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

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Message-ID: <DM5PR12MB1850FF1DC4DC1714E31AADB5C0589@DM5PR12MB1850.namprd12.prod.outlook.com>
Date:   Tue, 18 Jan 2022 05:36:01 +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

> 17.01.2022 10:02, Akhil R пишет:
> >> 10.01.2022 19:05, 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.

Thanks,
Akhil