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

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Date:   Tue, 1 Feb 2022 12:05:57 +0000
From:   Akhil R <akhilrajeev@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>
CC:     "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>,
        Pavan Kunapuli <pkunapuli@...dia.com>
Subject: RE: [PATCH v17 2/4] dmaengine: tegra: Add tegra gpcdma driver

> 31.01.2022 18:38, Akhil R пишет:
> > Does the below method look good? bytes_xfer is updated on every ISR and in
> > tx_status(), the wcount is read to calculate the intermittent value. If the
> transfer
> > get complete in between, use wcount as 0 to add sg_req.len to bytes_xfer
> >
> > static int tegra_dma_get_residual(struct tegra_dma_channel *tdc)
> > {
> >       unsigned long wcount = 0, status;
> >       unsigned int bytes_xfer, residual;
> >       struct tegra_dma_desc *dma_desc = tdc->dma_desc;
> >       struct tegra_dma_sg_req *sg_req = dma_desc->sg_req;
> >
> >       /*
> >        * Do not read from CHAN_XFER_COUNT if EOC bit is set
> >        * as the transfer would have already completed and
> >        * the register could have updated for next transfer
> >        * in case of cyclic transfers.
> >        */
> >       status = tdc_read(tdc, TEGRA_GPCDMA_CHAN_STATUS);
> >       if (!(status & TEGRA_GPCDMA_STATUS_ISE_EOC))
> >               wcount = tdc_read(tdc, TEGRA_GPCDMA_CHAN_XFER_COUNT);
> 
> You can't read WCOUNT after the STATUS without racing with the STATUS
> updates made by h/w. You should read the WCOUNT first and only then
> check the STATUS.
> 
> You should also check whether T20 tegra_dma_sg_bytes_xferred()
> workarounds apply to newer h/w. I see that the h/w base hasn't changed
> much since T20.
The calculation in T20 driver is not applicable here. The register shows the
actual number of words remaining to be transferred as far as I understand.

Thanks,
Akhil