[PATCH 3/5] dmaengine: Support for querying maximum trasnfer length (of an SG element)

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On 2017-09-21 20:14, Vinod Koul wrote:

On Tue, Sep 12, 2017 at 01:44:22PM +0300, Peter Ujfalusi wrote:

quoted

Certain DMA engines have limitation on the maximum size of a transfer they
can support. This size limitation is per SG element or for period length in
cyclic transfers.
In TI's eDMA and sDMA this limitation is not really a length limit, but it
is the number of bursts that we can support in one transfer.

With this callback the DMA drivers can provide hints to clients on how they
should set up their buffers (sglist, cyclic buffer). Without this the
clients must have open coded workarounds in place for each and every DMA
engine they might be interfacing with to have correct length for the
transfers.

Signed-off-by: Peter Ujfalusi <redacted>
---
 include/linux/dmaengine.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index 8319101170fc..739824b94c1b 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h

@@ -705,6 +705,9 @@ struct dma_filter {
  * @device_prep_dma_imm_data: DMA's 8 byte immediate data to the dst address
  * @device_config: Pushes a new configuration to a channel, return 0 or an error
  *	code
+ * @device_get_max_len: Get the maximum supported length in bytes of a slave
+ *	transfer based on the set dma_slave_config. The length limitation
+ *	applies to each SG element's length.
  * @device_pause: Pauses any transfer happening on a channel. Returns
  *	0 or an error code
  * @device_resume: Resumes any transfer on a channel previously

@@ -792,6 +795,8 @@ struct dma_device {
 
 	int (*device_config)(struct dma_chan *chan,
 			     struct dma_slave_config *config);
+	u32 (*device_get_max_len)(struct dma_chan *chan,
+				  enum dma_transfer_direction dir);
 	int (*device_pause)(struct dma_chan *chan);
 	int (*device_resume)(struct dma_chan *chan);
 	int (*device_terminate_all)(struct dma_chan *chan);

@@ -812,6 +817,15 @@ static inline int dmaengine_slave_config(struct dma_chan *chan,
 	return -ENOSYS;
 }
 
+static inline u32 dmaengine_slave_get_max_len(struct dma_chan *chan,
+					      enum dma_transfer_direction dir)
+{
+	if (chan->device->device_get_max_len)
+		return chan->device->device_get_max_len(chan, dir);

not another callback :)

on a serious note, why shouldn't this be one more capability in
dma_slave_caps. looking at next patch it seems static

It is not really static, the size in bytes depends on the dev_width and
the maxburst:
dev_width * burst * (SZ_64K - 1);

The number of (dev_width * burst) is static, yes. Other DMA engines
might have similar interpretation, but returning the maximum length in
bytes sounded more generic for other engines to be able to adopt.

Initially I had maxburst_cnt in struct dma_device for maximum burst
count within one SG, but it felt clumsy and not too intuitive either.

- P?ter