On 3/8/2018 6:24 AM, Robin Murphy wrote:
On 08/03/18 06:46, Karthik Ramasubramanian wrote:
On 3/6/2018 2:56 PM, Stephen Boyd wrote:
Quoting Karthik Ramasubramanian (2018-03-02 16:58:23)
+ return iova;
+}
+EXPORT_SYMBOL(geni_se_tx_dma_prep);
+
+/**
+ * geni_se_rx_dma_prep() - Prepare the Serial Engine for RX DMA
transfer
+ * @se: Pointer to the concerned Serial
Engine.
+ * @buf: Pointer to the RX buffer.
+ * @len: Length of the RX buffer.
+ *
+ * This function is used to prepare the buffers for DMA RX.
+ *
+ * Return: Mapped DMA Address of the buffer on success, NULL on
failure.
+ */
+dma_addr_t geni_se_rx_dma_prep(struct geni_se *se, void *buf,
size_t len)
+{
+ dma_addr_t iova;
+ struct geni_wrapper *wrapper = se->wrapper;
+ u32 val;
+
+ iova = dma_map_single(wrapper->dev, buf, len,
DMA_FROM_DEVICE);
+ if (dma_mapping_error(wrapper->dev, iova))
+ return (dma_addr_t)NULL;
Can't return a dma_mapping_error address to the caller and have them
figure it out?
Earlier we used to return the DMA_ERROR_CODE which has been removed
recently in arm64 architecture. If we return the dma_mapping_error,
then
the caller also needs the device which encountered the mapping error.
The serial interface drivers can use their parent currently to resolve
the mapping error. Once the wrapper starts mapping using IOMMU context
bank, then the serial interface drivers do not know which device to use
to know if there is an error.
Having said that, the dma_ops suggestion might help with handling this
situation. I will look into it further.
Ok, thanks.
+{
+ struct geni_wrapper *wrapper = se->wrapper;
+
+ if (iova)
+ dma_unmap_single(wrapper->dev, iova, len,
DMA_FROM_DEVICE);
+}
+EXPORT_SYMBOL(geni_se_rx_dma_unprep);
Instead of having the functions exported, could we set the dma_ops on
all child devices of the wrapper that this driver populates and then
implement the DMA ops for those devices here? I assume that there's
never another DMA master between the wrapper and the serial engine,
so I
think it would work.
This suggestion looks like it will work.
It would be a good idea to check with some other people on the dma_ops
suggestion. Maybe add the DMA mapping subsystem folks to help out here
I have added the DMA mapping subsystem folks to help out here.
To present an overview, we have a wrapper controller which is composed
of several serial engines. The serial engines are programmed with
UART, I2C or SPI protocol and support DMA transfer. When the serial
engines perform DMA transfer, the wrapper controller device is used to
perform the mapping. The reason wrapper device is used is because of
IOMMU and there is only one IOMMU context bank to perform the
translation for the entire wrapper controller. So the wrapper
controller exports map and unmap functions to the individual protocol
drivers.
There is a suggestion to make the parent wrapper controller implement
the dma_map_ops, instead of exported map/unmap functions and populate
those dma_map_ops on all the children serial engines. Can you please
provide your inputs regarding this suggestion?
Implementing dma_map_ops inside a driver for real hardware is almost
always the wrong thing to do.
Based on what I could infer about the hardware from looking through the
whole series in the linux-arm-msm archive, this is probably more like a
multi-channel DMA controller where each "channel" has a configurable
serial interface on the other end, as opposed to an actual bus where the
serial engines are individually distinct AHB masters routed through the
wrapper. If that's true, then using the QUP platform device for DMA API
calls is the appropriate thing to do. Personally I'd be inclined not to
abstract the dma_{map,unmap} calls at all, and just have the protocol
drivers make them directly using dev->parent/wrapper->dev/whatever, but
if you do want to abstract those then just give the abstraction a saner
interface, i.e. pass the DMA handle by reference and return a regular
int for error/success status.
Robin.
Thank you Robin & Christoph for your inputs. The wrapper driver used to
provide the recommended abstraction until v2 of this patch series. In v3
it was tweaked to address a comment. If there are no objections, I will
revive it back.
Regards,
Karthik.
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