On Wed, May 27, 2020 at 01:50:15AM +0300, Serge Semin wrote:
> Some devices may lack the support of the hardware accelerated SG list
> entries automatic walking through and execution. In this case a burden of
> the SG list traversal and DMA engine re-initialization lies on the
> DMA engine driver (normally implemented by using a DMA transfer completion
> IRQ to recharge the DMA device with a next SG list entry). But such
> solution may not be suitable for some DMA consumers. In particular SPI
> devices need both Tx and Rx DMA channels work synchronously in order
> to avoid the Rx FIFO overflow. In case if Rx DMA channel is paused for
> some time while the Tx DMA channel works implicitly pulling data into the
> Rx FIFO, the later will be eventually overflown, which will cause the data
> loss. So if SG list entries aren't automatically fetched by the DMA
> engine, but are one-by-one manually selected for execution in the
> ISRs/deferred work/etc., such problem will eventually happen due to the
> non-deterministic latencies of the service execution.
>
> In order to let the DMA consumer know about the DMA device capabilities
> regarding the hardware accelerated SG list traversal we introduce the
> max_sg_list capability. It is supposed to be initialized by the DMA engine
> driver with 0 if there is no limitation for the number of SG entries
> atomically executed and with non-zero value if there is such constraints,
> so the upper limit is determined by the number set to the property.
Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
But see below.
> Suggested-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Signed-off-by: Serge Semin <Sergey.Semin@xxxxxxxxxxxxxxxxxxxx>
> Cc: Alexey Malahov <Alexey.Malahov@xxxxxxxxxxxxxxxxxxxx>
> Cc: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Rob Herring <robh+dt@xxxxxxxxxx>
> Cc: linux-mips@xxxxxxxxxxxxxxx
> Cc: devicetree@xxxxxxxxxxxxxxx
>
> ---
>
> Changelog v3:
> - This is a new patch created as a result of the discussion with Vinud and
> Andy in the framework of DW DMA burst and LLP capabilities.
> ---
> drivers/dma/dmaengine.c | 1 +
> include/linux/dmaengine.h | 8 ++++++++
> 2 files changed, 9 insertions(+)
>
> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> index b332ffe52780..ad56ad58932c 100644
> --- a/drivers/dma/dmaengine.c
> +++ b/drivers/dma/dmaengine.c
> @@ -592,6 +592,7 @@ int dma_get_slave_caps(struct dma_chan *chan, struct dma_slave_caps *caps)
> caps->directions = device->directions;
> caps->min_burst = device->min_burst;
> caps->max_burst = device->max_burst;
> + caps->max_sg_nents = device->max_sg_nents;
> caps->residue_granularity = device->residue_granularity;
> caps->descriptor_reuse = device->descriptor_reuse;
> caps->cmd_pause = !!device->device_pause;
> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> index 0c7403b27133..6801200c76b6 100644
> --- a/include/linux/dmaengine.h
> +++ b/include/linux/dmaengine.h
> @@ -467,6 +467,9 @@ enum dma_residue_granularity {
> * should be checked by controller as well
> * @min_burst: min burst capability per-transfer
> * @max_burst: max burst capability per-transfer
> + * @max_sg_nents: max number of SG list entries executed in a single atomic
> + * DMA tansaction with no intermediate IRQ for reinitialization. Zero
> + * value means unlimited number if entries.
if -> of ?
> * @cmd_pause: true, if pause is supported (i.e. for reading residue or
> * for resume later)
> * @cmd_resume: true, if resume is supported
> @@ -481,6 +484,7 @@ struct dma_slave_caps {
> u32 directions;
> u32 min_burst;
> u32 max_burst;
> + u32 max_sg_nents;
> bool cmd_pause;
> bool cmd_resume;
> bool cmd_terminate;
> @@ -773,6 +777,9 @@ struct dma_filter {
> * should be checked by controller as well
> * @min_burst: min burst capability per-transfer
> * @max_burst: max burst capability per-transfer
> + * @max_sg_nents: max number of SG list entries executed in a single atomic
> + * DMA tansaction with no intermediate IRQ for reinitialization. Zero
> + * value means unlimited number if entries.
Ditto.
> * @residue_granularity: granularity of the transfer residue reported
> * by tx_status
> * @device_alloc_chan_resources: allocate resources and return the
> @@ -844,6 +851,7 @@ struct dma_device {
> u32 directions;
> u32 min_burst;
> u32 max_burst;
> + u32 max_sg_nents;
> bool descriptor_reuse;
> enum dma_residue_granularity residue_granularity;
>
> --
> 2.26.2
>
--
With Best Regards,
Andy Shevchenko
