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

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Date:   Mon, 24 Jan 2022 00:07:37 +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

23.01.2022 19:49, Akhil R пишет:
>> 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.
> 
> Okay. So, the lock should be held throughout the function.
> Do you think tegra_dma_pause should also hold a lock
> and remove irq_synchronize? That function also writes
> to CSR register.

Interrupt handler shall not unpause channel in a case of race condition,
it should handle completed transfer and check whether channel is paused
before issuing next transfer.
So yes, pause also needs a lock.