linux-kernel - Re: [PATCH v3 09/10] dmaengine: dw: Introduce max burst length hw config

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Message-ID: <20200528145224.GT1634618@smile.fi.intel.com>
Date:   Thu, 28 May 2020 17:52:24 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Vinod Koul <vkoul@...nel.org>, Viresh Kumar <vireshk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Serge Semin <fancer.lancer@...il.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Arnd Bergmann <arnd@...db.de>,
        Rob Herring <robh+dt@...nel.org>, linux-mips@...r.kernel.org,
        devicetree@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 09/10] dmaengine: dw: Introduce max burst length hw
 config

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;

> +	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