linux-kernel - Re: [PATCH v17 2/4] dmaengine: tegra: Add tegra gpcdma driver

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Date:   Mon, 31 Jan 2022 02:23:28 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Akhil R <akhilrajeev@...dia.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Krishna Yarlagadda <kyarlagadda@...dia.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
        Rajesh Gumasta <rgumasta@...dia.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "vkoul@...nel.org" <vkoul@...nel.org>
Cc:     Pavan Kunapuli <pkunapuli@...dia.com>
Subject: Re: [PATCH v17 2/4] dmaengine: tegra: Add tegra gpcdma driver

30.01.2022 19:43, Akhil R пишет:
>> 30.01.2022 13:26, Dmitry Osipenko пишет:
>>> 30.01.2022 13:05, Dmitry Osipenko пишет:
>>>> Still nothing prevents interrupt handler to fire during the pause.
>>>>
>>>> What you actually need to do is to disable/enable interrupt. This
>>>> will prevent the interrupt racing and then pause/resume may look like this:
>>>
>>> Although, seems this won't work, unfortunately. I see now that
>>> device_pause() doesn't have might_sleep().
>>>
>>
>> Ah, I see now that the pause/unpause is actually a separate control and doesn't
>> conflict with "start next transfer".
>>
>> So you just need to set/unset the pause under lock. And don't touch
>> tdc->dma_desc. That's it.
>>
>> static int tegra_dma_device_pause(struct dma_chan *dc) {
>>         struct tegra_dma_channel *tdc = to_tegra_dma_chan(dc);
>>         unsigned long flags;
>>         u32 val;
>>
>>         if (!tdc->tdma->chip_data->hw_support_pause)
>>                 return -ENOSYS;
>>
>>         spin_lock_irqsave(&tdc->vc.lock, flags);
>>
>>         val = tdc_read(tdc, TEGRA_GPCDMA_CHAN_CSRE);
>>         val |= TEGRA_GPCDMA_CHAN_CSRE_PAUSE;
>>         tdc_write(tdc, TEGRA_GPCDMA_CHAN_CSRE, val);
>>
>>         spin_unlock_irqrestore(&tdc->vc.lock, flags);
>>
>>         return 0;
>> }
>>
>> static int tegra_dma_device_resume(struct dma_chan *dc) {
>>         struct tegra_dma_channel *tdc = to_tegra_dma_chan(dc);
>>         unsigned long flags;
>>         u32 val;
>>
>>         if (!tdc->tdma->chip_data->hw_support_pause)
>>                 return -ENOSYS;
>>
>>         spin_lock_irqsave(&tdc->vc.lock, flags);
>>
>>         val = tdc_read(tdc, TEGRA_GPCDMA_CHAN_CSRE);
>>         val &= ~TEGRA_GPCDMA_CHAN_CSRE_PAUSE;
>>         tdc_write(tdc, TEGRA_GPCDMA_CHAN_CSRE, val);
>>
>>         spin_unlock_irqrestore(&tdc->vc.lock, flags);
>>
>>         return 0;
>> }
> 
> The reason I separated out register writes was to conveniently call those
> in dma_start() and terminate_all(). Do you see any issue there?
> The recommended way of terminating a transfer in between is to pause
> it before disabling the channel.

This is a sample code, feel free to adjust it as needed.

> dma_desc could be NULL while these functions are called. pause() or
> resume() is unneeded if there isn't any transfer going on. Moreover,
> if we are to calculate the xfer_size, the check would be mandatory.

For now looks like it should be better to move the xfer_size updating to
other places, like terminate_all() and tx_status().

I also see now that you have
residue_granularity=DMA_RESIDUE_GRANULARITY_BURST, while tx_status()  in
fact uses segment granularity. This needs to be corrected.

You also must add locking to tx_status(), to protect tdc->dma_desc
pointer updates.