[PATCH v6] i2c: imx: support DMA defer probing

[Carlos Song]

> -----Original Message-----
> From: Andi Shyti <andi.shyti at kernel.org>
> Sent: Tuesday, December 24, 2024 7:14 AM
> To: Carlos Song <carlos.song at nxp.com>
> Cc: o.rempel at pengutronix.de; kernel at pengutronix.de; shawnguo at kernel.org;
> s.hauer at pengutronix.de; festevam at gmail.com; linux-i2c at vger.kernel.org;
> imx at lists.linux.dev; linux-arm-kernel at lists.infradead.org;
> linux-kernel at vger.kernel.org; Clark Wang <xiaoning.wang at nxp.com>; Ahmad
> Fatoum <a.fatoum at pengutronix.de>
> Subject: [EXT] Re: [PATCH v6] i2c: imx: support DMA defer probing
> 
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> 
> Hi Carlos,
> 
> ...
> 
> > @@ -1802,6 +1803,18 @@ static int i2c_imx_probe(struct platform_device
> *pdev)
> >       if (ret == -EPROBE_DEFER)
> >               goto clk_notifier_unregister;
> >
> > +     /* As we can always fall back to PIO, let's ignore the error setting up
> DMA. */
> > +     ret = i2c_imx_dma_request(i2c_imx, phy_addr);
> > +     if (ret) {
> > +             if (ret == -EPROBE_DEFER)
> > +                     goto clk_notifier_unregister;
> > +             else if (ret == -ENODEV)
> > +                     dev_dbg(&pdev->dev, "Only use PIO mode\n");
> > +             else
> > +                     dev_err(&pdev->dev, "Failed to setup DMA (%pe),
> only use PIO mode\n",
> > +                             ERR_PTR(ret));
> 
> My question here is not just about the use of dev_err vs dev_err_probe, but why
> don't we exit the probe if we get an error.
> 
> We should use PIO only in case of ENODEV, in all the other cases I think we
> should just leave. E.g. why don't we exit if we meet ret == -ENOMEM?

Hi, Andi

Thank you! From my point, I2C is critical bus so it should be available as much as possible.
-ENOMEM or other unknown errors all are from i2c_imx_dma_request(). So error happened in enable DMA mode process.

As I comment at previous mail[1]:
DMA mode should be optional for i2c-imx[2], i2c-imx can accept DMA mode not enabled.
Even though DMA mode can not be enabled by some known/unknown issue, I2C still can work in PIO mode in all time for all cases.
As a result, don't exit the I2C probe and only print error to show i2c DMA error.

This patch just is used to make i2c-imx can support defer probe to use DMA resources as much as possible.

If meet a DMA error then exit i2c probe, which means binding I2C to DMA, this is not what we expect. Once the DMA encounters a problem,
the entire I2C bus and peripherals will not be able to start, this is not a small damage, so we use current logic.

[1]: https://lore.kernel.org/imx/AM0PR0402MB39374E34FD6133B5E3D414D7E82F2@AM0PR0402MB3937.eurprd04.prod.outlook.com/
[2]:
commit ce1a78840ff7ab846065d5b65eaac959bafe1949
Author: Yao Yuan <yao.yuan at freescale.com>
Date:   Tue Nov 18 18:31:06 2014 +0800

    i2c: imx: add DMA support for freescale i2c driver

    Add dma support for i2c. This function depend on DMA driver.
    You can turn on it by write both the dmas and dma-name properties in dts node.
    DMA is optional, even DMA request unsuccessfully, i2c can also work well.

    Signed-off-by: Yuan Yao <yao.yuan at freescale.com>
    Signed-off-by: Wolfram Sang <wsa at the-dreams.de>

Carlos
> 
> Andi