Re: [PATCH v2 5/6] dmaengine: dw: Introduce max burst length hw config

[Serge Semin]

On Fri, May 08, 2020 at 02:41:53PM +0300, Andy Shevchenko wrote:
> On Fri, May 08, 2020 at 01:53:03PM +0300, Serge Semin wrote:
> > IP core of the DW DMA controller may be synthesized with different
> > max burst length of the transfers per each channel. According to Synopsis
> > having the fixed maximum burst transactions length may provide some
> > performance gain. At the same time setting up the source and destination
> > multi size exceeding the max burst length limitation may cause a serious
> > problems. In our case the system just hangs up. In order to fix this
> > lets introduce the max burst length platform config of the DW DMA
> > controller device and don't let the DMA channels configuration code
> > exceed the burst length hardware limitation. Depending on the IP core
> > configuration the maximum value can vary from channel to channel.
> > It can be detected either in runtime from the DWC parameter registers
> > or from the dedicated dts property.
> 
> I'm wondering what can be the scenario when your peripheral will ask something
> which is not supported by DMA controller?

I may misunderstood your statement, because seeing your activity around my
patchsets including the SPI patchset and sometimes very helpful comments,
this question answer seems too obvious to see you asking it.

No need to go far for an example. See the DW APB SSI driver. Its DMA module
specifies the burst length to be 16, while not all of ours channels supports it.
Yes, originally it has been developed for the Intel Midfield SPI, but since I
converted the driver into a generic code we can't use a fixed value. For instance
in our hardware only two DMA channels of total 16 are capable of bursting up to
16 bytes (data items) at a time, the rest of them are limited with up to 4 bytes
burst length. While there are two SPI interfaces, each of which need to have two
DMA channels for communications. So I need four channels in total to allocate to
provide the DMA capability for all interfaces. In order to set the SPI controller
up with valid optimized parameters the max-burst-length is required. Otherwise we
can end up with buffers overrun/underrun.

> 
> Peripheral needs to supply a lot of configuration parameters specific to the
> DMA controller in use (that's why we have struct dw_dma_slave).
> So, seems to me the feasible approach is supply correct data in the first place.

How to supply a valid data if clients don't know the DMA controller limitations
in general?

> 
> If you have specific channels to acquire then you probably need to provide a
> custom xlate / filter functions. Because above seems a bit hackish workaround
> of dynamic channel allocation mechanism.

No, I don't have a specific channel to acquire and in general you may use any
returned from the DMA subsystem (though some platforms may need a dedicated
channels to use, in this case xlate / filter is required). In our SoC any DW DMAC
channel can be used for any DMA-capable peripherals like SPI, I2C, UART. But the
their DMA settings must properly and optimally configured. It can be only done
if you know the DMA controller parameters like max burst length, max block-size,
etc.

So no. The change proposed by this patch isn't workaround, but a useful feature,
moreover expected to be supported by the generic DMA subsystem.

> 
> But let's see what we can do better. Since maximum is defined on the slave side
> device, it probably needs to define minimum as well, otherwise it's possible
> that some hardware can't cope underrun bursts.

There is no need to define minimum if such limit doesn't exists except a
natural 1. Moreover it doesn't exist for all DMA controllers seeing noone has
added such capability into the generic DMA subsystem so far.

-Sergey

> 
> Vinod, what do you think?
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>