RE: [EXT] Re: [PATCH] i2c: imx: Defer probing if EDMA not available

Peng Ma
2019-12-13T10:34:03Z

>-----Original Message-----
>From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
>Sent: 2019年12月12日 18:59
>To: Peng Ma <peng.ma@nxp.com>
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>Subject: Re: [EXT] Re: [PATCH] i2c: imx: Defer probing if EDMA not available
>
>Caution: EXT Email
>
>On Thu, Dec 12, 2019 at 03:09:32AM +0000, Peng Ma wrote:
>> Hello Russell,
>>
>> Thanks very much for your strict guidance and comments.
>> I realized it is hard to us that we want to i2c used edma when edma
>> probe after i2c probe.
>
>I have no problem with that aim. I'm just very concerned by the proposed
>implementation, especially when it has already been proven to cause
>regressions in the kernel. I seem to remember that the infinite loop caused
>other issues, such as the system being unable to complete booting.
>
>> I look forward to discussing with you as below, if you like.
>> Thanks.
>>
>> You say I could do this:
>> "So, if you want to do this (and yes, I'd also encourage it to be
>> conditional on EDMA being built-in, as I2C is commonly used as a way
>> to get at RTCs, which are read before kernel modules can be loaded)
>> then you MUST move
>> i2c_imx_dma_request() before
>> i2c_add_numbered_adapter() to avoid the infinite loop."
>>
>> Even if I do this, It's hard to avoid the infinite loop of i2c probe caused by
>EDMA(build-in) initialization failure.
>
>It isn't clear what you mean here.
>
>If EDMA fails to probe (because fsl_edma_probe() returns an error other than
>EPROBE_DEFER) then of_dma_find_controller() will return NULL. That will be
>propagated down through i2c_imx_dma_request(). This is no different from the
>case where EDMA is built as a module. It is also no different from the case
>where EDMA hasn't yet been probed.
>
Hello Russell,

The result of my test is not like that, It is still with probe loop, the test config as follows:
1.EDMA build-in
2.return -EINVAL top of fsl_edma_probe when edma probe
3.i2c probe with original patch, I put the i2c_imx_dma_request in front of i2c_add_numbered_adapter or used original patch.

I send you the function of_dma_request_slave_channel could explain it last mail,
"Return -EPROBE_DEFER" depends on:
1. edma not probe or probe failed
2. There is edma node in DTS and I2C with edma property

>If i2c_imx_dma_request() is placed after i2c_add_numbered_adapter(), and
>EPROBE_DEFER is propagated out of i2c_imx_probe(), then _yes_, it will cause
>an infinite loop, because you are replicating the exact conditions that caused
>the attempt to propagate i2c_imx_dma_request()'s return value to be reverted
>last time - which brought the kernel to a grinding halt.
>
>If i2c_imx_dma_request() is placed before i2c_add_numbered_adapter(), then
>there is no infinite deferred probing loop - yes, i2c_imx_probe() will be called as
>a result of other drivers successfully probing, and each time it will return
>EPROBE_DEFER, but the _key_ point is that the action of i2c_imx_probe() will
>not _self trigger_ the deferred probing _and_ place itself onto the deferred
>probe list.
>
>Please, rather than continuing to send emails arguing over this point,
>investigate the stated issue with some practical tests:
>
>1. Make i2c_imx_probe() propagate i2c_imx_dma_request()'s return value,
> as it did in the original patch.
>2. Build i2c-imx into the kernel.
>3. Build edma as a module.
>4. Build and test boot the kernel and check what happens.
>5. Move i2c_imx_dma_request() before i2c_add_numbered_adapter() 6. Build
>and test boot the resulting kernel and note any differences.
>
[Peng Ma] the i2c probe loop still exist but not infinite loop with below cases:
1:
Used original patch
Build i2c-imx into the kernel
Build edma into the kernel
2:
Used original patch
Build i2c-imx into the kernel
Build edma into the kernel
Move i2c_imx_dma_request() before i2c_add_numbered_adapter()
3:
Used original patch
Build i2c-imx into the kernel
Build edma as a module
Move i2c_imx_dma_request() before i2c_add_numbered_adapter()

I saw the commit e8c220fac415d9f4a994b0c2871b835feac1eb4e you said
1. i2c_imx_probe() is called and successfully registers an I2C
adapter via i2c_add_numbered_adapter()

2. As a part of i2c_add_numbered_adapter() new I2C slave devices
are added from DT which results in a call to
driver_deferred_probe_trigger()

3. i2c_imx_probe() continues and calls i2c_imx_dma_request() which
due to lack of proper DMA driver returns -EPROBE_DEFER

4. i2c_imx_probe() fails, removes I2C adapter and returns
-EPROBE_DEFER, which places it into deferred probe list

5. Deferred probe work triggered in #2 above kicks in and calls
i2c_imx_probe() again thus bringing us to step #1"
Can I understand 4, you mean just remove I2C adapter the i2c slave devices not be removed? Then the i2c slave devices will probe their drivers triggered the driver_deferred_probe_trigger().
If so, My test is not like this, when the i2c probe failed the i2c slave devices will be removed and I2C adapter will be removed too.

Best Regards,
Peng
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