linux-kernel - Re: [PATCH v16 2/4] dmaengine: tegra: Add tegra gpcdma driver

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Date:   Sat, 22 Jan 2022 21:31:39 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Akhil R <akhilrajeev@...dia.com>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.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 v16 2/4] dmaengine: tegra: Add tegra gpcdma driver

21.01.2022 19:24, Akhil R пишет:
>>>>>>> +static int tegra_dma_terminate_all(struct dma_chan *dc)
>>>>>>> +{
>>>>>>> +     struct tegra_dma_channel *tdc = to_tegra_dma_chan(dc);
>>>>>>> +     unsigned long flags;
>>>>>>> +     LIST_HEAD(head);
>>>>>>> +     int err;
>>>>>>> +
>>>>>>> +     if (tdc->dma_desc) {
>>>>>>
>>>>>> Needs locking protection against racing with the interrupt handler.
>>>>> tegra_dma_stop_client() waits for the in-flight transfer
>>>>> to complete and prevents any additional transfer to start.
>>>>> Wouldn't it manage the race? Do you see any potential issue there?
>>>>
>>>> You should consider interrupt handler like a process running in a
>>>> parallel thread. The interrupt handler sets tdc->dma_desc to NULL, hence
>>>> you'll get NULL dereference in tegra_dma_stop_client().
>>>
>>> Is it better if I remove the below part from tegra_dma_stop_client() so
>>> that dma_desc is not accessed at all?
>>>
>>> +     wcount = tdc_read(tdc, TEGRA_GPCDMA_CHAN_XFER_COUNT);
>>> +     tdc->dma_desc->bytes_transferred +=
>>> +                     tdc->dma_desc->bytes_requested - (wcount * 4);
>>>
>>> Because I don't see a point in updating the value there. dma_desc is set
>>> to NULL in the next step in terminate_all() anyway.
>>
>> That isn't going help you much because you also can't release DMA
>> descriptor while interrupt handler still may be running and using that
>> descriptor.
> 
> Does the below functions look good to resolve the issue, provided
> tegra_dma_stop_client() doesn't access dma_desc?

Stop shall not race with the start.

> +static int tegra_dma_terminate_all(struct dma_chan *dc)
> +{
> +       struct tegra_dma_channel *tdc = to_tegra_dma_chan(dc);
> +       unsigned long flags;
> +       LIST_HEAD(head);
> +       int err;
> +
> +       err = tegra_dma_stop_client(tdc);
> +       if (err)
> +               return err;
> +
> +       tegra_dma_stop(tdc);
> +
> +       spin_lock_irqsave(&tdc->vc.lock, flags);
> +       tegra_dma_sid_free(tdc);
> +       tdc->dma_desc = NULL;
> +
> +       vchan_get_all_descriptors(&tdc->vc, &head);
> +       spin_unlock_irqrestore(&tdc->vc.lock, flags);
> +
> +       vchan_dma_desc_free_list(&tdc->vc, &head);
> +
> +       return 0;
> +}
> 
> +static irqreturn_t tegra_dma_isr(int irq, void *dev_id)
> +{
> +       struct tegra_dma_channel *tdc = dev_id;
> +       struct tegra_dma_desc *dma_desc = tdc->dma_desc;
> +       struct tegra_dma_sg_req *sg_req;
> +       u32 status;
> +
> +       /* Check channel error status register */
> +       status = tdc_read(tdc, TEGRA_GPCDMA_CHAN_ERR_STATUS);
> +       if (status) {
> +               tegra_dma_chan_decode_error(tdc, status);
> +               tegra_dma_dump_chan_regs(tdc);
> +               tdc_write(tdc, TEGRA_GPCDMA_CHAN_ERR_STATUS, 0xFFFFFFFF);
> +       }
> +
> +       status = tdc_read(tdc, TEGRA_GPCDMA_CHAN_STATUS);
> +       if (!(status & TEGRA_GPCDMA_STATUS_ISE_EOC))
> +               return IRQ_HANDLED;
> +
> +       tdc_write(tdc, TEGRA_GPCDMA_CHAN_STATUS,
> +                 TEGRA_GPCDMA_STATUS_ISE_EOC);
> +
> +       spin_lock(&tdc->vc.lock);
> +       if (!dma_desc)
All checks and assignments must be done inside of critical section.


> +               goto irq_done;