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

-----Original Message-----
From: Andi Shyti <andi.shyti@kernel.org>
Sent: Tuesday, December 24, 2024 7:14 AM
To: Carlos Song <redacted>
Cc: o.rempel@pengutronix.de; kernel@pengutronix.de; shawnguo@kernel.org;
s.hauer@pengutronix.de; festevam@gmail.com; linux-i2c@vger.kernel.org;
imx@lists.linux.dev; linux-arm-kernel@lists.infradead.org;
linux-kernel@vger.kernel.org; Clark Wang [off-list ref]; Ahmad
Fatoum [off-list ref]
Subject: [EXT] Re: [PATCH v6] i2c: imx: support DMA defer probing

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Hi Carlos,

...

quoted

@@ -1802,6 +1803,18 @@ static int i2c_imx_probe(struct platform_device

*pdev)

quoted

      if (ret == -EPROBE_DEFER)
              goto clk_notifier_unregister;

+     /* As we can always fall back to PIO, let's ignore the error setting up

DMA. */

quoted

+     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",

quoted

+                             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/ (local)
[2]:
commit ce1a78840ff7ab846065d5b65eaac959bafe1949
Author: Yao Yuan [off-list ref]
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 [off-list ref]
    Signed-off-by: Wolfram Sang [off-list ref]

Carlos

Andi