linux-kernel - Re: [PATCH v4 02/10] dmaengine: Actions: Add support for S700 DMA engine

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Message-ID: <20200629095446.GH2599@vkoul-mobl>
Date:   Mon, 29 Jun 2020 15:24:46 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     André Przywara <andre.przywara@....com>
Cc:     Amit Singh Tomar <amittomer25@...il.com>, afaerber@...e.de,
        manivannan.sadhasivam@...aro.org, dan.j.williams@...el.com,
        cristian.ciocaltea@...il.com, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-actions@...ts.infradead.org
Subject: Re: [PATCH v4 02/10] dmaengine: Actions: Add support for S700 DMA
 engine

On 24-06-20, 10:35, André Przywara wrote:
> On 24/06/2020 07:15, Vinod Koul wrote:
> > On 09-06-20, 15:47, Amit Singh Tomar wrote:
> > 
> >> @@ -372,6 +383,7 @@ static inline int owl_dma_cfg_lli(struct owl_dma_vchan *vchan,
> >>  				  struct dma_slave_config *sconfig,
> >>  				  bool is_cyclic)
> >>  {
> >> +	struct owl_dma *od = to_owl_dma(vchan->vc.chan.device);
> >>  	u32 mode, ctrlb;
> >>  
> >>  	mode = OWL_DMA_MODE_PW(0);
> >> @@ -427,14 +439,26 @@ static inline int owl_dma_cfg_lli(struct owl_dma_vchan *vchan,
> >>  	lli->hw[OWL_DMADESC_DADDR] = dst;
> >>  	lli->hw[OWL_DMADESC_SRC_STRIDE] = 0;
> >>  	lli->hw[OWL_DMADESC_DST_STRIDE] = 0;
> >> -	/*
> >> -	 * Word starts from offset 0xC is shared between frame length
> >> -	 * (max frame length is 1MB) and frame count, where first 20
> >> -	 * bits are for frame length and rest of 12 bits are for frame
> >> -	 * count.
> >> -	 */
> >> -	lli->hw[OWL_DMADESC_FLEN] = len | FCNT_VAL << 20;
> >> -	lli->hw[OWL_DMADESC_CTRLB] = ctrlb;
> >> +
> >> +	if (od->devid == S700_DMA) {
> >> +		/* Max frame length is 1MB */
> >> +		lli->hw[OWL_DMADESC_FLEN] = len;
> >> +		/*
> >> +		 * On S700, word starts from offset 0x1C is shared between
> >> +		 * frame count and ctrlb, where first 12 bits are for frame
> >> +		 * count and rest of 20 bits are for ctrlb.
> >> +		 */
> >> +		lli->hw[OWL_DMADESC_CTRLB] = FCNT_VAL | ctrlb;
> >> +	} else {
> >> +		/*
> >> +		 * On S900, word starts from offset 0xC is shared between
> >> +		 * frame length (max frame length is 1MB) and frame count,
> >> +		 * where first 20 bits are for frame length and rest of
> >> +		 * 12 bits are for frame count.
> >> +		 */
> >> +		lli->hw[OWL_DMADESC_FLEN] = len | FCNT_VAL << 20;
> >> +		lli->hw[OWL_DMADESC_CTRLB] = ctrlb;
> > 
> > Unfortunately this wont scale, we will keep adding new conditions for
> > newer SoC's! So rather than this why not encode max frame length in
> > driver_data rather than S900_DMA/S700_DMA.. In future one can add values
> > for newer SoC and not code above logic again.
> 
> What newer SoCs? I don't think we should try to guess the future here.

In a patch for adding new SoC, quite ironical I would say!

> We can always introduce further abstractions later, once we actually
> *know* what we are looking at.

Rather if we know we are adding abstractions, why not add in a way that
makes it scale better rather than rework again

> Besides, I don't understand what you are after. The max frame length is
> 1MB in both cases, it's just a matter of where to put FCNT_VAL, either
> in FLEN or in CTRLB. And having an extra flag for that in driver data
> sounds a bit over the top at the moment.

Maybe, maybe not. I would rather make it support N SoC when adding
support for second one rather than keep adding everytime a new SoC is
added...

-- 
~Vinod