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SubjectRe: [PATCH v3 09/10] dmaengine: dw: Introduce max burst length hw config
On Thu, May 28, 2020 at 05:52:24PM +0300, Andy Shevchenko wrote:
> On Wed, May 27, 2020 at 01:50:20AM +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 DMA transaction 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.
> >
> > Note the maximum burst length parameter can be detected either in runtime
> > from the DWC parameter registers or from the dedicated DT property.
> > Depending on the IP core configuration the maximum value can vary from
> > channel to channel so by overriding the channel slave max_burst capability
> > we make sure a DMA consumer will get the channel-specific max burst
> > length.
>
> ...
>
> > static void dwc_caps(struct dma_chan *chan, struct dma_slave_caps *caps)
> > {
> > + struct dw_dma_chan *dwc = to_dw_dma_chan(chan);
> >
>

> Perhaps,
>
> /* DesignWare DMA supports burst value from 0 */
> caps->min_burst = 0;

Regarding min_burst being zero. I don't fully understand what it means.
It means no burst or burst with minimum length or what?
In fact DW DMA burst length starts from 1. Remember the burst-length run-time
parameter we were arguing about? Anyway the driver makes sure that both
0 and 1 requested burst length are setup as burst length of 1 in the
CTLx.SRC_MSIZE, CTLx.DST_MSIZE fields.

I agree with the rest of your comments below.

-Sergey

>
> > + caps->max_burst = dwc->max_burst;
> > }
>
> ...
>
> > + *maxburst = clamp(*maxburst, 0U, dwc->max_burst);
>
> Shouldn't we do the same for iDMA 32-bit? Thus, perhaps do it in the core.c?
>
> > *maxburst = *maxburst > 1 ? fls(*maxburst) - 2 : 0;
>
> > + if (!of_property_read_u32_array(np, "snps,max-burst-len", mb,
> > + nr_channels)) {
> > + for (tmp = 0; tmp < nr_channels; tmp++)
> > + pdata->max_burst[tmp] = mb[tmp];
>
> I think we may read directly to the array. This ugly loops were introduced due
> to type mismatch. (See below)
>
> > + } else {
> > + for (tmp = 0; tmp < nr_channels; tmp++)
> > + pdata->max_burst[tmp] = DW_DMA_MAX_BURST;
> > + }
>
> And this will be effectively memset32().
>
> > unsigned char nr_masters;
> > unsigned char data_width[DW_DMA_MAX_NR_MASTERS];
> > unsigned char multi_block[DW_DMA_MAX_NR_CHANNELS];
> > + unsigned int max_burst[DW_DMA_MAX_NR_CHANNELS];
>
> I think we have to stop with this kind of types and use directly what is in the
> properties, i.e.
>
> u32 max_burst[...];
>
> --
> With Best Regards,
> Andy Shevchenko
>
>

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