_tail); Thank you Vinod for your time! Kelvin
