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

From: Russell King - ARM Linux admin <[email-masked]>
Date: Thu, 12 Dec 2019 10:58:57 +0000

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.

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.

-- 
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