linux-kernel - Re: [PATCH v5] i2c: imx: support DMA defer probing

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Message-ID:
 <AM0PR0402MB3937E87BBDC5CEA1C51E4E3AE8072@AM0PR0402MB3937.eurprd04.prod.outlook.com>
Date: Fri, 20 Dec 2024 10:46:17 +0000
From: Carlos Song <carlos.song@....com>
To: Ahmad Fatoum <a.fatoum@...gutronix.de>, Oleksij Rempel
	<o.rempel@...gutronix.de>
CC: Andi Shyti <andi.shyti@...nel.org>, Frank Li <frank.li@....com>,
	"kernel@...gutronix.de" <kernel@...gutronix.de>, "shawnguo@...nel.org"
	<shawnguo@...nel.org>, "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
	"festevam@...il.com" <festevam@...il.com>, "linux-i2c@...r.kernel.org"
	<linux-i2c@...r.kernel.org>, "imx@...ts.linux.dev" <imx@...ts.linux.dev>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Clark Wang <xiaoning.wang@....com>
Subject: Re: [PATCH v5] i2c: imx: support DMA defer probing



> -----Original Message-----
> From: Ahmad Fatoum <a.fatoum@...gutronix.de>
> Sent: Friday, December 20, 2024 5:32 PM
> To: Carlos Song <carlos.song@....com>; Oleksij Rempel
> <o.rempel@...gutronix.de>
> Cc: Andi Shyti <andi.shyti@...nel.org>; Frank Li <frank.li@....com>;
> kernel@...gutronix.de; shawnguo@...nel.org; s.hauer@...gutronix.de;
> festevam@...il.com; linux-i2c@...r.kernel.org; imx@...ts.linux.dev;
> linux-arm-kernel@...ts.infradead.org; linux-kernel@...r.kernel.org; Clark Wang
> <xiaoning.wang@....com>
> Subject: [EXT] Re: [PATCH v5] i2c: imx: support DMA defer probing
>
> Caution: This is an external email. Please take care when clicking links or
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>
>
> Hi Carlos,
>
> On 20.12.24 10:23, Carlos Song wrote:
> >>> Hi, the ret is from i2c_imx_dma_request() and look like that ret has
> >>> been converted by PTR_ERR, So the ret error has been human readable
> >> version?
> >>
> >> I am not sure I understand the question. ERR_PTR() makes an error
> >> pointer and %pe formats that pointer as error message. So you don't
> >> need to change any function return types unless needed, just at the
> >> end print it with %pe instead of %d (and after error pointer conversion if
> needed).
> >
> > Sorry, I don't know if I understand it incorrectly.
> > I review other driver code, most choose to return error value but not an error
> pointer.
> > Shouldn't error value be more readable than error pointers?
> > When we see -110 we know TIMEOUT and we see -12 we know NO MEM.
>
> I know -110, but -12 I need to look up :) Both are cryptic to end users, which is
> why %pe was added on top of the existing %p:
>
> If CONFIG_SYMBOLIC_ERRNAME is enabled %pe expands to an error string, e.g.
> "ENOMEM" or "ETIMEDOUT". If it's disabled, you get the same error number
> that was printed raw before.
>
> Cheers,
> Ahmad
>

Wow! Looks so cool.
Thank you very much for your patient explanation! I agree it.

Also I will change the comment from your suggestion[1]:

"
  /*
   * As we can always fall back to PIO, let's ignore the error setting up
   * DMA and see if we run into errors while setting up PIO mode.
   */
"
In fact, other errors are also from DMA setting not from setting PIO mode.
So can I comment simply like this?

        /* 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));
        }

[1]https://lore.kernel.org/imx/89a3b1c9-2be2-4e7f-a0c6-abbf8b88957b@pengutronix.de/

BR
Carlos
> >
> > i2c_imx_dma_request is using PTR_ERR to convert pointer to error value[1].
> > I don't know why need to use ERR_PTR to reconvert the value to pointer:
> >
> > dev_err(&pdev->dev, "Failed to setup DMA (%pe), only use PIO mode\n",
> > ERR_PTR(ret));
> >
> > Is there some strong reason?
> >
> > [1]
> > https://lore/
> > .kernel.org%2Fimx%2FAM0PR0402MB3937419BBB58B75FB8F8DE2DE8072%
> 40AM0PR04
> >
> 02MB3937.eurprd04.prod.outlook.com%2F&data=05%7C02%7Ccarlos.song%4
> 0nxp
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