linux-kernel - Re: [PATCH v1 4/5] dmaengine: Add driver for NVIDIA Tegra AHB DMA controller

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Message-ID: <b601b829-d87f-99e0-dcf9-ad3f9a7195df@gmail.com>
Date:   Thu, 28 Sep 2017 17:06:03 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Vinod Koul <vinod.koul@...el.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        Prashant Gaikwad <pgaikwad@...dia.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Rob Herring <robh+dt@...nel.org>, linux-tegra@...r.kernel.org,
        devicetree@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 4/5] dmaengine: Add driver for NVIDIA Tegra AHB DMA
 controller

On 28.09.2017 12:29, Vinod Koul wrote:
>> +	default:
>> +		return -EINVAL;
>> +	}
>> +
>> +	ahb_seq  = burst << TEGRA_AHBDMA_CHANNEL_AHB_BURST_SHIFT;
>> +	ahb_seq |= TEGRA_AHBDMA_CHANNEL_ADDR_WRAP;
>> +	ahb_seq |= TEGRA_AHBDMA_CHANNEL_INTR_ENB;
>> +
>> +	writel_relaxed(ahb_seq,
>> +		       ahbdma_chan->regs + TEGRA_AHBDMA_CHANNEL_AHB_SEQ);
>> +
>> +	writel_relaxed(ahb_addr,
>> +		       ahbdma_chan->regs + TEGRA_AHBDMA_CHANNEL_AHB_PTR);
> 
> oh no, you don't write to HW here. This can be called anytime when you have
> txn running! You should save these and use them in prep_ calls.
> 

BTW, some of the DMA drivers have exactly the same problem. I now see that it is
actually documented explicitly in provider.txt, but that's inconsistent across
the actual drivers.

-- 
Dmitry