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

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Message-ID: <451e8c12-7b45-4b12-af10-63f60ba6e92e@pengutronix.de>
Date: Fri, 20 Dec 2024 10:31:59 +0100
From: Ahmad Fatoum <a.fatoum@...gutronix.de>
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" <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

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

> 
> 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/imx/AM0PR0402MB3937419BBB58B75FB8F8DE2DE8072@AM0PR0402MB3937.eurprd04.prod.outlook.com/
>>
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