Date:   Thu, 27 Jan 2022 16:29:36 +0000
From:   Akhil R <akhilrajeev@...dia.com>
To:     Dmitry Osipenko <digetx@...il.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.
Thanks for the points. I collected more details on this scenario and found that
DMA can only be paused if there is an in-flight transfer (i.e no interrupt will be
pending). And there cannot be an interrupt after DMA is paused. Hence, there
is no case of ISR unpausing a paused transfer. So, I guess, the check or a lock
might not be required there.

One thing I am thinking of is to wait for DMA to be busy so that it can be paused.
I would add these changes and send out a new patch.

Thanks,
Akhil

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